Molecular lesion

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Molecular_lesion 

Ball and Stick Model of Double Helical DNA

A molecular lesion or point lesion is damage to the structure of a biological molecule such as DNA, RNA, or protein. This damage may result in the reduction or absence of normal function, and in rare cases the gain of a new function. Lesions in DNA may consist of breaks or other changes in chemical structure of the helix, ultimately preventing transcription. Meanwhile, lesions in proteins consist of both broken bonds and improper folding of the amino acid chain. While many nucleic acid lesions are general across DNA and RNA, some are specific to one, such as thymine dimers being found exclusively in DNA. Several cellular repair mechanisms exist, ranging from global to specific, in order to prevent lasting damage resulting from lesions.

Causes

There are two broad causes of nucleic acid lesions, endogenous and exogenous factors. Endogenous factors, or endogeny, refer to the resulting conditions that develop within an organism. This is in contrast with exogenous factors which originate from outside the organism. DNA and RNA lesions caused by endogenous factors generally occur more frequently than damage caused by exogenous ones.

Endogenous Factors

Endogenous sources of specific DNA damage include pathways like hydrolysis, oxidation, alkylation, mismatch of DNA bases, depurination, depyrimidination, double-strand breaks (DSS), and cytosine deamination. DNA lesions can also naturally occur from the release of specific compounds such as reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive carbonyl species (RCS), lipid peroxidation products, adducts, and alkylating agents through metabolic processes. ROS is one of the major endogenous sources of DNA damage and the most studied oxidative DNA adduct is 8-oxo-dG. Other adducts known to form are etheno-, propano-, and malondialdehyde-derived DNA adducts. The aldehydes formed from lipid peroxidation also pose another threat to DNA. Proteins such as “damage-up” proteins (DDPs) can promote endogenous DNA lesions by either increasing the amount of reactive oxygen by transmembrane transporters, losing chromosomes by replisome binding, and stalling replication by transcription factors. For RNA lesions specifically, the most abundant types of endogenous damage include oxidation, alkylation, and chlorination. Phagocytic cells produce radical species that include hypochlorous acid (HOCl), nitric oxide (NO•), and peroxynitrite (ONOO−) to fight infections, and many cell types use nitric oxide as a signaling molecule. However, these radical species can also cause the pathways that form RNA lesions.

Thymine Photodimer Caused by UV Light

Exogenous Factors

Ultraviolet Radiation

UV light, specifically non-ionizing shorter-wavelength radiation such as UVC and UVB, causes direct DNA damage by initiating a synthesis reaction between two thymine molecules. The resulting dimer is very stable. Although they can be removed through excision repairs, when UV damage is extensive, the entire DNA molecule breaks down and the cell dies. If the damage is not too extensive, precancerous or cancerous cells are created from healthy cells.

Chemotherapy drugs

Chemotherapeutics, by design, induce DNA damage and are targeted towards rapidly dividing cancer cells. However, these drugs can not tell the difference between sick and healthy cells, resulting in the damage of normal cells.

Alkylating agents

Alkylating agents are a type of chemotherapeutic drug which keeps the cell from undergoing mitosis by damaging its DNA. They work in all phases of the cell cycle. The use of alkylating agents may result in leukemia due to them being able to target the cells of the bone marrow.

Cancer causing agents

Carcinogens are known to cause a number of DNA lesions, such as single-strand breaks, double- strand breaks, and covalently bound chemical DNA adducts. Tobacco products are one of the most prevalent cancer-causing agents of today. Other DNA damaging, cancer-causing agents include asbestos, which can cause damage through physical interaction with DNA or by indirectly setting off a reactive oxygen species, excessive nickel exposure, which can repress the DNA damage-repair pathways, aflatoxins, which are found in food, and many more.

Lesions of Nucleic Acids

Types of Structural DNA Molecular Lesion Damage

Oxidative lesions

Oxidative lesions are an umbrella category of lesions caused by reactive oxygen species (ROS), reactive nitrogen species (RNS), other byproducts of cellular metabolism, and exogenous factors such as ionizing or ultraviolet radiation. Byproducts of oxidative respiration are the main source of reactive species which cause a background level of oxidative lesions in the cell. DNA and RNA are both affected by this, and it has been found that RNA oxidative lesions are more abundant in humans compared to DNA. This may be due cytoplasmic RNA having closer proximity to the electron transport chain. Known oxidative lesions characterized in DNA and RNA are many in number, as oxidized products are unstable and may resolve quickly. The hydroxyl radical and singlet oxygen are common reactive oxygen species responsible for these lesions. 8-oxo-guanine (8-oxoG) is the most abundant and well characterized oxidative lesion, found in both RNA and DNA. Accumulation of 8-oxoG may cause dire damage within the mitochondria and is thought to be a key player in the aging process. RNA oxidation has direct consequences in the production of proteins. mRNA affected by oxidative lesions is still recognized by ribosome, but the ribosome will undergo stalling and dysfunction. This results in proteins having either decreased expression or truncation, leading to aggregation and general dysfunction.

Structural rearrangements

• Depurination is caused by hydrolysis and results in loss if the purine base of a nucleic acid. DNA is more prone to this, as the transition state in the depurination reaction has much greater energy in RNA.
• Tautomerization is a chemical reaction that is primarily relevant in the behavior of amino acids and nucleic acids. Both of which are correlated to DNA and RNA. The process of tautomerization of DNA bases occurs during DNA replication. The ability for the wrong tautomer of one of the standard nucleic bases to mispair causes a mutation during the process of DNA replication which can be cytotoxic or mutagenic to the cell. These mispairings can result in transition, transversion, frameshift, deletion, and/or duplication mutations. Some diseases that result from tautomerization induced DNA lesions include Kearns-Sayre syndrome, Fragile X syndrome, Kennedy disease, and Huntington’s disease.
• Cytosine deamination commonly occurs under physiological conditions and essentially is the deamination of cytosine. This process yields uracil as its product, which is not a base pair found within DNA. This process causes extensive DNA damage. The rate of this process is slowed down significantly in double-stranded DNA compared to single-stranded DNA.

Single and Double Stranded Breaks

Single-strand breaks (SSBs) occur when one strand of the DNA double helix experiences breakage of a single nucleotide accompanied by damaged 5’- and/or 3’-termini at this point. One common source of SSBs is due to oxidative attack by physiological reactive oxygen species (ROS) such as hydrogen peroxide. H₂O₂ causes SSBs three times more frequently than double-strand breaks (DSBs). Alternative methods of SSB acquisition include direct disintegration of the oxidized sugar or through DNA base-excision repair (BER) of damaged bases. Additionally, cellular enzymes may perform erroneous activity leading to SSBs or DSBs by a variety of mechanisms. One such example would be when the cleavage complex formed by DNA topoisomerase 1 (TOP1) relaxes DNA during transcription and replication through the transient formation of a nick. While TOP1 normally reseals this nick shortly after, these cleavage complexes may collide with RNA or DNA polymerases or be proximal to other lesions, leading to TOP1-linked SSBs or TOP1-linked DSBs.

Chemical Adducts

A DNA adduct is a segment of DNA that binds to a chemical carcinogen. Some adducts that cause lesions to DNA included oxidatively modified bases, propano-, etheno-, and MDA-induced adducts. 5‐Hydroxymethyluracil is an example of an oxidatively modified base where oxidation of the methyl group of thymine occurs. This adduct interferes with the binding of transcription factors to DNA which can trigger apoptosis or result in deletion mutations. Propano adducts are derived by species generated by lipid peroxidation. For example, HNE is a major toxic product of the process. It regulates the expression of genes that are involved in cell cycle regulation and apoptosis. Some of the aldehydes from lipid peroxidation can be converted to epoxy aldehydes by oxidation reactions. These epoxy aldehydes can damage DNA by producing etheno adducts. An increase in this type of DNA lesion exhibits conditions resulting in oxidative stress which is known to be associated with an increased risk of cancer. Malondialdehyde (MDA) is another highly toxic product from lipid peroxidation and also in the synthesis of prostaglandin. MDA reacts with DNA to form the M1dG adduct which causes DNA lesions.

Disease Effects

Many systems are in place to repair DNA and RNA lesions but it is possible for lesions to escape these measures. This may lead to mutations or large genome abnormalities, which can threaten the cell or organism’s ability to live. Several cancers are a result of DNA lesions. Even repair mechanisms to heal the damage may end up causing more damage. Mismatch repair defects, for example, cause instability that predisposes to colorectal and endometrial carcinomas.

DNA lesions in neurons may lead to neurodegenerative disorders such as Alzheimer’s, Huntington’s, and Parkinson’s diseases. These come as a result of neurons generally being associated with high mitochondrial respiration and redox species production, which can damage nuclear DNA. Since these cells often cannot be replaced after being damaged, the damage done to them leads to dire consequences. Other disorders stemming from DNA lesions and their association with neurons include but are not limited to Fragile X syndrome, Friedrich’s ataxia, and spinocerebellar ataxias.

During replication, usually DNA polymerases are unable to go past the lesioned area, however, some cells are equipped with special polymerases which allow for translesion synthesis (TLS). TLS polymerases allow for the replication of DNA past lesions and risk generating mutations at a high frequency. Common mutations that occur after undergoing this process are point mutations and frameshift mutations. Several diseases come as a result of this process including several cancers and Xeroderma pigmentosum.

The effect of oxidatively damaged RNA has resulted in a number of human diseases and is especially associated with chronic degeneration. This type of damage has been observed in many neurodegenerative diseases such as Amyotrophic lateral sclerosis, Alzheimer’s, Parkinson’s, dementia with Lewy bodies, and several prion diseases. It is important to note that this list is rapidly growing and data suggests that RNA oxidation occurs early in the development of these diseases, rather than as an effect of cellular decay. RNA and DNA lesions are both associated with the development of diabetes mellitus type 2.

Repair Mechanisms

DNA Damage Response

When DNA is damaged such as due to a lesion, a complex signal transduction pathway is activated which is responsible for recognizing the damage and instigating the cell’s response for repair. Compared to the other lesion repair mechanisms, DDR is the highest level of repair and is employed for the most complex lesions. DDR consists of various pathways, the most common of which are the DDR kinase signaling cascades. These are controlled by phosphatidylinositol 3-kinase-related kinases (PIKK), and range from DNA-dependent protein kinase (DNA-PKcs) and ataxia telangiectasia-mutated (ATM) most involved in repairing DSBs to the more versatile Rad3-related (ATR). ATR is crucial to human cell viability, while ATM mutations cause the severe disorder ataxia-telangiectasia leading to neurodegeneration, cancer, and immunodeficiency. These three DDR kinases all recognize damage via protein-protein interactions which localize the kinases to the areas of damage. Next, further protein-protein interactions and posttranslational modifications (PTMs) complete the kinase activation, and a series of phosphorylation events takes place. DDR kinases perform repair regulation at three levels - via PTMs, at the level of chromatin, and at the level of the nucleus.

BER Pathway

Base Excision Repair

Base excision repair (BER) is responsible for removing damaged bases in DNA. This mechanism specifically works on excising small base lesions which do not distort the DNA double helix, in contrast to the nucleotide excision repair pathway which is employed in correcting more prominent distorting lesions. DNA glycosylases initiate BER by both recognizing the faulty or incorrect bases and then removing them, forming AP sites lacking any purine or pyrimidine. AP endonuclease then cleaves the AP site, and the single-strand break is either processed by short-patch BER to replace a single nucleotide long-patch BER to create 2-10 replacement nucleotides.

Single Stranded Break Repair

DNA DSB Repair System

Single stranded breaks (SSBs) can severely threaten genetic stability and cell survival if not quickly and properly repaired, so cells have developed fast and efficient SSB repair (SSBR) mechanisms. While global SSBR systems extract SSBs throughout the genome and during interphase, S-phase specific SSBR processes work together with homologous recombination at the replication forks.

Double Stranded Break Repair

Double stranded breaks (DSB) are a threat to all organisms as they can cause cell death and cancer. They can be caused exogenously as a result of radiation and endogenously from errors in replication or encounters with DNA lesions by the replication fork. DSB repair occurs through a variety of different pathways and mechanisms in order to correctly repair these errors.

Nucleotide Excision and Mismatch Repair

Nucleotide excision repair  is one of the main mechanisms used to remove bulky adducts from DNA lesions caused by chemotherapy drugs, environmental mutagens, and most importantly UV radiation. This mechanism functions by releasing a short damage containing oligonucleotide from the DNA site, and then that gap is filled in and repaired by NER. NER recognizes a variety of structurally unrelated DNA lesions due to the flexibility of the mechanism itself, as NER is highly sensitive to changes in the DNA helical structure. Bulky adducts seem to trigger NER. The XPC-RAD23-CETN2 heterotrimer involved with NER has a critical role in DNA lesion recognition. In addition to other general lesions in the genome, UV damaged DNA binding protein complex (UV-DDB)  also has an important role in both recognition and repair of UV-induced DNA photolesions.

Mismatch repair (MMR) mechanisms within the cell correct base mispairs that occur during replication using a variety of pathways. It has a high affinity for targeting DNA lesions with specificity, as alternations in base pair stacking that occur at DNA lesion sites affect the helical structure. This is likely one of many signals that triggers MMR.

Space rendezvous

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Space_rendezvous 

Astronaut Christopher Cassidy uses a rangefinder to determine distance between the Space Shuttle Endeavour and the International Space Station

Lunar Module Eagle ascent stage rendezvous with the command module Columbia in lunar orbit after returning from a landing

A space rendezvous (/ˈrɒndeɪvuː/) is a set of orbital maneuvers during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance (e.g. within visual contact). Rendezvous requires a precise match of the orbital velocities and position vectors of the two spacecraft, allowing them to remain at a constant distance through orbital station-keeping. Rendezvous may or may not be followed by docking or berthing, procedures which bring the spacecraft into physical contact and create a link between them.

The same rendezvous technique can be used for spacecraft "landing" on natural objects with a weak gravitational field, e.g. landing on one of the Martian moons would require the same matching of orbital velocities, followed by a "descent" that shares some similarities with docking.

History

In its first human spaceflight program Vostok, the Soviet Union launched pairs of spacecraft from the same launch pad, one or two days apart (Vostok 3 and 4 in 1962, and Vostok 5 and 6 in 1963). In each case, the launch vehicles' guidance systems inserted the two craft into nearly identical orbits; however, this was not nearly precise enough to achieve rendezvous, as the Vostok lacked maneuvering thrusters to adjust its orbit to match that of its twin. The initial separation distances were in the range of 5 to 6.5 kilometers (3.1 to 4.0 mi), and slowly diverged to thousands of kilometers (over a thousand miles) over the course of the missions.

In 1963 Buzz Aldrin submitted his doctoral thesis titled, Line-Of-Sight Guidance Techniques For Manned Orbital Rendezvous. As a NASA astronaut, Aldrin worked to "translate complex orbital mechanics into relatively simple flight plans for my colleagues."

First attempt failed

NASA's first attempt at rendezvous was made on June 3, 1965, when US astronaut Jim McDivitt tried to maneuver his Gemini 4 craft to meet its spent Titan II launch vehicle's upper stage. McDivitt was unable to get close enough to achieve station-keeping, due to depth-perception problems, and stage propellant venting which kept moving it around. However, the Gemini 4 attempts at rendezvous were unsuccessful largely because NASA engineers had yet to learn the orbital mechanics involved in the process. Simply pointing the active vehicle's nose at the target and thrusting was unsuccessful. If the target is ahead in the orbit and the tracking vehicle increases speed, its altitude also increases, actually moving it away from the target. The higher altitude then increases orbital period due to Kepler's third law, putting the tracker not only above, but also behind the target. The proper technique requires changing the tracking vehicle's orbit to allow the rendezvous target to either catch up or be caught up with, and then at the correct moment changing to the same orbit as the target with no relative motion between the vehicles (for example, putting the tracker into a lower orbit, which has a shorter orbital period allowing it to catch up, then executing a Hohmann transfer back to the original orbital height).

As GPO engineer André Meyer later remarked, "There is a good explanation for what went wrong with rendezvous." The crew, like everyone else at MSC, "just didn't understand or reason out the orbital mechanics involved. As a result, we all got a whole lot smarter and really perfected rendezvous maneuvers, which Apollo now uses."

First successful rendezvous

Gemini 7 photographed from Gemini 6 in 1965

Rendezvous was first successfully accomplished by US astronaut Wally Schirra on December 15, 1965. Schirra maneuvered the Gemini 6 spacecraft within 1 foot (30 cm) of its sister craft Gemini 7. The spacecraft were not equipped to dock with each other, but maintained station-keeping for more than 20 minutes. Schirra later commented:

Somebody said ... when you come to within three miles (5 km), you've rendezvoused. If anybody thinks they've pulled a rendezvous off at three miles (5 km), have fun! This is when we started doing our work. I don't think rendezvous is over until you are stopped – completely stopped – with no relative motion between the two vehicles, at a range of approximately 120 feet (37 m). That's rendezvous! From there on, it's stationkeeping. That's when you can go back and play the game of driving a car or driving an airplane or pushing a skateboard – it's about that simple.

Schirra used another metaphor to describe the difference between the two nations' achievements:

[The Russian "rendezvous"] was a passing glance—the equivalent of a male walking down a busy main street with plenty of traffic whizzing by and he spots a cute girl walking on the other side. He's going 'Hey wait' but she's gone. That's a passing glance, not a rendezvous.

First docking

Main article: Docking and berthing of spacecraft

Gemini 8 Agena target vehicle

Gemini 8 docking with the Agena in March 1966

The first docking of two spacecraft was achieved on March 16, 1966 when Gemini 8, under the command of Neil Armstrong, rendezvoused and docked with an uncrewed Agena Target Vehicle. Gemini 6 was to have been the first docking mission, but had to be cancelled when that mission's Agena vehicle was destroyed during launch.

The Soviets carried out the first automated, uncrewed docking between Cosmos 186 and Cosmos 188 on October 30, 1967.

The first Soviet cosmonaut to attempt a manual docking was Georgy Beregovoy who unsuccessfully tried to dock his Soyuz 3 craft with the uncrewed Soyuz 2 in October 1968. He was able to bring his craft from 200 meters (660 ft) to as close as 30 centimetres (1 ft), but was unable to dock before exhausting his maneuvering fuel.

The first successful crewed docking occurred on January 16, 1969 when Soyuz 4 and Soyuz 5 docked, collecting the two crew members of Soyuz 5, which had to perform an extravehicular activity to reach Soyuz 4.

In March 1969 Apollo 9 achieved the first internal transfer of crew members between two docked spacecraft.

The first rendezvous of two spacecraft from different countries took place in 1975, when an Apollo spacecraft docked with a Soyuz spacecraft as part of the Apollo–Soyuz mission.

The first multiple space docking took place when both Soyuz 26 and Soyuz 27 were docked to the Salyut 6 space station during January 1978.

Uses

Damaged solar arrays on Mir's Spektr module following a collision with an uncrewed Progress spacecraft in September 1997 as part of Shuttle-Mir. The Progress spacecraft were used for re-supplying the station. In this space rendezvous gone wrong, the Progress collided with Mir, beginning a depressurization that was halted by closing the hatch to Spektr.

A rendezvous takes place each time a spacecraft brings crew members or supplies to an orbiting space station. The first spacecraft to do this was Soyuz 11, which successfully docked with the Salyut 1 station on June 7, 1971. Human spaceflight missions have successfully made rendezvous with six Salyut stations, with Skylab, with Mir and with the International Space Station (ISS). Currently Soyuz spacecraft are used at approximately six month intervals to transport crew members to and from ISS. With the introduction of NASA's Commercial Crew Program, the US is able to use their own launch vehicle along with the Soyuz, an updated version of SpaceX's Cargo Dragon; Crew Dragon. 

Robotic spacecraft are also used to rendezvous with and resupply space stations. Soyuz and Progress spacecraft have automatically docked with both Mir and the ISS using the Kurs docking system, Europe's Automated Transfer Vehicle also used this system to dock with the Russian segment of the ISS. Several uncrewed spacecraft use NASA's berthing mechanism rather than a docking port. The Japanese H-II Transfer Vehicle (HTV), SpaceX Dragon, and Orbital Sciences' Cygnus spacecraft all maneuver to a close rendezvous and maintain station-keeping, allowing the ISS Canadarm2 to grapple and move the spacecraft to a berthing port on the US segment. However the updated version of Cargo Dragon will no longer need to berth but instead will autonomously dock directly to the space station. The Russian segment only uses docking ports so it is not possible for HTV, Dragon and Cygnus to find a berth there.

Space rendezvous has been used for a variety of other purposes, including recent service missions to the Hubble Space Telescope. Historically, for the missions of Project Apollo that landed astronauts on the Moon, the ascent stage of the Apollo Lunar Module would rendezvous and dock with the Apollo Command/Service Module in lunar orbit rendezvous maneuvers. Also, the STS-49 crew rendezvoused with and attached a rocket motor to the Intelsat VI F-3 communications satellite to allow it to make an orbital maneuver.

Possible future rendezvous may be made by a yet to be developed automated Hubble Robotic Vehicle (HRV), and by the CX-OLEV, which is being developed for rendezvous with a geosynchronous satellite that has run out of fuel. The CX-OLEV would take over orbital stationkeeping and/or finally bring the satellite to a graveyard orbit, after which the CX-OLEV can possibly be reused for another satellite. Gradual transfer from the geostationary transfer orbit to the geosynchronous orbit will take a number of months, using Hall effect thrusters.

Alternatively the two spacecraft are already together, and just undock and dock in a different way:

• Soyuz spacecraft from one docking point to another on the ISS or Salyut
• In the Apollo spacecraft, a maneuver known as transposition, docking, and extraction was performed an hour or so after Trans Lunar Injection of the sequence third stage of the Saturn V rocket / LM inside LM adapter / CSM (in order from bottom to top at launch, also the order from back to front with respect to the current motion), with CSM crewed, LM at this stage uncrewed:
  • the CSM separated, while the four upper panels of the LM adapter were disposed of
  • the CSM turned 180 degrees (from engine backward, toward LM, to forward)
  • the CSM connected to the LM while that was still connected to the third stage
  • the CSM/LM combination then separated from the third stage

NASA sometimes refers to "Rendezvous, Proximity-Operations, Docking, and Undocking" (RPODU) for the set of all spaceflight procedures that are typically needed around spacecraft operations where two spacecraft work in proximity to one another with intent to connect to one another.

Phases and methods

Command and service module Charlie Brown as seen from Lunar Module Snoopy

Orbital rendezvous. 1/ Both spacecraft must be in the same orbital plane. ISS flies in a higher orbit (lower speed), ATV flies in a lower orbit and catches up with ISS. 2/At the moment when the ATV and the ISS make an alpha angle (about 2°), the ATV crosses the elliptical orbit to the ISS.

The standard technique for rendezvous and docking is to dock an active vehicle, the "chaser", with a passive "target". This technique has been used successfully for the Gemini, Apollo, Apollo/Soyuz, Salyut, Skylab, Mir, ISS, and Tiangong programs.

To properly understand spacecraft rendezvous it is essential to understand the relation between spacecraft velocity and orbit. A spacecraft in a certain orbit cannot arbitrarily alter its velocity. Each orbit correlates to a certain orbital velocity. If the spacecraft fires thrusters and increases (or decreases) its velocity it will obtain a different orbit, one that correlates to the higher (or lower) velocity. For circular orbits, higher orbits have a lower orbital velocity. Lower orbits have a higher orbital velocity.

For orbital rendezvous to occur, both spacecraft must be in the same orbital plane, and the phase of the orbit (the position of the spacecraft in the orbit) must be matched. For docking, the speed of the two vehicles must also be matched. The "chaser" is placed in a slightly lower orbit than the target. The lower the orbit, the higher the orbital velocity. The difference in orbital velocities of chaser and target is therefore such that the chaser is faster than the target, and catches up with it.

Once the two spacecraft are sufficiently close, the chaser's orbit is synchronized with the target's orbit. That is, the chaser will be accelerated. This increase in velocity carries the chaser to a higher orbit. The increase in velocity is chosen such that the chaser approximately assumes the orbit of the target. Stepwise, the chaser closes in on the target, until proximity operations (see below) can be started. In the very final phase, the closure rate is reduced by use of the active vehicle's reaction control system. Docking typically occurs at a rate of 0.1 ft/s (0.030 m/s) to 0.2 ft/s (0.061 m/s).

Rendezvous phases

Space rendezvous of an active, or "chaser", spacecraft with an (assumed) passive spacecraft may be divided into several phases, and typically starts with the two spacecraft in separate orbits, typically separated by more than 10,000 kilometers (6,200 mi):

Phase	Separation distance	Typical phase duration
Drift Orbit A (out of sight, out of contact)	>2 λmax	1 to 20 days
Drift Orbit B (in sight, in contact)	2 λmax to 1 kilometer (3,300 ft)	1 to 5 days
Proximity Operations A	1,000–100 meters (3,280–330 ft)	1 to 5 orbits
Proximity Operations B	100–10 meters (328–33 ft)	45 – 90 minutes
Docking	<10 meters (33 ft)	<5 minutes

A variety of techniques may be used to effect the translational and rotational maneuvers necessary for proximity operations and docking.

Methods of approach

The two most common methods of approach for proximity operations are in-line with the flight path of the spacecraft (called V-bar, as it is along the velocity vector of the target) and perpendicular to the flight path along the line of the radius of the orbit (called R-bar, as it is along the radial vector, with respect to Earth, of the target). The chosen method of approach depends on safety, spacecraft / thruster design, mission timeline, and, especially for docking with the ISS, on the location of the assigned docking port.

V-bar approach

The V-bar approach is an approach of the "chaser" horizontally along the passive spacecraft's velocity vector. That is, from behind or from ahead, and in the same direction as the orbital motion of the passive target. The motion is parallel to the target's orbital velocity. In the V-bar approach from behind, the chaser fires small thrusters to increase its velocity in the direction of the target. This, of course, also drives the chaser to a higher orbit. To keep the chaser on the V-vector, other thrusters are fired in the radial direction. If this is omitted (for example due to a thruster failure), the chaser will be carried to a higher orbit, which is associated with an orbital velocity lower than the target's. Consequently, the target moves faster than the chaser and the distance between them increases. This is called a natural braking effect, and is a natural safeguard in case of a thruster failure.

STS-104 was the third Space Shuttle mission to conduct a V-bar arrival at the International Space Station. The V-bar, or velocity vector, extends along a line directly ahead of the station. Shuttles approach the ISS along the V-bar when docking at the PMA-2 docking port.

R-bar approach

The R-bar approach consists of the chaser moving below or above the target spacecraft, along its radial vector. The motion is orthogonal to the orbital velocity of the passive spacecraft. When below the target the chaser fires radial thrusters to close in on the target. By this it increases its altitude. However, the orbital velocity of the chaser remains unchanged (thruster firings in the radial direction have no effect on the orbital velocity). Now in a slightly higher position, but with an orbital velocity that does not correspond to the local circular velocity, the chaser slightly falls behind the target. Small rocket pulses in the orbital velocity direction are necessary to keep the chaser along the radial vector of the target. If these rocket pulses are not executed (for example due to a thruster failure), the chaser will move away from the target. This is a natural braking effect. For the R-bar approach, this effect is stronger than for the V-bar approach, making the R-bar approach the safer one of the two. Generally, the R-bar approach from below is preferable, as the chaser is in a lower (faster) orbit than the target, and thus "catches up" with it. For the R-bar approach from above, the chaser is in a higher (slower) orbit than the target, and thus has to wait for the target to approach it.

Astrotech proposed meeting ISS cargo needs with a vehicle which would approach the station, "using a traditional nadir R-bar approach." The nadir R-bar approach is also used for flights to the ISS of H-II Transfer Vehicles, and of SpaceX Dragon vehicles.

Z-bar approach

An approach of the active, or "chaser", spacecraft horizontally from the side and orthogonal to the orbital plane of the passive spacecraft—that is, from the side and out-of-plane of the orbit of the passive spacecraft—is called a Z-bar approach.

User experience design

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/User_experience_design 

User experience design (UX design, UXD, UED, or XD) is the process of defining the experience a user would go through when interacting with a company, its services, and its products. Design decisions in UX design are often driven by research, data analysis, and test results rather than aesthetic preferences and opinions. Unlike user interface design, which focuses solely on the design of a computer interface, UX design encompasses all aspects of a user's perceived experience with a product or website, such as its usability, usefulness, desirability, brand perception, and overall performance. UX design is also an element of the customer experience (CX), which encompasses all aspects and stages of a customer's experience and interaction with a company.

History

The field of user experience design is a conceptual design discipline and has its roots in human factors and ergonomics, a field that, since the late 1940s, has focused on the interaction between human users, machines, and the contextual environments to design systems that address the user's experience. With the proliferation of workplace computers in the early 1990s, user experience started to become a positive insight for designers. Donald Norman, a professor and researcher in design, usability, and cognitive science, coined the term "user experience," and brought it to a wider audience.

I invented the term because I thought human interface and usability were too narrow. I wanted to cover all aspects of the person's experience with the system including industrial design graphics, the interface, the physical interaction and the manual. Since then the term has spread widely, so much so that it is starting to lose its meaning.

— Donald Norman

There is a debate occurring in the experience design community regarding its focus, provoked in part by design scholar and practitioner, Don Norman. Norman claims that when designers describe people only as customers, consumers, and users, designers risk diminishing their ability to do good design.

Elements

Research

Main article: User research

User experience design draws from design approaches like human-computer interaction and user-centered design, and includes elements from similar disciplines like interaction design, visual design, information architecture, user research, and others.

Another portion of the research is understanding the end-user and the purpose of the application. Though this might seem clear to the designer, stepping back and empathizing with the user will yield the best results.

It helps to identify and prove or disprove assumptions, find commonalities across target audience members, and recognize their needs, goals, and mental models.

Visual design

Visual design, also commonly known as graphic design, user interface design, communication design, and visual communication, represents the aesthetics or look-and-feel of the front end of any user interface. Graphic treatment of interface elements is often perceived as the visual design. The purpose of visual design is to use visual elements like colors, images, and symbols to convey a message to its audience. Fundamentals of Gestalt psychology and visual perception give a cognitive perspective on how to create effective visual communication.

Information architecture

Main article: Information architecture

Information architecture is the art and science of structuring and organizing the information in products and services to support usability and findability.

In the context of information architecture, information is separate from both knowledge and data, and lies nebulously between them. It is information about objects. The objects can range from websites, to software applications, to images et al. It is also concerned with metadata: terms used to describe and represent content objects such as documents, people, process, and organizations. Information architecture also encompasses how the pages and navigation are structured.

Interaction design

Main article: Interaction design

It is well recognized that the component of interaction design is an essential part of user experience (UX) design, centering on the interaction between users and products. The goal of interaction design is to create a product that produces an efficient and delightful end-user experience by enabling users to achieve their objectives in the best way possible.

The current high emphasis on user-centered design and the strong focus on enhancing user experience have made interaction designers critical in conceptualizing products to match user expectations and meet the standards of the latest UI patterns and components.

In the last few years, the role of interaction designer has shifted from being just focused on specifying UI components and communicating them to the engineers to a situation in which designers have more freedom to design contextual interfaces based on helping meet the user's needs. Therefore, User Experience Design evolved into a multidisciplinary design branch that involves multiple technical aspects from motion graphics design and animation to programming.

Usability

Main article: Usability

Usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.

Usability is attached to all tools used by humans and is extended to both digital and non-digital devices. Thus, it is a subset of user experience but not wholly contained. The section of usability that intersects with user experience design is related to humans' ability to use a system or application. Good usability is essential to positive user experience but does not alone guarantee it.

Accessibility

Main article: Accessibility

Accessibility of a system describes its ease of reach, use, and understanding. In terms of user experience design, it can also be related to the overall comprehensibility of the information and features. It helps shorten the learning curve associated with the system. Accessibility in many contexts can be related to the ease of use for people with disabilities and comes under usability. In addition, accessible design is the concept of services, products, or facilities in which designers should accommodate and consider for the needs of people with disabilities. According to the Web Content Accessibility Guidelines (WCAG), all content must follow by the four main principles of POUR: Perceivable, Operable, Understandable and Robust.

WCAG compliance

Main article: Web Content Accessibility Guidelines

Web Content Accessibility Guidelines (WCAG) 2.0 covers a wide range of recommendations for making Web content more accessible. This makes web content more usable to users in general. Making content more usable and readily accessible to all types of users enhances a user's overall user experience.

Human–computer interaction

Main article: Human–computer interaction

Human–computer interaction is concerned with the design, evaluation and implementation of interactive computing systems for human use and with the study of major phenomena surrounding them.

Getting ready to design

After research, the designer uses the modeling of the users and their environments. User modeling or personas are composite archetypes based on behavior patterns uncovered during research. Personas provide designers a precise way of thinking and communicating about how groups of users behave, how they think, what they want to accomplish and why. Once created, personas help the designer to understand the users' goals in specific contexts, which is particularly useful during ideation and for validating design concepts. Other types of models include workflow models, artifact models, and physical models.

Design

When the designer has a solid understand of the user's needs and goals, they begin to sketch out the interaction framework (also known as wireframes). This stage defines the high-level structure of screen layouts, as well as the product's flow, behavior, and organization. There are many kinds of materials that can be involved during this iterative phase, from whiteboards to paper prototypes. As the interaction framework establishes an overall structure for product behavior, a parallel process focused on the visual and industrial designs. The visual design framework defines the experience attributes, visual language, and the visual style.

Once a solid and stable framework is established, wireframes are translated from sketched storyboards to full-resolution screens that depict the user interface at the pixel level. At this point, it is critical for the programming team to collaborate closely with the designer. Their input is necessary to create a finished design that can and will be built while remaining true to the concept.

Test and iterate

Usability testing is carried out by giving users various tasks to perform on the prototypes. Any issues or problems faced by the users are collected as field notes and these notes are used to make changes in the design and reiterate the testing phase. Aside from monitoring issues, questions asked by users are also noted in order to identify potential points of confusion. Usability testing is, at its core, a means to "evaluate, not create".

UX deliverables

UX designers perform a number of different tasks and therefore use a range of deliverables to communicate their design ideas and research findings to stakeholders. Regarding UX specification documents, these requirements depend on the client or the organization involved in designing a product. The four major deliverables are: a title page, an introduction to the feature, wireframes, and a version history. Depending on the type of project, the specification documents can also include flow models, cultural models, personas, user stories, scenarios and any prior user research.

The deliverables that UX designers will produce as part of their job include wireframes, prototypes, user flow diagrams, specification and tech docs, websites and applications, mockups, presentations, personas, user profiles, videos, and to a lesser degree reports. Documenting design decisions, in the form of annotated wireframes, gives the developer the necessary information they may need to successfully code the project.

Follow-up to project launch

Requires:

• User testing/usability testing
• A/B testing
• Information architecture
• Sitemaps and user flows
• Additional wireframing as a result of test results and fine-tuning

UX stakeholders

A user experience designer is considered a UX practitioner, along with the following job titles: user experience researcher, information architect, interaction designer, human factors engineer, business analyst, consultant, creative director, interaction architect, and usability specialist.

Interaction designers

Interaction designers (IxD) are responsible for understanding and specifying how the product should behave. This work overlaps with the work of both visual and industrial designers in a couple of important ways. When designing physical products, interaction designers must work with industrial designers early on to specify the requirements for physical inputs and to understand the behavioral impacts of the mechanisms behind them. Interaction designers cross paths with visual designers throughout the project. Visual designers guide the discussions of the brand and emotive aspects of the experience, Interaction designers communicate the priority of information, flow, and functionality in the interface.

Technical communicators

Historically, technical and professional communication (TPC) has been as an industry that practices writing and communication. However, recently UX design has become more prominent in TPC as companies look to develop content for a wide range of audiences and experiences. It is now an expectation that technical and professional skills should be coupled with UX design. According to Verhulsdonck, Howard, and Tham, "...it is not enough to write good content. According to industry expectations, next to writing good content, it is now also crucial to design good experiences around that content." Technical communicators must now consider different platforms such as social media and apps, as well as different channels like web and mobile. In a similar manner, coupling TPC with UX design allows technical communicators to garner evidence on target audiences. UX Writers, a branch of technical communicators, specialize in crafting content for mobile platforms while executing a user-centered approach.

User interface designers

User interface (UI) design is the process of making interfaces in software or computerized devices with a focus on looks or style. Designers aim to create designs users will find easy to use and pleasurable. UI design typically refers to graphical user interfaces but also includes others, such as voice-controlled ones.

Visual designers

The visual designer ensures that the visual representation of the design effectively communicates the data and hints at the expected behavior of the product. At the same time, the visual designer is responsible for conveying the brand ideals in the product and for creating a positive first impression; this responsibility is shared with the industrial designer if the product involves hardware. In essence, a visual designer must aim for maximum usability combined with maximum desirability. Visual designer need not be good in artistic skills but must deliver the theme in a desirable manner.

Testing the design

Main article: Usability testing

Usability testing is the most common method used by designers to test their designs. The basic idea behind conducting a usability test is to check whether the design of a product or brand works well with the target users. While carrying out usability testing, two things are being tested: whether the design of the product is successful and, if not, how it can be improved.

While designers are testing, they are testing the design and not the user. Further, every design is evolving, with both UX design and design thinking moving in the direction of Agile software development. The designers carry out usability testing as early and often as possible, ensuring that every aspect of the final product has been tested.

Moon rock

From Wikipedia, the free encyclopedia

Olivine basalt collected from the rim of Hadley Rille by the crew of Apollo 15

Moon rock or lunar rock is rock originating from Earth's Moon. This includes lunar material collected during the course of human exploration of the Moon, and rock that has been ejected naturally from the Moon's surface and landed on Earth as meteorites.

Sources

Moon rocks on Earth come from four sources: those collected by six United States Apollo program crewed lunar landings from 1969 to 1972; those collected by three Soviet uncrewed Luna probes in the 1970s; those collected by the Chinese Lunar Exploration Program's uncrewed probes; and rocks that were ejected naturally from the lunar surface before falling to Earth as lunar meteorites.

Apollo program

Six Apollo missions collected 2,200 samples of material weighing 381 kilograms (840 lb), processed into more than 110,000 individually cataloged samples.

Mission	Site	Sample mass returned	Year
Apollo 11	Mare Tranquillitatis	21.55 kg (47.51 lb)	1969
Apollo 12	Ocean of Storms	34.30 kg (75.62 lb)	1969
Apollo 14	Fra Mauro formation	42.80 kg (94.35 lb)	1971
Apollo 15	Hadley-Apennine	76.70 kg (169.10 lb)	1971
Apollo 16	Descartes Highlands	95.20 kg (209.89 lb)	1972
Apollo 17	Taurus-Littrow	110.40 kg (243.40 lb)	1972

Luna program

Three Luna spacecraft returned with 301 grams (10.6 oz) of samples.

Mission	Site	Sample mass returned	Year
Luna 16	Mare Fecunditatis	101 g (3.6 oz)	1970
Luna 20	Mare Fecunditatis	30 g (1.1 oz)	1972
Luna 24	Mare Crisium	170 g (6.0 oz)	1976

The Soviet Union abandoned its attempts at a crewed lunar program in the 1970s, but succeeded in landing three robotic Luna spacecraft with the capability to collect and return small samples to Earth. A combined total of less than half a kilogram of material was returned.

In 1993, three small rock fragments from Luna 16, weighing 200 mg, were sold for US$ 442,500 at Sotheby's (equivalent to $896,417 in 2022). In 2018, the same three Luna 16 rock fragments sold for US$ 855,000 at Sotheby's.

Chang'e

See also: Changesite–(Y)

Chang'e 5, the fifth lunar exploration mission of the Chinese Lunar Exploration Program, returned with ~1,731 g (61.1 oz) of samples.

Mission	Site	Sample mass returned	Year
Chang'e 5	Mons Rümker	1,731 g (3.8 lb)	2020

Lunar meteorites

More than 370 lunar meteorites have been collected on Earth, representing more than 30 different meteorite finds (no falls), with a total mass of over 1,090 kilograms (2,400 lb). Some were discovered by scientific teams (such as ANSMET) searching for meteorites in Antarctica, with most of the remainder discovered by collectors in the desert regions of northern Africa and Oman. A Moon rock known as "NWA 12691", which weighs 13.5 kilograms (30 lb), was found in the Sahara Desert at the Algerian and Mauritanian borders in January 2017, and later went on sale for $2.5 million in 2020.

Dating

Rocks from the Moon have been measured by radiometric dating techniques. They range in age from about 3.16 billion years old for the basaltic samples derived from the lunar maria, up to about 4.44 billion years old for rocks derived from the highlands. Based on the age-dating technique of "crater counting," the youngest basaltic eruptions are believed to have occurred about 1.2 billion years ago, but scientists do not possess samples of these lavas. In contrast, the oldest ages of rocks from the Earth are between 3.8 and 4.28 billion years.

Composition

Main article: Geology of the Moon

 

Common lunar minerals

Mineral	Elements	Lunar rock appearance
Plagioclase feldspar	Calcium (Ca) Aluminium (Al) Silicon (Si) Oxygen (O)	White to transparent gray; usually as elongated grains.
Pyroxene	Iron (Fe), Magnesium (Mg) Calcium (Ca) Silicon (Si) Oxygen (O)	Maroon to black; the grains appear more elongated in the maria and more square in the highlands.
Olivine	Iron (Fe) Magnesium (Mg) Silicon (Si) Oxygen (O)	Greenish color; generally, it appears in a rounded shape.
Ilmenite	Iron (Fe), Titanium (Ti) Oxygen (O)	Black, elongated square crystals.

Moon rocks fall into two main categories: those found in the lunar highlands (terrae), and those in the maria. The terrae consist dominantly of mafic plutonic rocks. Regolith breccias with similar protoliths are also common. Mare basalts come in three distinct series in direct relation to their titanium content: high-Ti basalts, low-Ti basalts, and Very Low-Ti (VLT) basalts.

Almost all lunar rocks are depleted in volatiles and are completely lacking in hydrated minerals common in Earth rocks. In some regards, lunar rocks are closely related to Earth's rocks in their isotopic composition of the element oxygen. The Apollo Moon rocks were collected using a variety of tools, including hammers, rakes, scoops, tongs, and core tubes. Most were photographed prior to collection to record the condition in which they were found. They were placed inside sample bags and then a Special Environmental Sample Container for return to the Earth to protect them from contamination. In contrast to the Earth, large portions of the lunar crust appear to be composed of rocks with high concentrations of the mineral anorthite. The mare basalts have relatively high iron values. Furthermore, some of the mare basalts have very high levels of titanium (in the form of ilmenite).

Highlands rocks

Processing facility in Lunar Sample Building at JSC

Slice of Moon rock at the National Air and Space Museum in Washington, DC

Mineral composition of Highland rocks

	Plagioclase	Pyroxene	Olivine	Ilmenite
Anorthosite	90%	5%	5%	0%
Norite	60%	35%	5%	0%
Troctolite	60%	5%	35%	0%

Primary igneous rocks in the lunar highlands compose three distinct groups: the ferroan anorthosite suite, the magnesian suite, and the alkali suite.

Lunar breccias, formed largely by the immense basin-forming impacts, are dominantly composed of highland lithologies because most mare basalts post-date basin formation (and largely fill these impact basins).

• The ferroan anorthosite suite consists almost exclusively of the rock anorthosite (>90% calcic plagioclase) with less common anorthositic gabbro (70-80% calcic plagioclase, with minor pyroxene). The ferroan anorthosite suite is the most common group in the highlands, and is inferred to represent plagioclase flotation cumulates of the lunar magma ocean, with interstitial mafic phases formed from trapped interstitial melt or rafted upwards with the more abundant plagioclase framework. The plagioclase is extremely calcic by terrestrial standards, with molar anorthite contents of 94–96% (An94–96). This reflects the extreme depletion of the bulk Moon in alkalis (Na, K) as well as water and other volatile elements. In contrast, the mafic minerals in this suite have low Mg/Fe ratios that are inconsistent with calcic plagioclase compositions. Ferroan anorthosites have been dated using the internal isochron method at circa 4.4 Ga.
• The magnesian suite (or "Mg-suite") consists of dunites (>90% olivine), troctolites (olivine-plagioclase), and gabbros (plagioclase-pyroxene) with relatively high Mg/Fe ratios in the mafic minerals and a range of plagioclase compositions that are still generally calcic (An86–93). These rocks represent later intrusions into the highlands crust (ferroan anorthosite) at round 4.3–4.1 Ga. An interesting aspect of this suite is that analysis of the trace element content of plagioclase and pyroxene requires equilibrium with a KREEP-rich magma, despite the refractory major element contents.
• The alkali suite is so-called because of its high alkali content—for Moon rocks. The alkali suite consists of alkali anorthosites with relatively sodic plagioclase (An70–85), norites (plagioclase-orthopyroxene), and gabbronorites (plagioclase-clinopyroxene-orthopyroxene) with similar plagioclase compositions and mafic minerals more iron-rich than the magnesian suite. The trace element content of these minerals also indicates a KREEP-rich parent magma. The alkali suite spans an age range similar to the magnesian suite.
• Lunar granites are relatively rare rocks that include diorites, monzodiorites, and granophyres. They consist of quartz, plagioclase, orthoclase or alkali feldspar, rare mafics (pyroxene), and rare zircon. The alkali feldspar may have unusual compositions unlike any terrestrial feldspar, and they are often Ba-rich. These rocks apparently form by the extreme fractional crystallization of magnesian suite or alkali suite magmas, although liquid immiscibility may also play a role. U-Pb date of zircons from these rocks and from lunar soils have ages of 4.1–4.4 Ga, more or less the same as the magnesian suite and alkali suite rocks. In the 1960s, NASA researcher John A. O'Keefe and others linked lunar granites with tektites found on Earth although many researchers refuted these claims. According to one study, a portion of lunar sample 12013 has a chemistry that closely resembles javanite tektites found on Earth.
• Lunar breccias range from glassy vitrophyre melt rocks, to glass-rich breccia, to regolith breccias. The vitrophyres are dominantly glassy rocks that represent impact melt sheets that fill large impact structures. They contain few clasts of the target lithology, which is largely melted by the impact. Glassy breccias form from impact melt that exit the crater and entrain large volumes of crushed (but not melted) ejecta. It may contain abundant clasts that reflect the range of lithologies in the target region, sitting in a matrix of mineral fragments plus glass that welds it all together. Some of the clasts in these breccias are pieces of older breccias, documenting a repeated history of impact brecciation, cooling, and impact. Regolith breccias resemble the glassy breccias but have little or no glass (melt) to weld them together. As noted above, the basin-forming impacts responsible for these breccias pre-date almost all mare basalt volcanism, so clasts of mare basalt are very rare. When found, these clasts represent the earliest phase of mare basalt volcanism preserved.

Mare basalts

Mineral composition of mare basalts

	Plagioclase	Pyroxene	Olivine	Ilmenite
High titanium content	30%	54%	3%	18%
Low titanium content	30%	60%	5%	5%
Very low titanium content	35%	55%	8%	2%

Mare basalts are named as such because they frequently constitute large portions of the lunar maria. These typically contain 18–21 percent FeO by weight, and 1–13 percent TiO₂. They are similar to terrestrial basalts, but have many important differences; for example, mare basalts show a large negative europium anomaly. The type location is Mare Crisium sampled by Luna 24.

• KREEP Basalts (and borderline VHK (Very High K) basalts) have extraordinary potassium content. These contain 13–16 percent Al₂O₃, 9–15 percent FeO, and are enriched in magnesium and incompatible elements (potassium, phosphorus and rare earth elements) 100–150 times compared to ordinary chondrite meteorites. These are commonly encountered around the Oceanus Procellarum, and are identified in remote sensing by their high (about 10 ppm) thorium contents. Most of incompatible elements in KREEP basalts are incorporated in the grains of the phosphate minerals apatite and merrillite.

Curation and availability

Genesis Rock returned by the Apollo 15 mission.

The main repository for the Apollo Moon rocks is the Lunar Sample Laboratory Facility at the Lyndon B. Johnson Space Center in Houston, Texas. For safekeeping, there is also a smaller collection stored at White Sands Test Facility in Las Cruces, New Mexico. Most of the rocks are stored in nitrogen to keep them free of moisture. They are handled only indirectly, using special tools.

Some Moon rocks from the Apollo missions are displayed in museums, and a few allow visitors to touch them. One of these, called the Touch Rock, is displayed in the Smithsonian National Air and Space Museum in Washington, D.C. The idea of having touchable Moon rocks at a museum was suggested by Apollo scientist Farouk El-Baz, who was inspired by his childhood pilgrimage to Mecca where he touched the Black Stone (which in Islam is believed to be sent down from the heavens).

Moon rocks collected during the course of lunar exploration are currently considered priceless. In 2002, a safe was stolen from the Lunar Sample Building that contained minute samples of lunar and Martian material. The samples were recovered, and NASA estimated their value during the ensuing court case at about $1 million for 10 oz (280 g) of material.

Naturally transported Moon rocks in the form of lunar meteorites are sold and traded among private collectors.

Goodwill Moon rocks

Main article: Lunar sample displays

Honduras plaque

Apollo 17 astronauts Eugene Cernan and Harrison Schmitt picked up a rock "composed of many fragments, of many sizes, and many shapes, probably from all parts of the Moon". This rock was later labeled sample 70017. President Nixon ordered that fragments of that rock should be distributed in 1973 to all 50 US states and 135 foreign heads of state. The fragments were presented encased in an acrylic sphere, mounted on a wood plaque which included the recipients' flag which had also flown aboard Apollo 17. Many of the presentation Moon rocks are now unaccounted for, having been stolen or lost.

Discoveries

Three minerals were discovered from the Moon: armalcolite, tranquillityite, and pyroxferroite. Armalcolite was named for the three astronauts on the Apollo 11 mission: Armstrong, Aldrin and Collins.

Stolen and missing Moon rocks

Main article: Stolen and missing Moon rocks

Because of their rarity on Earth, and the difficulty of obtaining more, Moon rocks have been frequent targets of theft and vandalism, and many have gone missing or were stolen.