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Sunday, May 9, 2021

Strategic Defense Initiative

From Wikipedia, the free encyclopedia
 
Strategic Defense Initiative Organization
Sdilogo.svg
Agency overview
Formed1984
Dissolved1993 (renamed)
Superseding agency
JurisdictionFederal government of the United States

The Strategic Defense Initiative (SDI), nicknamed the "Star Wars program", was a proposed missile defense system intended to protect the United States from attack by ballistic strategic nuclear weapons (intercontinental ballistic missiles and submarine-launched ballistic missiles). The concept was first announced on March 23, 1983 by President Ronald Reagan, a vocal critic of the doctrine of mutually assured destruction (MAD), which he described as a "suicide pact", and called upon American scientists and engineers to develop a system that would render nuclear weapons obsolete.

The Strategic Defense Initiative Organization (SDIO) was set up in 1984 within the US Department of Defense to oversee development. A wide array of advanced weapon concepts, including lasers, particle beam weapons and ground- and space-based missile systems were studied, along with various sensor, command and control, and high-performance computer systems that would be needed to control a system consisting of hundreds of combat centers and satellites spanning the entire globe and involved in a very short battle. The United States holds a significant advantage in the field of comprehensive advanced missile defense systems through decades of extensive research and testing and a number of these concepts and obtained technologies and insights were transferred to subsequent programs.

Under the SDIO's Innovative Sciences and Technology Office, headed by physicist and engineer Dr. James Ionson, the investment was predominantly made in basic research at national laboratories, universities, and in industry; these programs have continued to be key sources of funding for top research scientists in the fields of high-energy physics, supercomputing/computation, advanced materials, and many other critical science and engineering disciplines and funding which indirectly supports other research work by top scientists.

In 1987, the American Physical Society concluded that the technologies being considered were decades away from being ready for use, and at least another decade of research was required to know whether such a system was even possible. After the publication of the APS report, SDI‘s budget was repeatedly cut. By the late 1980s, the effort had been re-focused on the "Brilliant Pebbles" concept using small orbiting missiles not unlike a conventional air-to-air missile, which was expected to be much less expensive to develop and deploy.

SDI was controversial in some sectors, and was criticized for threatening to destabilize the MAD-approach potentially rendering the Soviet nuclear arsenal useless and to possibly re-ignite "an offensive arms race". Through declassified papers of American intelligence agencies the wider implications and effects of the program were examined and revealed that the potential neutralization of its arsenal and resulting loss of a balancing power factor SDI was a cause of grave concern for the Soviet Union and her primary successor state Russia. By the early 1990s, with the Cold War ending and nuclear arsenals being rapidly reduced, political support for SDI collapsed. SDI officially ended in 1993, when the Clinton Administration redirected the efforts towards theatre ballistic missiles and renamed the agency the Ballistic Missile Defense Organization (BMDO).

History

National BMD

The US Army had considered the issue of ballistic missile defense (BMD) as early as late in World War II. Studies on the topic suggested attacking a V-2 rocket would be difficult because the flight time was so short that it would leave little time to forward information through command and control networks to the missile batteries that would attack them. Bell Labs pointed out that although longer-range missiles flew much faster, their longer flight times would address the timing issue and their very high altitudes would make long-range detection by radar easier.

This led to a series of projects including Nike Zeus, Nike-X, Sentinel and ultimately the Safeguard Program, all aimed at deploying a nationwide defensive system against attacks by Soviet ICBMs. The reason for so many programs was the rapidly changing strategic threat; the Soviets claimed to be producing missiles "like sausages", and ever-more missiles would be needed to defend against this growing fleet. Low-cost countermeasures like radar decoys required additional interceptors to counter. An early estimate suggested one would have to spend $20 on defense for every $1 the Soviets spent on offense. The addition of MIRV in the late 1960s further upset the balance in favor of offense systems. This cost-exchange ratio was so favorable that it appeared the only thing building a defense would do would be to cause an arms race.

The Extended Range Nike Zeus/Spartan missile of the late-1960s was designed to provide full-country defense as part of the Sentinel-Safeguard programs. Projected to cost $40 billion ($315 billion in 2021) it would have offered minimal protection and damage prevention in an all-out attack.

When initially faced with this problem, Dwight D. Eisenhower asked ARPA to consider alternative concepts. Their Project Defender studied all sorts of systems, before abandoning most of them to concentrate on Project BAMBI. BAMBI used a series of satellites carrying interceptor missiles that would attack the Soviet ICBMs shortly after launch. This boost phase intercept rendered MIRV impotent; a successful attack would destroy all of the warheads. Unfortunately, the operational cost of such a system would be enormous, and the US Air Force continually rejected such concepts. Development was cancelled in 1963.

Through this period, the entire topic of BMD became increasingly controversial. Early deployment plans were met with little interest, but by the late 1960s, public meetings on the Sentinel system were met by thousands of angry protesters. After thirty years of effort, only one such system would be built; a single base of the original Safeguard system became operational in April 1975, only to shut down in February 1976.

A Soviet military A-35 anti-ballistic missile system was deployed around Moscow to intercept enemy ballistic missiles targeting the city or its surrounding areas. The A-35 was the only Soviet ABM system allowed under the 1972 Anti-Ballistic Missile Treaty. In development since the 1960s and in operation from 1971 until the 1990s, it featured the nuclear-tipped A350 exoatmospheric interceptor missile.

Lead up to SDI

The bright spikes extending below the initial fireball of one of 1952's Operation Tumbler–Snapper test shots, are known as the "rope trick effect". They are caused by the intense flash of thermal/soft X-rays released by the explosion heating the steel tower guy-wires white hot. The development of the W71 and the Project Excalibur x-ray laser were based on enhancing the destructive effects of these x-rays.

George Shultz, Reagan's secretary of state, suggested that a 1967 lecture by physicist Edward Teller (the so-called "father of the hydrogen bomb") was an important precursor to SDI. In the lecture, Teller talked about the idea of defending against nuclear missiles using nuclear weapons, principally the W65 and W71, with the latter being a contemporary enhanced thermal/X-ray device used actively on the Spartan missile in 1975. Held at Lawrence Livermore National Laboratory (LLNL), the 1967 lecture was attended by Reagan shortly after he became the governor of California.

Development of laser weapons in the Soviet Union began in 1964–1965. Though classified at the time, a detailed study on a Soviet space-based laser system began no later than 1976 as the Skif, a 1 MW Carbon dioxide laser along with the anti-satellite Kaskad, an in-orbit missile platform.

A revolver cannon (Rikhter R-23) was mounted on the 1974 Soviet Salyut 3 space station, a satellite that successfully test fired its cannon in orbit.

In 1979, Teller contributed to a Hoover Institution publication where he claimed that the US would be facing an emboldened USSR due to their work on civil defense. Two years later at a conference in Italy, he made the same claims about their ambitions, but with a subtle change; now he claimed that the reason for their boldness was their development of new space-based weapons. According to the popular opinion at the time, and one shared by author Frances FitzGerald; there was absolutely no evidence that such research was being carried out. What had really changed was that Teller was now selling his latest nuclear weapon, the X-ray laser. Finding limited success in his efforts to get funding for the project, his speech in Italy was a new attempt to create a missile gap.

In 1979, Reagan visited the NORAD command base, Cheyenne Mountain Complex, where he was first introduced to the extensive tracking and detection systems extending throughout the world and into space; however, he was struck by their comments that while they could track the attack down to the individual targets, there was nothing one could do to stop it. Reagan felt that in the event of an attack this would place the president in a terrible position, having to choose between immediate counterattack or attempting to absorb the attack and then maintain an upper hand in the post-attack era. Shultz suggests that this feeling of helplessness, coupled with the defensive ideas proposed by Teller a decade earlier, combined to form the impetus of the SDI.

In the fall of 1979, at Reagan's request, Lieutenant General Daniel O. Graham, the former head of the DIA, briefed Reagan on an updated BAMBI he called High Frontier, a missile shield composed of multi-layered ground- and space-based weapons that could track, intercept, and destroy ballistic missiles, which would theoretically be possible because of emerging technologies. It was designed to replace the MAD doctrine that Reagan and his aides described as a suicide pact. In September 1981, Graham formed a small, Virginia-based think tank called High Frontier to continue research on the missile shield. The Heritage Foundation provided High Frontier with space to conduct research, and Graham published a 1982 report entitled, "High Frontier: A New National Strategy" that examined in greater detail how the system would function.

Graham was not alone in considering the anti-missile problem. Since the late 1970s, a group had been pushing for the development of a high-powered chemical laser that would be placed in orbit and attack ICBMs, the Space Based Laser (SBL). More recently, new developments under Project Excalibur by Teller's "O-Group" at LLNL suggested that a single X-ray laser could shoot down dozens of missiles with a single shot. Graham organized a meeting space at the Heritage Foundation in Washington and the groups began to meet in order to present their plans to the incoming president.

The group met with Reagan several times during 1981 and 1982, apparently with little effect, while the buildup of new offensive weaponry like the B-1 Lancer and MX missile continued; however, in early 1983, the Joint Chiefs of Staff met with the president and outlined the reasons why they might consider shifting some of the funding from the offensive side to new defensive systems.

According to a 1983 US Interagency Intelligence Assessment, there was good evidence that in the late 1960s the Soviets were devoting serious thought to both explosive and non-explosive nuclear power sources for lasers.

Project and proposals

President Reagan delivering the March 23, 1983 speech initiating SDI

Announcement

On March 23, 1983, Reagan announced SDI in a nationally televised speech, stating "I call upon the scientific community who gave us nuclear weapons to turn their great talents to the cause of mankind and world peace: to give us the means of rendering these nuclear weapons impotent and obsolete."

Strategic Defense Initiative Organization (SDIO)

In 1984, the Strategic Defense Initiative Organization (SDIO) was established to oversee the program, which was headed by Lt. General James Alan Abrahamson USAF, a past Director of the NASA Space Shuttle program.

In addition to the ideas presented by the original Heritage group, a number of other concepts were also considered. Notable among these were particle-beam weapons, updated versions of nuclear shaped charges, and various plasma weapons. Additionally, the SDIO invested in computer systems, component miniaturization, and sensors.

Initially, the program focused on large scale systems designed to defeat a massive Soviet offensive strike. For this mission, SDIO concentrated almost entirely on the "high tech" solutions like lasers. Graham's proposal was repeatedly rejected by members of the Heritage group as well as within SDIO; when asked about it in 1985, Abrahamson suggested that the concept was underdeveloped and was not being considered.

By 1986, many of the promising ideas were failing. Teller's X-ray laser, run under Project Excalibur, failed several key tests in 1986 and was soon being suggested solely for the anti-satellite role. The particle beam concept was demonstrated to basically not work, as was the case with several other concepts. Only the Space Based Laser seemed to have any hope of developing in the short term, but it was growing in size due to its fuel consumption.

APS report

The American Physical Society (APS) had been asked by the SDIO to provide a review of the various concepts. They put together an all-star panel including many of the inventors of the laser, one of which was a Nobel laureate. Their initial report was presented in 1986, but due to classification issues it was not released to the public (in redacted form) until early 1987.

The report considered all of the systems then under development, and concluded none of them were anywhere near ready for deployment. Specifically, they noted that all of the systems had to improve their energy output by at least 100 times, and in some cases as much as a million. In other cases, like Excalibur, they dismissed the concept entirely. Their summary stated simply:

We estimate that all existing candidates for directed energy weapons (DEWs) require two or more orders of magnitude, (powers of 10) improvements in power output and beam quality before they may be seriously considered for application in ballistic missile defense systems.

In a best case scenario, they concluded that none of the systems could be deployed as an anti-missile system until into the next century.

Strategic Defense System

Faced with this report, and the press storm that followed, the SDIO changed direction. Beginning in late 1986, Abrahamson proposed that SDI would be based on the system he had previously dismissed, a version of High Frontier now renamed the "Strategic Defense System, Phase I Architecture". The name implied that the concept would be replaced by more advanced systems in future phases.

Strategic Defense System, or SDS, was largely the Smart Rocks concept with an added layer of ground-based missiles in the US. These missiles were intended to attack the enemy warheads that the Smart Rocks had missed. In order to track them when they were below the radar horizon, SDS also added a number of additional satellites flying at low altitude that would feed tracking information to both the space-based "garages" as well as the ground-based missiles. The ground-based systems operational today trace their roots back to this concept.

While SDS was being proposed, Lawrence Livermore National had introduced a new concept known as Brilliant Pebbles. This was essentially the combination of the sensors on the garage satellites and the low-orbit tracking stations on the Smart Rocks missile. Advancements in new sensors and microprocessors allowed all of this to be packaged into the volume of a small missile nose cone. Over the next two years, a variety of studies suggested that this approach would be cheaper, easier to launch and more resistant to counterattack, and in 1990 Brilliant Pebbles was selected as the baseline model for the SDS Phase 1.

Global Protection Against Limited Strikes (GPALS)

While SDIO and SDS was ongoing, the Warsaw Pact was rapidly disintegrating, culminating in the destruction of the Berlin Wall in 1989. One of the many reports on SDS considered these events, and suggested that the massive defense against a Soviet launch would soon be unnecessary, but that short and medium range missile technology would likely proliferate as the former Soviet Union disintegrated and sold off their hardware. One of the core ideas behind the GPALS system was that the Soviet Union would not always be assumed as the aggressor and the United States would not always be assumed as the target.

Instead of a heavy defense aimed at ICBMs, this report suggested realigning the deployment for the Global Protection Against Limited Strikes (GPALS). Against such threats the Brilliant Pebbles would have limited performance, largely because the missiles fired for only a short period and the warheads did not rise high enough for them to be easily tracked by a satellite above them. To the original SDS, GPALS added a new mobile ground-based missile, and added more low-orbit satellites known as Brilliant Eyes to feed information to the Pebbles.

GPALS was approved by President George H.W. Bush in 1991. The new system would cut the proposed costs of the SDI system from $53 billion to $41 billion over a decade. Also, instead of making plans to protect against thousands of incoming missiles, the GPALS system sought to provide flawless protection from up to two hundred nuclear missiles. The GPALS system also was able to protect the United States from attacks coming from all different parts of the world.

Ballistic Missile Defense Organization (BMDO)

In 1993, the Clinton administration further shifted the focus to ground-based interceptor missiles and theater scale systems, forming the Ballistic Missile Defense Organization (BMDO) and closing the SDIO. The Ballistic Missile Defense Organization was renamed again by the George W. Bush administration as the Missile Defense Agency and focused onto limited National Missile Defense.

Ground-based programs

Extended Range Interceptor (ERINT) launch from White Sands Missile Range

Extended Range Interceptor (ERINT)

The Extended Range Interceptor (ERINT) program was part of SDI's Theater Missile Defense Program and was an extension of the Flexible Lightweight Agile Guided Experiment (FLAGE), which included developing hit-to-kill technology and demonstrating the guidance accuracy of a small, agile, radar-homing vehicle.

FLAGE scored a direct hit against a MGM-52 Lance missile in flight, at White Sands Missile Range in 1987. ERINT was a prototype missile similar to the FLAGE, but it used a new solid-propellant rocket motor that allowed it to fly faster and higher than FLAGE.

Under BMDO, ERINT was later chosen as the MIM-104 Patriot (Patriot Advanced Capability-3,PAC-3) missile.

Homing Overlay Experiment (HOE)

4 m (13 ft) diameter web deployed by Homing Overlay Experiment

Given concerns about the previous programs using nuclear-tipped interceptors, in the 1980s the US Army began studies about the feasibility of hit-to-kill vehicles, i.e. interceptor missiles that would destroy incoming ballistic missiles just by colliding with them head-on.

The Homing Overlay Experiment (HOE) was the first hit-to-kill system tested by the US Army, and also the first successful hit-to-kill intercept of a mock ballistic missile warhead outside the Earth's atmosphere.

The HOE used a Kinetic Kill Vehicle (KKV) to destroy a ballistic missile. The KKV was equipped with an infrared seeker, guidance electronics and a propulsion system. Once in space, the KKV could extend a folded structure similar to an umbrella skeleton of 4 m (13 ft) diameter to enhance its effective cross section. This device would destroy the ICBM reentry vehicle on collision.

Four test launches were conducted in 1983 and 1984 at Kwajalein Missile Range in the Republic of the Marshall Islands. For each test a Minuteman missile was launched from Vandenberg Air Force Base in California carrying a single mock re-entry vehicle targeted for Kwajalein lagoon more than 4,000 miles (6,400 km) away.

After test failures with the first three flight tests because of guidance and sensor problems, the DOD reported that the fourth and final test on June 10, 1984 was successful, intercepting the Minuteman RV with a closing speed of about 6.1 km/s at an altitude of more than 160 km.

Although the fourth test was described as a success, the New York Times in August 1993 reported that the HOE4 test was rigged to increase the likelihood of a successful hit. At the urging of Senator David Pryor, the General Accounting Office investigated the claims and concluded that though steps were taken to make it easier for the interceptor to find its target (including some of those alleged by the New York Times), the available data indicated that the interceptor had been successfully guided by its onboard infrared sensors in the collision, and not by an onboard radar guidance system as alleged. Per the GAO report, the net effect of the DOD enhancements increased the infrared signature of the target vessel by 110% over the realistic missile signature initially proposed for the HOE program, but nonetheless the GAO concluded the enhancements to the target vessel were reasonable given the objectives of the program and the geopolitical consequences of its failure. Further, the report concluded that the DOD's subsequent statements before Congress about the HOE program "fairly characterize[d]" the success of HOE4, but confirmed that the DOD never disclosed to Congress the enhancements made to the target vessel.

The technology developed for the HOE system was later used by the SDI and expanded into the Exoatmospheric Reentry-vehicle Interception System (ERIS) program.

ERIS and HEDI

Developed by Lockheed as part of the ground-based interceptor portion of SDI, the Exoatmospheric Reentry-vehicle Interceptor Subsystem (ERIS) began in 1985, with at least two tests occurring in the early 1990s. This system was never deployed, but the technology of the system was used in the Terminal High Altitude Area Defense (THAAD) system and the Ground-Based Interceptor currently deployed as part of the Ground-Based Midcourse Defense (GMD) system.

Directed-energy weapon (DEW) programs

X-ray laser

The 1984 SDI concept of a space based Nuclear reactor pumped laser or a chemical hydrogen fluoride laser satellite, Resulted in this 1984 artist's concept of a laser-equipped satellite firing on another, causing a momentum change in the target object by laser ablation. Before having to cool and re-aim at further possible targets.
 
This early artwork of the Nuclear detonation pumped laser array depicts an Excalibur engaging three targets, simultaneously. In most descriptions, each Excalibur could fire at dozens of targets, which would be hundreds or thousands of kilometers away.

An early focus of the SDI effort was an X-ray lasers powered by nuclear explosions. Nuclear explosions give off a huge burst of X-rays, which the Excalibur concept intended to focus using a lasing medium consisting of metal rods. Many such rods would be placed around a warhead, each one aimed at a different ICBM, thus destroying many ICBMs in a single attack. It would cost much less for the US to build another Excalibur than the Soviets would need to build enough new ICBMs to counter it. The idea was first based on satellites, but when it was pointed out that these could be attacked in space, the concept moved to a "pop-up" concept, rapidly launched from a submarine off the Soviet northern coast.

However, on March 26, 1983, the first test, known as the Cabra event, was performed in an underground shaft and resulted in marginally positive readings that could be dismissed as being caused by a faulty detector. Since a nuclear explosion was used as the power source, the detector was destroyed during the experiment and the results therefore could not be confirmed. Technical criticism based upon unclassified calculations suggested that the X-ray laser would be of at best marginal use for missile defense. Such critics often cite the X-ray laser system as being the primary focus of SDI, with its apparent failure being a main reason to oppose the program; however, the laser was never more than one of the many systems being researched for ballistic missile defense.

Despite the apparent failure of the Cabra test, the long term legacy of the X-ray laser program is the knowledge gained while conducting the research. A parallel developmental program advanced laboratory X-ray lasers for biological imaging and the creation of 3D holograms of living organisms. Other spin-offs include research on advanced materials like SEAgel and Aerogel, the Electron-Beam Ion Trap facility for physics research, and enhanced techniques for early detection of breast cancer.

Chemical laser

SeaLite Beam Director, commonly used as the output for the MIRACL

Beginning in 1985, the Air Force tested an SDIO-funded deuterium fluoride laser known as Mid-Infrared Advanced Chemical Laser (MIRACL) at White Sands Missile Range. During a simulation, the laser successfully destroyed a Titan missile booster in 1985, however the test setup had the booster shell pressurized and under considerable compression loads. These test conditions were used to simulate the loads a booster would be under during launch. The system was later tested on target drones simulating cruise missiles for the US Navy, with some success. After the SDIO closed, the MIRACL was tested on an old Air Force satellite for potential use as an anti-satellite weapon, with mixed results. The technology was also used to develop the Tactical High Energy Laser, (THEL) which is being tested to shoot down artillery shells.

During the mid-to-late 1980s a number of panel discussions on lasers and SDI took place at various laser conferences. Proceedings of these conferences include papers on the status of chemical and other high power lasers at the time.

The Missile Defense Agency's Airborne Laser program uses a chemical laser which has successfully intercepted a missile taking off, so an offshoot of SDI could be said to have successfully implemented one of the key goals of the program.

Neutral particle beam

In July 1989, the Beam Experiments Aboard a Rocket (BEAR) program launched a sounding rocket containing a neutral particle beam (NPB) accelerator. The experiment successfully demonstrated that a particle beam would operate and propagate as predicted outside the atmosphere and that there are no unexpected side-effects when firing the beam in space. After the rocket was recovered, the particle beam was still operational. According to the BMDO, the research on neutral particle beam accelerators, which was originally funded by the SDIO, could eventually be used to reduce the half-life of nuclear waste products using accelerator-driven transmutation technology.

Laser and mirror experiments

Technicians at the Naval Research Laboratory (NRL) work on the Low-powered Atmosphere Compensation Experiment (LACE) satellite.

The High Precision Tracking Experiment (HPTE), launched with the Space Shuttle Discovery on STS-51-G, was tested June 21, 1985 when a Hawaii-based low-power laser successfully tracked the experiment and bounced the laser off of the HPTE mirror.

The Relay mirror experiment (RME), launched in February 1990, demonstrated critical technologies for space-based relay mirrors that would be used with an SDI directed-energy weapon system. The experiment validated stabilization, tracking, and pointing concepts and proved that a laser could be relayed from the ground to a 60 cm mirror on an orbiting satellite and back to another ground station with a high degree of accuracy and for extended durations.

Launched on the same rocket as the RME, the Low-power Atmospheric Compensation Experiment (LACE) satellite was built by the United States Naval Research Laboratory (NRL) to explore atmospheric distortion of lasers and real-time adaptive compensation for that distortion. The LACE satellite also included several other experiments to help develop and improve SDI sensors, including target discrimination using background radiation and tracking ballistic missiles using Ultraviolet Plume Imaging (UVPI). LACE was also used to evaluate ground-based adaptive optics, a technique now used in civilian telescopes to remove atmospheric distortions.

Hypervelocity Railgun (CHECMATE)

Research out of hypervelocity railgun technology was done to build an information base about railguns so that SDI planners would know how to apply the technology to the proposed defense system. The SDI railgun investigation, called the Compact High Energy Capacitor Module Advanced Technology Experiment, had been able to fire two projectiles per day during the initiative. This represented a significant improvement over previous efforts, which were only able to achieve about one shot per month. Hypervelocity railguns are, at least conceptually, an attractive alternative to a space-based defense system because of their envisioned ability to quickly shoot at many targets. Also, since only the projectile leaves the gun, a railgun system can potentially fire many times before needing to be resupplied.

A hypervelocity railgun works very much like a particle accelerator insofar as it converts electrical potential energy into kinetic energy imparted to the projectile. A conductive pellet (the projectile) is attracted down the rails by electric current flowing through a rail. Through the magnetic forces that this system achieves, a force is exerted on the projectile moving it down the rail. Railguns can generate muzzle-velocities in excess of 2.4 kilometers per second.

Railguns face a host of technical challenges before they will be ready for battlefield deployment. First, the rails guiding the projectile must carry very high power. Each firing of the railgun produces tremendous current flow (almost half a million amperes) through the rails, causing rapid erosion of the rail's surfaces (through ohmic heating), and even vaporization of the rail surface. Early prototypes were essentially single-use weapons, requiring complete replacement of the rails after each firing. Another challenge with the railgun system is projectile survivability. The projectiles experience acceleration force in excess of 100,000 g. To be effective, the fired projectile must first survive the mechanical stress of firing and the thermal effects of a trip through the atmosphere at many times the speed of sound before its subsequent impact with the target. In-flight guidance, if implemented, would require the onboard navigation system to be built to the same level of sturdiness as the main mass of the projectile.

In addition to being considered for destroying ballistic missile threats, railguns were also being planned for service in space platform (sensor and battle station) defense. This potential role reflected defense planner expectations that the railguns of the future would be capable of not only rapid fire, but also of multiple firings (on the order of tens to hundreds of shots).

Space-based programs

Space-Based Interceptor (SBI)

Groups of interceptors were to be housed in orbital modules. Hover testing was completed in 1988 and demonstrated integration of the sensor and propulsion systems in the prototype SBI. It also demonstrated the ability of the seeker to shift its aiming point from a rocket's hot plume to its cool body, a first for infrared ABM seekers. Final hover testing occurred in 1992 using miniaturized components similar to what would have actually been used in an operational interceptor. These prototypes eventually evolved into the Brilliant Pebbles program.

Brilliant Pebbles

Brilliant Pebbles concept artwork

Brilliant Pebbles was a non-nuclear system of satellite-based interceptors designed to use high-velocity, watermelon-sized, teardrop-shaped projectiles made of tungsten as kinetic warheads. It was designed to operate in conjunction with the Brilliant Eyes sensor system. The project was conceived in November 1986 by Lowell Wood at Lawrence Livermore National Laboratory. Detailed studies were undertaken by several advisory boards, including the Defense Science Board and JASON, in 1989.

The Pebbles were designed in such a way that autonomous operation, without further external guidance from planned SDI sensor systems, was possible. This was attractive as a cost saving measure, as it would allow scaling back of those systems, and was estimated to save $7 to $13 billion versus the standard Phase I Architecture. Brilliant Pebbles later became the centerpiece of a revised architecture under the Bush Administration SDIO.

John H. Nuckolls, director of Lawrence Livermore National Laboratory from 1988 to 1994, described the system as "The crowning achievement of the Strategic Defense Initiative". Some of the technologies developed for SDI were used in numerous later projects. For example, the sensors and cameras that were developed and manufactured for Brilliant Pebbles systems became components of the Clementine mission and SDI technologies may also have a role in future missile defense efforts.

Though regarded as one of the most capable SDI systems, the Brilliant Pebbles program was canceled in 1994 by the BMDO.

Sensor programs

Delta 183 launch vehicle lifts off, carrying the SDI sensor experiment "Delta Star", March 24, 1989

SDIO sensor research encompassed visible light, ultraviolet, infrared, and radar technologies, and eventually led to the Clementine mission though that mission occurred just after the program transitioned to the BMDO. Like other parts of SDI, the sensor system initially was very large-scale, but after the Soviet threat diminished it was cut back.

Boost Surveillance and Tracking System (BSTS)

Boost Surveillance and Tracking System was part of the SDIO in the late 1980s, and was designed to assist detection of missile launches, especially during the boost phase; however, once the SDI program shifted toward theater missile defense in the early 1990s, the system left SDIO control and was transferred to the Air Force.

Space Surveillance and Tracking System (SSTS)

Space Surveillance and Tracking System was a system originally designed for tracking ballistic missiles during their mid-course phase. It was designed to work in conjunction with BSTS, but was later scaled down in favor of the Brilliant Eyes program.

Brilliant Eyes

Brilliant Eyes was a simpler derivative of the SSTS that focused on theater ballistic missiles rather than ICBMs and was meant to operate in conjunction with the Brilliant Pebbles system.

Brilliant Eyes was renamed Space and Missile Tracking System (SMTS) and scaled back further under BMDO, and in the late 1990s it became the low earth orbit component of the Air Force's Space Based Infrared System (SBIRS).

Other sensor experiments

The Delta 183 program used a satellite known as Delta Star to test several sensor related technologies. Delta Star carried a thermographic camera, a long-wave infrared imager, an ensemble of imagers and photometers covering several visible and ultraviolet bands as well as a laser detector and ranging device. The satellite observed several ballistic missile launches including some releasing liquid propellant as a countermeasure to detection. Data from the experiments led to advances in sensor technologies.

Countermeasures

An artist's concept of a ground / space-based hybrid laser weapon, 1984

In war-fighting, countermeasures can have a variety of meanings:

  1. The immediate tactical action to reduce vulnerability, such as chaff, decoys, and maneuvering.
  2. Counter strategies which exploit a weakness of an opposing system, such as adding more MIRV warheads which are less expensive than the interceptors fired against them.
  3. Defense suppression. That is, attacking elements of the defensive system.

Countermeasures of various types have long been a key part of warfighting strategy; however, with SDI they attained a special prominence due to the system cost, scenario of a massive sophisticated attack, strategic consequences of a less-than-perfect defense, outer spacebasing of many proposed weapons systems, and political debate.

Whereas the current United States national missile defense system is designed around a relatively limited and unsophisticated attack, SDI planned for a massive attack by a sophisticated opponent. This raised significant issues about economic and technical costs associated with defending against anti-ballistic missile defense countermeasures used by the attacking side.

For example, if it had been much cheaper to add attacking warheads than to add defenses, an attacker of similar economic power could have simply outproduced the defender. This requirement of being "cost effective at the margin" was first formulated by Paul Nitze in November 1985.

In addition, SDI envisioned many space-based systems in fixed orbits, ground-based sensors, command, control and communications facilities, etc. In theory, an advanced opponent could have targeted those, in turn requiring self-defense capability or increased numbers to compensate for attrition.

A sophisticated attacker having the technology to use decoys, shielding, maneuvering warheads, defense suppression, or other countermeasures would have multiplied the difficulty and cost of intercepting the real warheads. SDI design and operational planning had to factor in these countermeasures and the associated cost.

Response from the Soviet Union

SDI was high on Mikhail Gorbachev's agenda at the Geneva Summit.

SDI failed to dissuade the USSR from investing in development of ballistic missiles. The Soviet response to the SDI during the period of March 1983 through November 1985 provided indications of their view of the program both as a threat and as an opportunity to weaken NATO. SDI was likely seen not only as a threat to the physical security of the Soviet Union, but also as part of an effort by the United States to seize the strategic initiative in arms controls by neutralizing the military component of Soviet strategy. The Kremlin expressed concerns that space-based missile defenses would make nuclear war inevitable.

A major objective of that strategy was the political separation of Western Europe from the United States, which the Soviets sought to facilitate by aggravating allied concern over the SDI's potential implications for European security and economic interests. The Soviet predisposition to see deception behind the SDI was reinforced by their assessment of US intentions and capabilities and the utility of military deception in furthering the achievement of political goals.

Until the failing Soviet economy and the dissolution of the country between 1989 and 1991 which marks the end of the Cold War and with it the relaxation of the "arms race", warhead production had continued unabated in the USSR. Total deployed US and Soviet strategic weapons increased steadily from 1983 until the Cold War ended.

In 1986 Carl Sagan summarized what he heard Soviet commentators were saying about SDI, with a common argument being that it was equivalent to starting an economic war through a defensive arms race to further cripple the Soviet economy with extra military spending, while another interpretation was that it served as a disguise for the US wish to initiate a first strike on the Soviet Union.

Though classified at the time, a detailed study on a Soviet space-based LASER system began no later than 1976 as the Skif, a 1 MW Carbon dioxide laser along with the anti-satellite Kaskad, an in-orbit missile platform. With both devices reportedly designed to pre-emptively destroy any US satellites that might be launched in the future which could otherwise aid US missile defense.

DIA drawing of the Soviet Terra-3 laser in the USSR

Terra-3 was a Soviet laser testing centre, located on the Sary Shagan anti-ballistic missile (ABM) testing range in the Karaganda Region of Kazakhstan. It was originally built to test missile defense concepts, In 1984, officials within the United States Department of Defense (DoD) suggested it was the site of a prototypical anti-satellite weapon system.

In 1987 a disguised Mir space station module was lifted on the inaugural flight of the Energia booster as the Polyus and it has since been revealed that this craft housed a number of systems of the Skif laser, which were intended to be clandestinely tested in orbit, if it had not been for the spacecraft's attitude control system malfunctioning upon separation from the booster and it failing to reach orbit. More tentatively, it is also suggested that the Zarya module of the International Space Station, capable of station keeping and providing sizable battery power, was initially developed to power the Skif laser system.

The polyus was a prototype of the Skif orbital weapons platform designed to destroy Strategic Defense Initiative satellites with a megawatt carbon-dioxide laser. Soviet motivations behind attempting to launch components of the Skif laser in the form of Polyus were, according to interviews conducted years later, more for propaganda purposes in the prevailing climate of focus on US SDI, than as an effective defense technology, as the phrase "Space based laser" has a certain political capital.

In 2014, a declassified CIA paper states that "In response to SDI, Moscow threatened a variety of military countermeasures in lieu of developing a parallel missile defense system".

Controversy and criticism

SDI was not just lasers; in this Kinetic Energy Weapon test, a seven-gram Lexan projectile was fired from a light-gas gun at a velocity of 23,000 feet per second (7,000 m/s; 16,000 mph) at a cast aluminum block.

Historians from the Missile Defense Agency attribute the term "Star Wars" to a Washington Post article published March 24, 1983, the day after the speech, which quoted Democratic Senator Ted Kennedy describing the proposal as "reckless Star Wars schemes." Some critics used that term derisively, implying it was an impractical science fiction. In addition, the American media's liberal use of the moniker (despite President Reagan's request that they use the program's official name) did much to damage the program's credibility. In comments to the media on March 7, 1986, Acting Deputy Director of SDIO, Dr. Gerold Yonas, described the name "Star Wars" as an important tool for Soviet disinformation and asserted that the nickname gave an entirely wrong impression of SDI.

Jessica Savitch reported on the technology in episode No.111 of Frontline, "Space: The Race for High Ground" on PBS on November 4, 1983. The opening sequence shows Jessica Savitch seated next to a laser that she used to destroy a model of a communication satellite. The demonstration was perhaps the first televised use of a weapons grade laser. No theatrical effects were used. The model was actually destroyed by the heat from the laser. The model and the laser were realized by Marc Palumbo, a High Tech Romantic artist from the Center for Advanced Visual Studies at MIT.

Ashton Carter, then a board member at MIT, assessed SDI for Congress in 1984, saying there were a number of difficulties in creating an adequate missile defense shield, with or without lasers. Carter said X-rays have a limited scope because they become diffused through the atmosphere, much like the beam of a flashlight spreading outward in all directions. This means the X-rays needed to be close to the Soviet Union, especially during the critical few minutes of the booster phase, for the Soviet missiles to be both detectable to radar and targeted by the lasers themselves. Opponents disagreed, saying advances in technology, such as using very strong laser beams, and by "bleaching" the column of air surrounding the laser beam, could increase the distance that the X-ray would reach to successfully hit its target.

Physicists Hans Bethe and Richard Garwin, who worked with Edward Teller on both the atomic bomb and hydrogen bomb at Los Alamos, claimed a laser defense shield was unfeasible. They said that a defensive system was costly and difficult to build yet simple to destroy, and claimed that the Soviets could easily use thousands of decoys to overwhelm it during a nuclear attack. They believed that the only way to stop the threat of nuclear war was through diplomacy and dismissed the idea of a technical solution to the Cold War, saying that a defense shield could be viewed as threatening because it would limit or destroy Soviet offensive capabilities while leaving the American offense intact. In March 1984, Bethe coauthored a 106-page report for the Union of Concerned Scientists that concluded "the X-ray laser offers no prospect of being a useful component in a system for ballistic missile defense."

In response to this when Teller testified before Congress he stated that "instead of [Bethe] objecting on scientific and technical grounds, which he thoroughly understands, he now objects on the grounds of politics, on grounds of military feasibility of military deployment, on other grounds of difficult issues which are quite outside the range of his professional cognizance or mine."

On June 28, 1985, David Lorge Parnas resigned from SDIO's Panel on Computing in Support of Battle Management, arguing in eight short papers that the software required by the Strategic Defense Initiative could never be made to be trustworthy and that such a system would inevitably be unreliable and constitute a menace to humanity in its own right. Parnas said he joined the panel with the desire to make nuclear weapons "impotent and obsolete" but soon concluded that the concept was "a fraud".

SDI drew criticism from abroad as well. This 1986 Socialist German Workers Youth graffiti in Kassel, West Germany says "Keinen Krieg der Sterne! Stoppt SDI! SDAJ" or (No star wars! Stop SDI! SDAJ).

Treaty obligations

Another criticism of SDI was that it would require the United States to modify previously ratified treaties. The Outer Space Treaty of 1967, which requires "States Parties to the Treaty undertake not to place in orbit around the Earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner" and would forbid the US from pre-positioning in Earth orbit any devices powered by nuclear weapons and any devices capable of "mass destruction". Only the space stationed nuclear pumped X-ray laser concept would have violated this treaty, since other SDI systems, did not require the pre-positioning of nuclear explosives in space.

The Anti-Ballistic Missile Treaty and its subsequent protocol, which limited missile defenses to one location per country at 100 missiles each (which the USSR had and the US did not), would have been violated by SDI ground-based interceptors. The Nuclear Non-Proliferation Treaty requires "Each of the Parties to the Treaty undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control." Many viewed favoring deployment of ABM systems as an escalation rather than cessation of the nuclear arms race, and therefore a violation of this clause. On the other hand, many others did not view SDI as an escalation.

SDI and MAD

SDI was criticized for potentially disrupting the strategic doctrine of mutual assured destruction. MAD postulated that intentional nuclear attack was inhibited by the certainty of ensuing mutual destruction. Even if a nuclear first strike destroyed many of the opponent's weapons, sufficient nuclear missiles would survive to render a devastating counter-strike against the attacker. The criticism was that SDI could have potentially allowed an attacker to survive the lighter counter-strike, thus encouraging a first strike by the side having SDI. Another destabilizing scenario was countries being tempted to strike first before SDI was deployed, thereby avoiding a disadvantaged nuclear posture. Proponents of SDI argued that SDI development might instead cause the side that did not have the resources to develop SDI to, rather than launching a suicidal nuclear first strike attack before the SDI system was deployed, instead come to the bargaining table with the country that did have those resources and, hopefully, agree to a real, sincere disarmament pact that would drastically decrease all forces, both nuclear and conventional. Furthermore, the MAD argument was criticized on the grounds that MAD only covered intentional, full-scale nuclear attacks by a rational, non-suicidal opponent with similar values. It did not take into account limited launches, accidental launches, rogue launches, or launches by non-state entities or covert proxies.

During the Reykjavik talks with Mikhail Gorbachev in 1986, Ronald Reagan addressed Gorbachev's concerns about imbalance by stating that SDI technology could be provided to the entire world – including the Soviet Union – to prevent the imbalance from occurring. Gorbachev answered dismissively. When Reagan prompted technology sharing again, Gorbachev stated "we cannot assume an obligation relative to such a transition", referring to the cost of implementing such a program. 

A military officer who was involved in covert operations at the time has told journalist Seymour Hersh that much of the publicity about the program was deliberately false and intended to expose Soviet spies:

For example, the published stories about our Star Wars programme were replete with misinformation and forced the Russians to expose their sleeper agents inside the American government by ordering them to make a desperate attempt to find out what the US was doing. But we could not risk exposure of the administration's role and take the chance of another McCarthy period. So there were no prosecutions. We dried up and eliminated their access and left the spies withering on the vine ... Nobody on the Joint Chiefs of Staff ever believed we were going to build Star Wars, but if we could convince the Russians that we could survive a first strike, we win the game.

Non-ICBM delivery

Another criticism of SDI was that it would not be effective against non-space faring weapons, namely cruise missiles, bombers, short-range ballistic missile submarines and non-conventional delivery methods; however, it was never intended to act as a defense against non-space faring weapons.

Whistleblower

In 1992, scientist Aldric Saucier was given whistleblower protection after he was fired and complained about "wasteful spending on research and development" at the SDI.[94] Saucier also lost his security clearance.

Dream argument

From Wikipedia, the free encyclopedia
 
The Dream of Life, by unknown Mannerist painter, ca. 1533

The dream argument is the postulation that the act of dreaming provides preliminary evidence that the senses we trust to distinguish reality from illusion should not be fully trusted, and therefore, any state that is dependent on our senses should at the very least be carefully examined and rigorously tested to determine whether it is in fact reality.

Synopsis

While one dreams, one does not normally realize one is dreaming. On more rare occasions, the dream may be contained inside another dream with the very act of realizing that one is dreaming, itself, being only a dream that one is not aware of having. This has led philosophers to wonder whether it is possible for one ever to be certain, at any given point in time, that one is not in fact dreaming, or whether indeed it could be possible for one to remain in a perpetual dream state and never experience the reality of wakefulness at all.

In Western philosophy this philosophical puzzle was referred to by Plato (Theaetetus 158b-d), Aristotle (Metaphysics 1011a6), and the Academic Skeptics. It is now best known from René Descartes' Meditations on First Philosophy. The dream argument has become one of the most prominent skeptical hypotheses.

In Eastern philosophy this type of argument is sometimes referred to as the "Zhuangzi paradox":

He who dreams of drinking wine may weep when morning comes; he who dreams of weeping may in the morning go off to hunt. While he is dreaming he does not know it is a dream, and in his dream he may even try to interpret a dream. Only after he wakes does he know it was a dream. And someday there will be a great awakening when we know that this is all a great dream. Yet the stupid believe they are awake, busily and brightly assuming they understand things, calling this man ruler, that one herdsman—how dense! Confucius and you are both dreaming! And when I say you are dreaming, I am dreaming, too. Words like these will be labeled the Supreme Swindle. Yet, after ten thousand generations, a great sage may appear who will know their meaning, and it will still be as though he appeared with astonishing speed.

The Yogachara philosopher Vasubandhu (4th to 5th century C.E.) referenced the argument in his "Twenty verses on appearance only."

The dream argument came to feature prominently in Mahayana and Tibetan Buddhist philosophy. Some schools of thought (e.g., Dzogchen) consider perceived reality to be literally unreal. As Chögyal Namkhai Norbu puts it: "In a real sense, all the visions that we see in our lifetime are like a big dream . . . ." In this context, the term 'visions' denotes not only visual perceptions, but also appearances perceived through all senses, including sounds, smells, tastes, and tactile sensations, and operations on perceived mental objects.

Simulated reality

Dreaming provides a springboard for those who question whether our own reality may be an illusion. The ability of the mind to be tricked into believing a mentally generated world is the "real world" means at least one variety of simulated reality is a common, even nightly event.

Those who argue that the world is not simulated must concede that the mind—at least the sleeping mind—is not itself an entirely reliable mechanism for attempting to differentiate reality from illusion.

Whatever I have accepted until now as most true has come to me through my senses. But occasionally I have found that they have deceived me, and it is unwise to trust completely those who have deceived us even once.

— René Descartes

Critical discussion

In the past, philosophers John Locke and Thomas Hobbes have separately attempted to refute Descartes's account of the dream argument. Locke claimed that you cannot experience pain in dreams. Various scientific studies conducted within the last few decades provided evidence against Locke's claim by concluding that pain in dreams can occur, but on very rare occasions. Philosopher Ben Springett has said that Locke might respond to this by stating that the agonizing pain of stepping in to a fire is non-comparable to stepping in to a fire in a dream. Hobbes claimed that dreams are susceptible to absurdity while the waking life is not.

Many contemporary philosophers have attempted to refute dream skepticism in detail (see, e.g., Stone (1984)). Ernest Sosa (2007) devoted a chapter of a monograph to the topic, in which he presented a new theory of dreaming and argued that his theory raises a new argument for skepticism, which he attempted to refute. In A Virtue Epistemology: Apt Belief and Reflective Knowledge, he states: "in dreaming we do not really believe; we only make-believe." Jonathan Ichikawa (2008) and Nathan Ballantyne & Ian Evans (2010) have offered critiques of Sosa's proposed solution. Ichikawa argued that as we cannot tell whether our beliefs in waking life are truly beliefs and not imaginings, like in a dream, we are still not able to tell whether we are awake or dreaming.

Norman Malcolm in his monograph "Dreaming" (published in 1959) elaborated on Wittgenstein's question as to whether it really mattered if people who tell dreams "really had these images while they slept, or whether it merely seems so to them on waking". He argues that the sentence "I am asleep" is a senseless form of words; that dreams cannot exist independently of the waking impression; and that scepticism based on dreaming "comes from confusing the historical and dream telling senses...[of]...the past tense" (page 120). In the chapter: "Do I Know I Am Awake ?" he argues that we do not have to say: "I know that I am awake" simply because it would be absurd to deny that one is awake.

The dream hypothesis is also used to develop other philosophical concepts, such as Valberg's personal horizon: what this world would be internal to if this were all a dream.

Ethics of uncertain sentience

From Wikipedia, the free encyclopedia
 
Ethical questions around whether crustaceans, such as lobsters, are sentient and can experience pain, are a topic of much debate

The ethics of uncertain sentience refers to questions surrounding the treatment of and moral obligations towards individuals whose sentience—the capacity to subjectively sense and feel—and resulting ability to experience pain is uncertain; the topic has been particularly discussed within the field of animal ethics, with the precautionary principle frequently invoked in response.

Views

Animal ethics

David Foster Wallace in his 2005 essay "Consider the Lobster" investigated the potential sentience and capacity of crustaceans to experience pain and the resulting ethical implications of eating them. In 2014, the philosopher Robert C. Jones explored the ethical question that Wallace raised, arguing that "[e]ven if one remains skeptical of crustacean sentience, when it comes to issues of welfare it would be most prudent to employ the precautionary principle regarding our treatment of these animals, erring on the side of caution". Maximilian Padden Elder takes a similar view regarding the capacity for fishes to feel pain, arguing that the "precautionary principle is the moral ethic one ought to adopt in the face of uncertainty".

In the 2015 essay "Reconsider the Lobster", Jeff Sebo quotes Wallace's discussion of the difficulty of establishing whether an animal can experience pain. Sebo calls the question of how to treat individuals of uncertain sentience, the "sentience problem" and argues that this problem which "Wallace raises deserves much more philosophical attention than it currently receives." Sebo asserts that there are two motivating assumptions behind the problem: "sentientism about moral status"—the idea that if an individual is sentient, that they deserve moral consideration—and "uncertainty about other minds", which refers to scientific and philosophical uncertainty about which individuals are sentient.

In response to the problem, Sebo lays out three different potential approaches: the incautionary principle, which postulates that in cases of uncertainty about sentience it is morally permissible to treat individuals as if they are not sentient; the precautionary principle, which suggests that in such cases we have a moral obligation to treat them as if they are sentient; and the expected value principle, which asserts that we are "morally required to multiply our credence that they are by the amount of moral value they would have if they were, and to treat the product of this equation as the amount of moral value that they actually have". Sebo advocates for the latter position.

Jonathan Birch, in answer to the question surrounding animal sentience, advocates for a practical framework based on the precautionary principle, arguing that the framework aligns well with conventional practices in animal welfare science.

Simon Knutsson and Christian Munthe argue that from the perspective of virtue ethics, that when it comes to animals of uncertain sentience, such as "fish, invertebrates such as crustaceans, snails and insects", that it is a "requirement of a morally decent (or virtuous) person that she at least pays attention to and is cautious regarding the possibly morally relevant aspects of such animals".

Shelley A. Adamo argues that although the precautionary principle in relation to potential invertebrate sentience is the safest option, that it's not cost-free, as thoughtless legislation employed following the precautionary principle could be economically costly and that, as a result, we should be cautious about adopting it.

Environmental ethics

Kai Chan advocates for an environmental ethic, which is a form of ethical extensionism applied to all living beings because "there is a non-zero probability of sentience and consciousness" and that "we cannot justify excluding beings from consideration on the basis of uncertainty of their sentience".

Ethics of artificial intelligence

Nick Bostrom and Eliezer Yudkowsky argue that if an artificial intelligence is sentient, that it is wrong to inflict unnecessary pain on them, in the same way that it is wrong to inflict pain on an animal, unless there are "sufficiently strong morally overriding reasons to do so". They also advocate for the "Principle of Substrate Non-Discrimination", which asserts that "[i]f two beings have the same functionality and the same conscious experience, and differ only in the substrate of their implementation, then they have the same moral status."

Neuroethics

Adam J. Shriver argues for "precise, precautionary, and probabilistic approaches to sentience" and asserts that the evidence provided by neuroscience has differing relevance to each; he concludes that basic protections for animals should be guided by the precautionary principle and that although neuroscientific evidence in certain instances is not necessary to indicate that individuals of certain species require protections, "ongoing search for the neural correlates of sentience must be pursued in order to avoid harms that occur from mistaken accounts."

Heme

From Wikipedia, the free encyclopedia
 
Binding of oxygen to a heme prosthetic group.

Heme, or haem (spelling differences) is a substance precursive to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.

In microbiological terms, heme is coordination complex "consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to one or two axial ligands." The definition is loose, and many depictions omit the axial ligands. Among the metalloporphyrins deployed by metalloproteins as prosthetic groups, heme is one of the most widely used and defines a family of proteins known as hemoproteins. Hemes are most commonly recognized as components of hemoglobin, the red pigment in blood, but are also found in a number of other biologically important hemoproteins such as myoglobin, cytochromes, catalases, heme peroxidase, and endothelial nitric oxide synthase.

The word haem is derived from Greek αἷμα haima meaning "blood".

Space-filling model of the Fe-protoporphyrin IX subunit of heme B. Axial ligands omitted. Color scheme: grey=iron, blue=nitrogen, black=carbon, white=hydrogen, red=oxygen

Function

The heme group of succinate dehydrogenase bound to histidine, an electron carrier in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. From PDB: 1YQ3​.

Hemoproteins have diverse biological functions including the transportation of diatomic gases, chemical catalysis, diatomic gas detection, and electron transfer. The heme iron serves as a source or sink of electrons during electron transfer or redox chemistry. In peroxidase reactions, the porphyrin molecule also serves as an electron source, being able to delocalize radical electrons in the conjugated ring. In the transportation or detection of diatomic gases, the gas binds to the heme iron. During the detection of diatomic gases, the binding of the gas ligand to the heme iron induces conformational changes in the surrounding protein. In general, diatomic gases only bind to the reduced heme, as ferrous Fe(II) while most peroxidases cycle between Fe(III) and Fe(IV) and hemeproteins involved in mitochondrial redox, oxidation-reduction, cycle between Fe(II) and Fe(III).

It has been speculated that the original evolutionary function of hemoproteins was electron transfer in primitive sulfur-based photosynthesis pathways in ancestral cyanobacteria-like organisms before the appearance of molecular oxygen.

Hemoproteins achieve their remarkable functional diversity by modifying the environment of the heme macrocycle within the protein matrix. For example, the ability of hemoglobin to effectively deliver oxygen to tissues is due to specific amino acid residues located near the heme molecule. Hemoglobin reversibly binds to oxygen in the lungs when the pH is high, and the carbon dioxide concentration is low. When the situation is reversed (low pH and high carbon dioxide concentrations), hemoglobin will release oxygen into the tissues. This phenomenon, which states that hemoglobin's oxygen binding affinity is inversely proportional to both acidity and concentration of carbon dioxide, is known as the Bohr effect. The molecular mechanism behind this effect is the steric organization of the globin chain; a histidine residue, located adjacent to the heme group, becomes positively charged under acidic conditions (which are caused by dissolved CO2 in working muscles, etc.), releasing oxygen from the heme group.

Types

Major hemes

There are several biologically important kinds of heme:


Heme A Heme B Heme C Heme O
PubChem number 7888115 444098 444125 6323367
Chemical formula C49H56O6N4Fe C34H32O4N4Fe C34H36O4N4S2Fe C49H58O5N4Fe
Functional group at C3 Porphyrine General Formula V.1.svg –CH(OH)CH2Far –CH=CH2 –CH(cystein-S-yl)CH3 –CH(OH)CH2Far
Functional group at C8 –CH=CH2 –CH=CH2 –CH(cystein-S-yl)CH3 –CH=CH2
Functional group at C18 –CH=O –CH3 –CH3 –CH3
Structure of Fe-porphyrin subunit of heme B.
 
Structure of Fe-porphyrin subunit of heme A. Heme A is synthesized from heme B. In two sequential reactions a 17-hydroxyethylfarnesyl moiety is added at the 2-position and an aldehyde is added at the 8-position.

The most common type is heme B; other important types include heme A and heme C. Isolated hemes are commonly designated by capital letters while hemes bound to proteins are designated by lower case letters. Cytochrome a refers to the heme A in specific combination with membrane protein forming a portion of cytochrome c oxidase.

Other hemes

The following carbon numbering system of porphyrins is an older numbering used by biochemists and not the 1–24 numbering system recommended by IUPAC which is shown in the table above.
  • Heme l is the derivative of heme B which is covalently attached to the protein of lactoperoxidase, eosinophil peroxidase, and thyroid peroxidase. The addition of peroxide with the glutamyl-375 and aspartyl-225 of lactoperoxidase forms ester bonds between these amino acid residues and the heme 1- and 5-methyl groups, respectively. Similar ester bonds with these two methyl groups are thought to form in eosinophil and thyroid peroxidases. Heme l is one important characteristic of animal peroxidases; plant peroxidases incorporate heme B. Lactoperoxidase and eosinophil peroxidase are protective enzymes responsible for the destruction of invading bacteria and virus. Thyroid peroxidase is the enzyme catalyzing the biosynthesis of the important thyroid hormones. Because lactoperoxidase destroys invading organisms in the lungs and excrement, it is thought to be an important protective enzyme.
  • Heme m is the derivative of heme B covalently bound at the active site of myeloperoxidase. Heme m contains the two ester bonds at the heme 1- and 5-methyl groups also present in heme l of other mammalian peroxidases, such as lactoperoxidase and eosinophil peroxidase. In addition, a unique sulfonamide ion linkage between the sulfur of a methionyl amino-acid residue and the heme 2-vinyl group is formed, giving this enzyme the unique capability of easily oxidizing chloride and bromide ions to hypochlorite and hypobromite. Myeloperoxidase is present in mammalian neutrophils and is responsible for the destruction of invading bacteria and viral agents. It perhaps synthesizes hypobromite by "mistake". Both hypochlorite and hypobromite are very reactive species responsible for the production of halogenated nucleosides, which are mutagenic compounds.
  • Heme D is another derivative of heme B, but in which the propionic acid side chain at the carbon of position 6, which is also hydroxylated, forms a γ-spirolactone. Ring III is also hydroxylated at position 5, in a conformation trans to the new lactone group. Heme D is the site for oxygen reduction to water of many types of bacteria at low oxygen tension.
  • Heme S is related to heme B by having a formal group at position 2 in place of the 2-vinyl group. Heme S is found in the hemoglobin of a few species of marine worms. The correct structures of heme B and heme S were first elucidated by German chemist Hans Fischer.

The names of cytochromes typically (but not always) reflect the kinds of hemes they contain: cytochrome a contains heme A, cytochrome c contains heme C, etc. This convention may have been first introduced with the publication of the structure of heme A.

Use of capital letters to designate the type of heme

The practice of designating hemes with upper case letters was formalized in a footnote in a paper by Puustinen and Wikstrom which explains under which conditions a capital letter should be used: "we prefer the use of capital letters to describe the heme structure as isolated. Lowercase letters may then be freely used for cytochromes and enzymes, as well as to describe individual protein-bound heme groups (for example, cytochrome bc, and aa3 complexes, cytochrome b5, heme c1 of the bc1 complex, heme a3 of the aa3 complex, etc)." In other words, the chemical compound would be designated with a capital letter, but specific instances in structures with lowercase. Thus cytochrome oxidase, which has two A hemes (heme a and heme a3) in its structure, contains two moles of heme A per mole protein. Cytochrome bc1, with hemes bH, bL, and c1, contains heme B and heme C in a 2:1 ratio. The practice seems to have originated in a paper by Caughey and York in which the product of a new isolation procedure for the heme of cytochrome aa3 was designated heme A to differentiate it from previous preparations: "Our product is not identical in all respects with the heme a obtained in solution by other workers by the reduction of the hemin a as isolated previously (2). For this reason, we shall designate our product heme A until the apparent differences can be rationalized.". In a later paper, Caughey's group uses capital letters for isolated heme B and C as well as A.

Synthesis

Heme synthesis in the cytoplasm and mitochondrion

The enzymatic process that produces heme is properly called porphyrin synthesis, as all the intermediates are tetrapyrroles that are chemically classified as porphyrins. The process is highly conserved across biology. In humans, this pathway serves almost exclusively to form heme. In bacteria, it also produces more complex substances such as cofactor F430 and cobalamin (vitamin B12).

The pathway is initiated by the synthesis of δ-aminolevulinic acid (dALA or δALA) from the amino acid glycine and succinyl-CoA from the citric acid cycle (Krebs cycle). The rate-limiting enzyme responsible for this reaction, ALA synthase, is negatively regulated by glucose and heme concentration. Mechanism of inhibition of ALAs by heme or hemin is by decreasing stability of mRNA synthesis and by decreasing the intake of mRNA in the mitochondria. This mechanism is of therapeutic importance: infusion of heme arginate or hematin and glucose can abort attacks of acute intermittent porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase.

The organs mainly involved in heme synthesis are the liver (in which the rate of synthesis is highly variable, depending on the systemic heme pool) and the bone marrow (in which rate of synthesis of Heme is relatively constant and depends on the production of globin chain), although every cell requires heme to function properly. However, due to its toxic properties, proteins such as Hemopexin (Hx) are required to help maintain physiological stores of iron in order for them to be used in synthesis. Heme is seen as an intermediate molecule in catabolism of hemoglobin in the process of bilirubin metabolism. Defects in various enzymes in synthesis of heme can lead to group of disorder called porphyrias, these include acute intermittent porphyria, congenital erythropoetic porphyria, porphyria cutanea tarda, hereditary coproporphyria, variegate porphyria, erythropoietic protoporphyria.

Synthesis for food

Impossible Foods, producers of plant-based meat substitute, use an accelerated heme synthesis process involving soybean root leghemoglobin and yeast, adding the resulting heme to items such as meatless (vegan) Impossible burger patties. The DNA for leghemoglobin production was extracted from the soybean root nodules and expressed in yeast cells to overproduce heme for use in the meatless burgers. This process claims to create a meaty flavor in the resulting products.

Degradation

Heme breakdown

Degradation begins inside macrophages of the spleen, which remove old and damaged erythrocytes from the circulation. In the first step, heme is converted to biliverdin by the enzyme heme oxygenase (HO). NADPH is used as the reducing agent, molecular oxygen enters the reaction, carbon monoxide (CO) is produced and the iron is released from the molecule as the ferrous ion (Fe2+). CO acts as a cellular messenger and functions in vasodilation.

In addition, heme degradation appears to be an evolutionarily-conserved response to oxidative stress. Briefly, when cells are exposed to free radicals, there is a rapid induction of the expression of the stress-responsive heme oxygenase-1 (HMOX1) isoenzyme that catabolizes heme (see below). The reason why cells must increase exponentially their capability to degrade heme in response to oxidative stress remains unclear but this appears to be part of a cytoprotective response that avoids the deleterious effects of free heme. When large amounts of free heme accumulates, the heme detoxification/degradation systems get overwhelmed, enabling heme to exert its damaging effects.

heme heme oxygenase-1 biliverdin + Fe2+
Heme b.svg   Biliverdin3.svg
H+ + NADPH + O2 NADP+ + CO
Biochem reaction arrow forward YYNN horiz med.svg


 
 

In the second reaction, biliverdin is converted to bilirubin by biliverdin reductase (BVR):

biliverdin biliverdin reductase bilirubin
Biliverdin3.svg   Bilirubin ZZ.png
H+ + NADPH NADP+
Biochem reaction arrow forward YYNN horiz med.svg


 
 

Bilirubin is transported into the liver by facilitated diffusion bound to a protein (serum albumin), where it is conjugated with glucuronic acid to become more water-soluble. The reaction is catalyzed by the enzyme UDP-glucuronosyltransferase.

bilirubin UDP-glucuronosyltransferase bilirubin diglucuronide
Bilirubin ZZ.png   Bilirubin diglucuronide.svg
2 UDP-glucuronide 2 UMP + 2 Pi
Biochem reaction arrow forward YYNN horiz med.svg


This form of bilirubin is excreted from the liver in bile. Excretion of bilirubin from liver to biliary canaliculi is an active, energy-dependent and rate-limiting process. The intestinal bacteria deconjugate bilirubin diglucuronide and convert bilirubin to urobilinogens. Some urobilinogen is absorbed by intestinal cells and transported into the kidneys and excreted with urine (urobilin, which is the product of oxidation of urobilinogen, and is responsible for the yellow colour of urine). The remainder travels down the digestive tract and is converted to stercobilinogen. This is oxidized to stercobilin, which is excreted and is responsible for the brown color of feces.

In health and disease

Under homeostasis, the reactivity of heme is controlled by its insertion into the “heme pockets” of hemoproteins. Under oxidative stress however, some hemoproteins, e.g. hemoglobin, can release their heme prosthetic groups. The non-protein-bound (free) heme produced in this manner becomes highly cytotoxic, most probably due to the iron atom contained within its protoporphyrin IX ring, which can act as a Fenton's reagent to catalyze in an unfettered manner the production of free radicals. It catalyzes the oxidation and aggregation of protein, the formation of cytotoxic lipid peroxide via lipid peroxidation and damages DNA through oxidative stress. Due to its lipophilic properties, it impairs lipid bilayers in organelles such as mitochondria and nuclei. These properties of free heme can sensitize a variety of cell types to undergo programmed cell death in response to pro-inflammatory agonists, a deleterious effect that plays an important role in the pathogenesis of certain inflammatory diseases such as malaria and sepsis. There is an association between high intake of heme iron sourced from meat and increased risk of colon cancer. The heme content of red meat is 10 times higher than that of white meat such as chicken.

Genes

The following genes are part of the chemical pathway for making heme:


Introduction to entropy

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