Metallic hydrogen

From Wikipedia, the free encyclopedia

Gas giants such as Jupiter (pictured above) and Saturn might contain large amounts of metallic hydrogen (depicted in grey) and metallic helium.^[1]

A diagram of Jupiter showing a model of the planet's interior, with a rocky core overlaid by a deep layer of liquid metallic hydrogen and an outer layer predominantly of molecular hydrogen. Jupiter's true interior composition is uncertain. For instance, the core may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. Furthermore, there is no clear physical boundary between the hydrogen layers—with increasing depth the gas increases smoothly in temperature and density, ultimately becoming liquid. Features are shown to scale except for the aurorae and the orbits of the Galilean moons.

Metallic hydrogen is a kind of degenerate matter, a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.^[2]

At high pressure and temperatures, metallic hydrogen might exist as a liquid rather than a solid, and researchers think it is present in large quantities in the hot and gravitationally compressed interiors of Jupiter, Saturn, and in some extrasolar planets.^[3]

In October 2016, there were claims that metallic hydrogen had been observed in the laboratory at a pressure of around 495 gigapascals (4,950,000 bar; 4,890,000 atm; 71,800,000 psi).^[4] In January 2017, scientists at Harvard University reported the first creation of metallic hydrogen in a laboratory, using a diamond anvil cell.^[5] Several researchers in the field doubt this claim.^[6] Some observations consistent with metallic behavior had been reported earlier, such as the observation of new phases of solid hydrogen under static conditions^[7][8] and, in dense liquid deuterium, electrical insulator-to-conductor transitions associated with an increase in optical reflectivity.^[9]

Theoretical predictions

Metallization of hydrogen under pressure

Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H₂ molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250000 atm; 3600000 psi) hydrogen would display metallic properties: instead of discrete H₂ molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout.^[2] Since then, producing metallic hydrogen in the laboratory has been described as "...the holy grail of high-pressure physics."^[10]

The initial prediction about the amount of pressure needed was eventually shown to be too low.^[11] Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures of 100 GPa and higher.

Liquid metallic hydrogen

Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa (3,900,000 atm), where hydrogen would be a liquid metal, even at low temperatures.^[12][13]

Superconductivity

In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C), far higher than any other known candidate material. This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.^[14]

Possibility of novel types of quantum fluid

Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids: superconducting superfluids and metallic superfluids. Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.^[15][16][17]

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy LiH₆ would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiH_n and possibly other related alloys of type Li_n.^[18]

Experimental pursuit

Shock-wave compression, 1996

In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced the first identifiably metallic hydrogen^[19] for about a microsecond at temperatures of thousands of kelvins, pressures of over 1000000 atm (100 GPa), and densities of approximately 0.6 g/cm³.^[20] The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen, thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2500000 atm (250 GPa), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes they expected. The researchers used a 1960s-era light-gas gun, originally employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 1400000 atm (140 GPa), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator but, as the pressure increases significantly, the band-gap gradually fell to 0.3 eV. Because the thermal energy of the fluid (the temperature became about 3000 K or 2730 °C due to compression of the sample) was above 0.3 eV, the hydrogen might be considered metallic.

Other experimental research, 1996–2004

Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998,^[21] and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (3200000–3400000 atm or 320–340 GPa) and temperatures of 100–300 K (−173–27 °C), hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also ongoing on deuterium.^[22] Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (Rydberg matter) are effective promoters to metallic hydrogen.^[23]

Pulsed laser heating experiment, 2008

The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating.^[24] Hydrogen-rich molecular silane (SiH₄) was claimed to be metallized and become superconducting by M.I. Eremets et al..^[25] This claim is disputed, and their results have not been repeated.^[26][27]

Observation of liquid metallic hydrogen, 2011

In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 2600000–3000000 atm (260–300 GPa).^[7] This claim was questioned by other researchers in 2012.^[28][29]

Z machine, 2015

In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium.^[30]

Claimed observation of solid metallic hydrogen, 2016

On October 5, 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims of experimental evidence that solid metallic hydrogen had been synthesised in the laboratory. This manuscript was available in October 2016,^[31] and a revised version was subsequently published in the journal Science in January 2017.^[4][5]

In the preprint version of the paper, Dias and Silvera writes:

With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7×10²³ particles/cm³, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

— Dias & Silvera (2016) ^[31]

Silvera stated that they did not repeat their experiment, since more tests could damage or destroy their existing sample, but assured the scientific community that more tests are coming.^[32][6] He also stated that the pressure would eventually be released, in order to find out whether the sample was metastable (i.e., whether it would persist in its metallic state even after the pressure was released).^[33]

Shortly after the claim was published in Science, Nature's news division published an article stating that some other physicists regarded the result with skepticism. Recently, prominent members of the high pressure research community have criticised the claimed results,^[34][35][36] questioning the claimed pressures or the presence of metallic hydrogen at the pressures claimed.

In February 2017, it was reported that the sample of claimed metallic hydrogen was lost, after the diamond anvils it was contained between broke.^[37]

In August 2017, Silvera and Dias issued an erratum^[38] to the Science article, regarding corrected reflectance values due to variations between the optical density of stressed natural diamonds and the synthetic diamonds used in their pre-compression diamond anvil cell.

Quark star

From Wikipedia, the free encyclopedia

A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.

It is well known that massive stars can collapse to form neutron stars, under extreme temperatures and pressures. In simple terms, neutrons usually have space separating them, due to degeneracy pressure keeping them apart. Under extreme conditions such as a neutron star, the pressure separating nucleons is overwhelmed by gravity, and the separation between them breaks down, causing them to be packed extremely densely and form an immensely hot and dense state known as neutron matter. Because these neutrons are made of quarks, it is hypothesized that under even more extreme conditions, the degeneracy pressure keeping the quarks apart within the neutrons might break down in much the same way, creating an ultra-dense phase of degenerate matter based on densely packed quarks. This is seen as plausible, but is very hard to prove, as scientists cannot easily create the conditions needed to investigate the properties of quark matter, so it is not yet certain whether or not it actually happens in the universe.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons (which are formed from quarks bound together) break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, they would be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical size, electromagnetic, or temperature measurements, compared to other neutron stars.

The hypothesis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze.^[1][2] Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 10¹² K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 10¹² K can not be recreated artificially, so there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

Creation

It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.^[3][4]

Stability and strange quark matter

Ordinary quark matter consisting of up and down quarks (also referred to as u and d quarks) has a very high Fermi energy compared to ordinary atomic matter and is only stable under extreme temperatures and/or pressures. This suggests that the only stable quark stars will be neutron stars with a quark matter core, while quark stars consisting entirely of ordinary quark matter will be highly unstable and dissolve spontaneously.^[5][6]

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of u and d quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle.^[5] This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter.^[7]

Strange stars

Quark stars made of strange quark matter are known as strange stars, and they form a subgroup under the quark star category.^[7]

Strange stars might exist without regard of the Bodmer–Witten assumption of stability at near-zero temperatures and pressures, as strange quark matter might form and remain stable at the core of neutron stars, in the same way as ordinary quark matter could.^[3] Such strange stars will naturally have a crust layer of neutron star material. The depth of the crust layer will depend on the physical conditions and circumstances of the entire star and on the properties of strange quark matter in general.^[8] Stars partially made up of quark matter (including strange quark matter) are also referred to as hybrid stars.^{[9][10][11][12]}

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, they could also be created in the early cosmic phase separations following the Big Bang.^[5] If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.^[5]

Characteristics

Quark stars have some special characteristics that separate them from ordinary neutron stars.

Under the physical conditions found inside neutron stars, with extremely high densities but temperatures well below 10¹² K, quark matter is predicted to exhibit some peculiar characteristics. It is expected to behave as a Fermi liquid and enter a so-called color-flavor-locked (CFL) phase of color superconductivity, where "color" refers to the six "charges" exhibited in the strong interaction, instead of the positive and the negative charges in electromagnetism. At slightly lower densities, corresponding to higher layers closer to the surface of the compact star, the quark matter will behave as a non-CFL quark liquid, a phase that is even more mysterious than CFL and might include color conductivity and/or several additional yet undiscovered phases. None of these extreme conditions can currently be recreated in laboratories so nothing can be inferred about these phases from direct experiments.^[13]

If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than the strong force that binds ordinary hadrons.

Strange stars

Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like—nearly indistinguishable from—ordinary neutron stars. This suggests that many, or even all, known neutron stars might in fact be strange stars. However, the investigating team of Prashanth Jaikumar, Sanjay Reddy, and Andrew W. Steiner made some fundamental assumptions that led to uncertainties in their results large enough that the case is not finally settled. More research, both observational and theoretical, remains to be done on strange stars in the future.^[14]

Other theoretical work^[15] contends that, "A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star"; and, addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars.

Observed overdense neutron stars

At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low,^{[citation needed]} so in the Milky Way there would only be a small population of quark stars. If it is correct however, that overdense neutron stars can turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as observers would be looking for the wrong type of star.

Quark stars and strange stars are entirely hypothetical as of 2018, but there are several candidates.

Observations released by the Chandra X-ray Observatory on April 10, 2002 detected two possible quark stars, designated RX J1856.5-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than it should be, suggesting that they are composed of material denser than neutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive;^[16] and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded.

Another star, XTE J1739-285,^[17] has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible quark star candidate.

In 2006, Y. L. Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star.^[18]

It was reported in 2008 that observations of supernovae SN2006gy, SN2005gj and SN2005ap also suggest the existence of quark stars.^[19] It has been suggested that the collapsed core of supernova SN1987A may be a quark star.^[20][21]

In 2015, Z.G. Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.^[22]

Other theorized quark formations

Apart from ordinary quark matter and strange quark matter, other types of quark-gluon plasma might theoretically occur or be formed inside neutron stars and quark stars. This includes the following, some of which has been observed and studied in laboratories:

• Jaffe 1977, suggested a four-quark state with strangeness (qsqs).
• Jaffe 1977 suggested the H dibaryon, a six-quark state with equal numbers of up-, down-, and strange quarks (represented as uuddss or udsuds).
• Bound multi-quark systems with heavy quarks (QQqq).
• In 1987, a pentaquark state was first proposed with a charm anti-quark (qqqsc).
• Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqqs).
• Light pentaquarks are grouped within an antidecuplet, the lightest candidate, Ө⁺.
  • This can also be described by the diquark model of Jaffe and Wilczek (QCD).
• Ө⁺⁺ and antiparticle Ө⁻⁻.
• Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
• Charmed pentaquark Ө_c(3100) (uuddc) state was detected by the H1 collaboration.^[23]
• Tetra quark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetra quark particle of Z(4430), was discovered and investigated in laboratories on Earth.^[24]

BFR (rocket)

From Wikipedia, the free encyclopedia

BFR

SpaceX rendering of BFR
Function	Mars colonization, Earth–lunar transport, intercontinental transport, orbital launcher[1]
Manufacturer	SpaceX
Country of origin	United States
Cost per launch	US$7 million (external estimate)[2]
Size
Height	106 m (348 ft)[1]
Diameter	9 m (30 ft)
Mass	4,400,000 kg (9,700,000 lb)
Stages	2
Capacity
Payload to LEO	250,000 kg (550,000 lb) expendable[3] 150,000 kg (330,000 lb) reusable[3]
Payload to Earth (return)	50,000 kg (110,000 lb)[1][3]
Launch history
Status	In development
Launch sites	Kennedy LC-39A (potential) South Texas (proposed)
First stage – Booster
Length	58 m (190 ft) [1]
Diameter	9 m (30 ft)
Gross mass	3,065,000 kg (6,757,000 lb)
Engines	31 × Raptor
Thrust	52.7 MN (11,800,000 lbf) sea level [1]
Specific impulse	330 s (3.2 km/s) each engine, sea level
Fuel	Subcooled CH 4 / LOX

Second stage – Spaceship
Length	48 m (157 ft) [1]
Diameter	9 m (30 ft)
Empty mass	85,000 kg (187,000 lb)
Gross mass	1,335,000 kg (2,943,000 lb)
Propellant mass	240,000 kg (530,000 lb) CH 4 860,000 kg (1,900,000 lb) LOX
Engines	7 × Raptor (4 × vacuum, 3 × sea level) [4]
Thrust	12.7 MN (2,900,000 lbf) total
Specific impulse	375 s (3.68 km/s) vacuum each, outer 4 engines 356 s (3.49 km/s) vacuum each, inner 3 engines 330 s (3.2 km/s) sea level [1] each, inner 3 engines
Fuel	Subcooled CH 4 / LOX

BFR^[1]:2:35 is SpaceX's privately funded next-generation launch vehicle and spacecraft announced by Elon Musk in September 2017.^[5][6] It includes reusable launch vehicles and spacecraft that are intended by SpaceX to replace all of the company's existing hardware by the early 2020s, ground infrastructure for rapid launch and relaunch, and zero-gravity propellant transfer technology to be deployed in low Earth orbit (LEO). The new vehicles are much larger than the existing SpaceX fleet. The large payload to Earth orbit of up to 150,000 kg (330,000 lb) makes it a super heavy-lift launch vehicle.

The BFR system is planned to replace the Falcon 9 and Falcon Heavy launch vehicles, as well as the Dragon spacecraft, initially aiming at the Earth-orbit launch market, but explicitly adding substantial capability to support long-duration spaceflight in the cislunar and Mars mission environments.^[1] SpaceX intends this approach to bring significant cost savings that will help the company justify the development expense of designing and building the BFR system.

SpaceX had initially envisioned a larger design known as the ITS launch vehicle, which was presented in September 2016 as part of Musk's vision for an interplanetary transport system.^[7] The ITS range of vehicles was designed with a 12-meter (39 ft) core diameter,^[8] and the BFR design was scaled down to 9 meters (30 ft).^[1] While the ITS had been solely aimed at Mars transit and other interplanetary uses, SpaceX pivoted in 2017 to a plan that would support all SpaceX launch service provider capabilities with a single range of vehicles: Earth-orbit, Lunar-orbit, interplanetary missions, and even intercontinental passenger transport on Earth.^[1][9]

Development work began in 2012 on the Raptor rocket engines which are to be used for both stages of the BFR launch vehicle, and engine testing began in 2016. New rocket engine designs typically have longer lead times than other major parts of new launch vehicles and spacecraft. Tooling for the main tanks has been ordered and a facility to build the vehicles is under construction; construction of the first ship is scheduled to begin in the second quarter of 2018,^[1] with first suborbital flights planned for 2019.^[10] The company publicly stated an aspirational goal for initial Mars-bound cargo flights of BFR launching as early as 2022, followed by the first crewed flight to Mars one synodic period later, in 2024.^[5]

History

As early as 2007, Elon Musk stated a personal goal of eventually enabling human exploration and settlement of Mars,^[11][12] although his personal public interest in Mars goes back at least to 2001.^[13] Bits of additional information about the mission architecture were released in 2011–2015, including a 2014 statement that initial colonists would arrive at Mars no earlier than the middle of the 2020s.^[14] Company statements in 2016 indicated that SpaceX was "being intentionally fuzzy about the timeline ... We're going to try and make as much progress as we can with a very constrained budget."^[15][16]
Musk stated in a 2011 interview that he hoped to send humans to Mars's surface within 10–20 years,^[12] and in late 2012 he stated that he envisioned a Mars colony of tens of thousands with the first colonists arriving no earlier than the middle of the 2020s.^[14][17][18]

Early development

In March 2012, news accounts asserted that a Raptor upper-stage engine had begun development, although details were not released at that time.^[19] In October 2012, Musk publicly stated a high-level plan to build a second reusable rocket system with capabilities substantially beyond the Falcon 9/Falcon Heavy launch vehicles on which SpaceX had by then spent several billion US dollars.^[20] This new vehicle was to be "an evolution of SpaceX's Falcon 9 booster ... 'much bigger'." But Musk indicated that SpaceX would not be speaking publicly about it until 2013.^[14][21]

In June 2013, Musk stated that he intended to hold off any potential initial public offering of SpaceX shares on the stock market until after the "Mars Colonial Transporter is flying regularly."^[22][23]

In August 2014, media sources speculated that the initial flight test of the Raptor-driven super-heavy launch vehicle could occur as early as 2020, in order to fully test the engines under orbital spaceflight conditions; however, any colonization effort was reported to continue to be "deep into the future".^[24][25]

In early 2015, Musk said that he hoped to release details in late 2015 of the "completely new architecture" for the system that would enable the colonization of Mars. Those plans were delayed,^{[26][27][28][16][29]} and the name of the system architecture was changed to "Interplanetary Transport System" (ITS) in mid-September 2016.^[7]

On 27 September 2016, at the 67th annual meeting of the International Astronautical Congress, Musk unveiled substantial details of the design for the transport vehicles. The details included the very large size (12 meters (39 ft) core diameter),^[8] construction material, number and type of engines, thrust, cargo and passenger payload capabilities, in-orbit propellant-tanker refills, representative transit times, and portions of the Mars-side and Earth-side infrastructure that SpaceX intends to build to support a set of three flight vehicles. The three distinct vehicles that made up the ITS launch vehicle in the 2016 design were the:^[1]

• ITS booster, the first-stage of the launch vehicle
• ITS spaceship, a second-stage and long-duration in-space spacecraft
• ITS tanker, an alternative second-stage designed to carry more propellant for refueling other vehicles in space

In addition, Musk championed a larger systemic vision, a vision for a bottom-up emergent order of other interested parties—whether companies, individuals, or governments—to utilize the new and radically lower-cost transport infrastructure that SpaceX would endeavor to build in order to help build a sustainable human civilization on Mars by innovating and meeting the demand that such a growing venture would occasion.^[30][31]

In the November 2016 plan, SpaceX indicated it would fly its earliest research spacecraft missions to Mars using its Falcon Heavy launch vehicle and a specialized modified Dragon spacecraft, called "Red Dragon" prior to the completion, and first launch, of any ITS launch vehicle. Later Mars missions using ITS were slated at that time to begin no earlier than 2022.^[32]

By February 2017, the earliest launch of any SpaceX mission to Mars was to be 2020, two years later than the previously mentioned 2018 Falcon Heavy/Dragon2 exploratory mission.^[33] In July 2017, SpaceX announced it no longer plans to use a propulsively-landed Red Dragon spacecraft on the early missions, as had been previously announced.^[34]

In July 2017, SpaceX made public its plan to build a much smaller launch vehicle and spacecraft before building the ITS launch vehicle that had been unveiled nine months earlier designed explicitly for the beyond-Earth-orbit (BEO) part of future SpaceX launch service offerings. Musk indicated that the architecture had "evolved quite a bit" since the November 2016 articulation of the comprehensive Mars architecture. A key driver of the new architecture was to be making the new system useful for substantial Earth-orbit and cislunar launches so that the new system might pay for itself, in part, through economic spaceflight activities in the near-Earth space zone.^[35] ITS development was put on hold and "Serious development of BFR" began in 2017.^[1]:15:22

Announcement of the BFR

BFR compared to other systems

On 29 September 2017 at the 68th annual meeting of the International Astronautical Congress in Adelaide, South Australia, SpaceX unveiled the new smaller vehicle architecture. Musk said "we are searching for the right name, but the code name, at least, is BFR."^[1] The new launch vehicle system is a 9-meter (30 ft) diameter technology, using methalox-fueled Raptor rocket engine technology directed initially at the Earth-orbit and cislunar environment, later, being used for Mars missions.^[1][5]

Aerodynamics of the BFR second stage changed from the 2016-design ITS launch vehicle. The new design is cylindrical with a small delta wing at the rear end which includes a split flap for pitch and roll control. The delta wing and split flaps are needed to expand the mission envelope to allow the ship to land in a variety of atmospheric densities (no, thin, or heavy atmosphere) with a wide range of payloads (small, heavy, or none) in the nose of the ship.^{[1]:18:05–19:25} The cylindrical shape is for mass optimization.

There are three versions of the ship: BFR crew, BFR tanker, BFR cargo. The cargo version can also be used to launch satellites to low Earth orbit.

After retanking in a high-elliptic Earth orbit the spaceship is being designed to be able to land on the Moon and return to Earth without further refueling.^[1]:31:50 The most surprising announcement was to use BFR as a point-to-point transfer system for people on Earth. Musk expects ticket price to be on par with a full-fare economy plane ticket for the same distance.

As of September 2017, Raptor engines had been tested for a combined total of 1200 seconds of test firing time over 42 main engine tests. The longest test was 100 seconds, which is limited by the size of the propellant tanks at the SpaceX ground test facility. The test engine operates at 20 MPa (200 bar; 2,900 psi) pressure. The flight engine is aimed for 25 MPa (250 bar; 3,600 psi), and SpaceX expects to achieve 30 MPa (300 bar; 4,400 psi) in later iterations.^[1]

Testing of the BFR vehicle is expected to begin with short suborbital hops of the full-scale ship, likely to be just a few hundred kilometers altitude and lateral distance.^[36]

By September 2017, SpaceX had already started building launch vehicle components. "The tooling for the main tanks has been ordered, the facility is being built, we will start construction of the first ship [in the second quarter of 2018.]" Musk is hoping to be ready for an initial Mars launch in five years, in order to make the 2022 Mars conjunction window.^[1] In November 2017, SpaceX president and COO Gwynne Shotwell indicated that approximately half of all current development work on BFR is on Raptor engine development.^[37]

The aspirational goal is the first two cargo missions to Mars in 2022, with the goal to "confirm water resources and identify hazards" while putting "power, mining, and life support infrastructure" in place for future flights, followed by four ships in 2024, two crewed BFR spaceships plus two cargo-only ships bringing additional equipment and supplies with the goal of setting up the propellant production plant.^[1]

Nomenclature

The descriptor for the large SpaceX Mars rocket has varied over the past five years that SpaceX has publicly-released information about the project. "BFR" is the current code name for SpaceX's privately funded launch vehicle announced by Elon Musk in September 2017.^{[1]:2:39[5][6][38][39][40]} SpaceX President Gwynne Shotwell has stated the BFR code stands for "Big Falcon Rocket".^[41] However, Elon Musk has explained that although this is the official name, he drew inspiration from the BFG weapon in the Doom video games.^[42]

From September 2016 through August 2017, the overall system was referred to by SpaceX as the Interplanetary Transport System and the launch vehicle itself as the ITS launch vehicle. Beginning in mid-2013, and prior to September 2016, SpaceX had referred to both the architecture and the vehicle as the Mars Colonial Transporter.

Scope of BFR missions

The BFR launch vehicle is planned to replace all existing SpaceX vehicles and spacecraft in the early 2020s. SpaceX cost estimation has led the company to conclude that BFR launches will be cheaper per launch than launches of the existing vehicles and even cheaper than launches of the retired Falcon 1. This is partly due to the full reusability of all parts of BFR, and partly due to precision landing of the booster on its launch mount and industry-leading launch operations. More specifically, both Falcon 9 and Falcon Heavy launch vehicles and the Dragon spacecraft flown in February 2018 will be replaced in the operational SpaceX fleet during the early 2020s.^{[43][1]:24:50–27:05}

Flight missions of BFR will thus aim at the:^[43]

• legacy Earth-orbit market
• long-duration spaceflight missions in the cislunar region
• Mars missions, both as cargo ships as well as passenger-carrying transport
• long-distance commercial travel on Earth: the ability to transport people on point-to-point suborbital flights between two points on Earth in under one hour.^[36][44] Musk refers to this as "Earth-to-Earth".^[45]

Description

The BFR design combines several elements that, according to Musk, will make long-duration, beyond Earth orbit (BEO) spaceflights possible. They will reduce the per-ton cost of launches to low Earth orbit (LEO) and of transportation between BEO destinations. They will also serve all usage for the conventional LEO market. This will allow SpaceX to focus the majority of their development resources on the next-generation launch vehicle.^[1][9][46]

The fully-reusable super-heavy-lift BFR will consist of a:^[1]

• "BFR booster": a reusable booster stage.
• a reusable, integrated second-stage-with-spaceship, which will be built in at least three versions:
  • "BFR spaceship": a large, long-duration spaceship capable of carrying passengers or cargo to interplanetary destinations, to LEO, or between destinations on Earth.
  • "BFR tanker": an Earth-orbit, cargo-only propellant tanker to support the refilling of propellants in orbit. The tanker will enable a long-duration spaceship to serve as the second stage of the launch vehicle while expending almost all of its propellant to reach LEO. After refilling in orbit, the spaceship will provide a significant amount of the energy needed to put it onto an interplanetary trajectory.
  • "BFR satellite delivery spacecraft": will have a large cargo bay door that can open in space to facilitate the placement of large and small spacecraft into orbit.

Combining the second-stage of a launch vehicle with a long-duration spaceship will be a unique type of space mission architecture. This architecture is dependent on the successful refilling of propellants in orbit.

The BFR spaceship, the BFR tanker, and the BFR satellite delivery spacecraft will have the same outer mold line. The second-stage-spaceship will be capable of returning to the launch location. While returning, it will be able to tolerate multiple engine-out events and land successfully with just one operating engine.^[1]

The functioning of the system during BEO launches to Mars will include propellant production on the Mars surface. This is necessary for the return trip and to reuse the spaceship at a minimal cost. Lunar destinations will be possible without Lunar-propellant depots, so long as the spaceship is refueled in a high-elliptical orbit before the Lunar transit begins.^[1]

The major characteristics of the launch vehicle will include the following.^[1][47][3]

• Both stages will be completely reusable.
• The booster will return to land on the launch mount. The second-stage/spaceship will have the ability to return to near the launch mount. Both will use retropropulsive landing and the reusable LV technologies developed earlier by SpaceX.
• The expected landing reliability will be on a par with major airliners.
• Rendezvous and docking will be automated.
• There will be on-orbit propellant transfers from BFR tankers to BFR spaceships.
• A spaceship and its payload will be able to transit to the Moon or fly to Mars after an on-orbit propellant loading.
• Heat-shields will be reusable.
• The BFR spaceship will have a pressurized volume of 825 m³ (29,100 cu ft), with up to 40 cabins, large common areas, central storage, a galley, and a solar storm shelter for Mars missions.

Specifications^[1][47]

Component Attribute	Complete	BFR booster	BFR spaceship/tanker/ sat-delivery vehicle
LEO Payload	150,000 kg (330,000 lb)[3]
Return Payload			50,000 kg (110,000 lb)[3]
Cargo Volume	825 m3 (29,100 cu ft)[3]	N/A	825 m3 (29,100 cu ft)[3] (spaceship)
Diameter	9 m (30 ft)[3]
Length	106 m (348 ft)	58 m (190 ft)	48 m (157 ft)[3]
Maximum weight	4,400,000 kg (9,700,000 lb)[3]		1,335,000 kg (2,943,000 lb)
Propellant Capacity			CH 4 – 240,000 kg (530,000 lb)
			O 2 – 860,000 kg (1,900,000 lb)
Empty weight			85,000 kg (187,000 lb)[3]
Engines		31 × SL Raptors	3 × SL + 4 × vacuum Raptors[4]
Thrust		52.7 MN (11,800,000 lbf)	12.7 MN (2,900,000 lbf) total

The Raptor engine will operate at 25 MPa (250 bar; 3,600 psi) of chamber pressure and achieve 30 MPa (300 bar; 4,400 psi) in later iterations. The engine will be designed with an extreme focus on reliability.^[1][47]

Manufacturing the BFR

Construction of the initial BFR vehicle will be in a new factory to be built on the Los Angeles waterfront^[48] in the San Pedro area.^[49]