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The Space Interferometry Mission, also known as SIM Lite (formerly known as SIM PlanetQuest), is a planned space telescope being developed by the U.S. National Aeronautics and Space Administrationmarker (NASA), in conjunction with contractor Northrop Grumman. One of the main goals of the mission is the hunt for Earth-sized planets orbiting stars other than the Sun. The project is being managed by NASA's Jet Propulsion Laboratorymarker (JPL) in Pasadena, Californiamarker. The initial contracts for SIM Lite were awarded in 1998, totaling US$200 million. Work on the SIM project required scientists and engineers to move through eight specific new technology milestones, and by November 2006, all eight had been completed.

SIM Lite was originally scheduled for a 2005 launch, aboard an Evolved Expendable Launch Vehicle (EELV). As a result of continued budget cuts, the launch date has been pushed back at least five times. NASA has set a preliminary launch date for 2015 and U.S. federal budget documents confirm that a launch date is expected "no earlier" than 2015. The budget cuts to SIM Lite are expected to continue through FY 2010. As of February 2007, many of the engineers working on the SIM program had moved on to other areas and projects, and NASA directed the project to allocate its resources toward engineering risk reduction. However, the preliminary budget for NASA for 2008 included zero money for SIM.

In December 2007 the Congress restored funding for fiscal year 2008 as part of an omnibus appropriations bill which the President later signed. At the same time the Congress directed NASA to move the mission forward to the development phase.

The spacecraft will use optical interferometry to accomplish several scientific goals. The technique of interferometry will collect light through multiple telescope mirrors, in this case two, as if they were one large mirror. In addition to hunting for extrasolar planets, SIM will attempt to help astronomers construct a map of the Milky Way Galaxy. Other important tasks will include collecting data to help pinpoint stellar mass for specific types of stars and assisting in the determination of the spatial distribution of dark matter in the universe. The last task will be accomplished by measuring galactic motion.

Development

Beginnings

This chart depicts the potential number of habitable planets that SIM PlanetQuest might detect.
The Space Interferometry Mission began as a four-month preliminary architecture study in March 1997. NASA selected TRW's Space & Electronics Group, Eastman Kodak and Hughes Danbury Optical Systems to conduct the study. In 1998, TRW Inc. was selected as the contractor for the SIM Lite project; Northrup Grumman acquired part of TRW in 2002 and took over the contract. Also selected was Lockheed Martin Missiles and Space located in Sunnyvale, Californiamarker. The two contracts, which included the mission formulation and implementation phases, were announced in September 1998 and worth a total of over US$200 million. The formulation phase of the mission included initial mission design and planning for the full scale implementation of the mission. At the time of the NASA announcement, launch was scheduled for 2005 and the mission was part of the Origins Program, a series of missions designed to answer questions such as how and why humans are on Earth.

In August 2000, NASA asked project managers to consider looking at the Space Shuttle, instead of the previously proposed EELV, as a launch vehicle. In late November 2000, NASA announced that the project's scientific team was selected. The group included notable names from the world of extrasolar planet research including Geoffrey Marcy. The entire group consisted of 10 principal investigators and five mission specialists. At the time of this NASA announcement launch was scheduled for 2009 and the mission was still part of the Origins Program.

New technologies

SIM's new technology is meant to lead to the development of telescopes powerful enough to take images of Earth-like extrasolar planets orbiting distant stars and to determine whether those planets are able to sustain life. NASA has already started developing future missions, such as the Terrestrial Planet Finder, that will build on SIM's technological legacy. The technological development phase of the mission was completed in November 2006 with the announcement that the eight, mission-technology-milestones set by NASA were reached. The milestones were necessary steps in the technological development before flight control instruments could begin to be designed. The completion of each milestone meant that new systems had to be developed for nanometer control as well as picometer knowledge technology; these systems enable the telescope to make its accurate measurements.
Engineers at JPL examine components on an optical bench that simulates the precision performance of NASA's future SIM PlanetQuest mission.
One of the new technologies developed for the mission were high-tech "rulers", capable of making measurements in increments a fraction of the width of a hydrogen atom, in addition, the rulers were developed to work as a network. The mission team also created "shock absorbers" to alleviate the effects of tiny vibrations in the spacecraft which would impede accurate measurements. Another one of the milestones involved combining the new "rulers" and "shock absorbers" to prove that the Space Interferometry Mission craft could detect the tiny wobbles in stars caused by Earth-sized planets. The fifth of the technology milestones required the demonstration of the Microarcsecond Metrology Testbed at a performance of 3,200 picometers over its wide angle field of regard. The wide angle field will be used to determine the fixed positions of stars. This level of performance demonstrated SIM PlanetQuest's ability to calculate the astrometric grid. Another key development was the ability to apply the measurement capability worked out in the wide angle milestone a step further, into narrow-angle measurements. The narrow angle field will be used by SIM to detect terrestrial planets; the team applied the same criteria to both the narrow and wide angle measurements. The final requirement before beginning work on flight controls was to make sure that all of the systems developed for the mission worked cohesively; this final NASA technology goal was completed last as it was dependent upon the others.

Status after 2006

Between the end of April and June 2006 the project completed three engineering milestones and from November 2–November 8, 2006 SIM PlanetQuest completed a "Spacecraft Internal Design Review." As of June 2008, all of the eight engineering milestones have been successfully completed.

As of January 31, 2007 the project's status was the Jet Propulsion Laboratory's "Phase B", or the "Preliminary Design" phase. According to the Laboratory, Phase B is officially the "Definition" phase of the project. "The definition phase converts the preliminary plan into a baseline technical solution. Requirements are defined, schedules are determined, and specifications are prepared to initiate system design and development." In addition, as part of Phase B, the SIM Lite project will go through a number of reviews by NASA including System Requirements Review, System Design Review, and Non-Advocate Review. During this phase, experiments will be proposed, peer reviewed, and eventually selected by NASA's Office of Space Science. Experiment selections are based on scientific value, cost, management, engineering, and safety. The project has been in Phase B since June 2003.

Launch

The launch date for the SIM Lite mission has been pushed back at least five times. At the program's outset, in 1998, the launch was scheduled for 2005. By 2000, the launch date had been delayed until 2009, a date that held through 2003; though some project scientists cited 2008 in late 2000. Between 2004 and 2006, contractor Northrop Grumman, the company designing and developing SIM, listed a launch date of 2011 on their website. With the release of the FY 2007 NASA budget, predictions changed again, this time to a date no earlier than 2015 or 2016. The delay of the launch date is primarily related to budget cuts within the SIM Lite program. The 2007 change represented a difference of about three years from the 2006 launch date, outlined in NASA's FY 2006 budget as being two years behind 2005 budget predictions.Other groups predict dates matching officially predicted launch dates; the Michelson Science Center at the California Institute of Technologymarker also sets the date at 2015. As of June 2008, NASA has postponed the launch date "indefinitely".

The launch date of the SIM mission cannot be predicted with any certainty. A May 2005 NASA operating plan put the mission into a replanning phase through the spring of 2006. No definitive mission schedule has been published on the SIM Lite website, maintained by the Jet Propulsion Laboratorymarker (JPL), as of April 2007; aside from the estimated launch date of 2015. When the launch does occur it is planned to be via an Evolved Expendable Launch Vehicle (EELV), likely an Atlas V 551 or Delta IV Heavy.

Budget

SIM Lite is considered the flagship mission of NASA's Exoplanet Exploration Program (formerly known as the Navigator Program). According to the 2007 Presidential Budget for NASAmarker, the program is, "a coherent series of increasingly challenging projects, each complementary to the others and each mission building on the results and capabilities of those that preceded it as NASA searches for habitable planets outside of the solar system." The program, in addition to the Space Interferometry Mission, includes the Keck Interferometermarker and the Large Binocular Telescopemarker Interferometer. When originally approved in 1996, the mission was given a $700 million cap (in 1996 dollars) which included launch costs and five years of operation. The first contracts, for the preliminary architecture study, were worth $200,000 each.
NASA's budget outlined plans for the three projects for fiscal year (FY) 2007. Of the three missions, SIM Lite was delayed further and the Keck Interferometer saw budget cuts. The 2007 NASA budget stipulated, "SIM Phase B activity will continue while new cost and schedule plans are developed, consistent with recent funding decisions." The funding decisions included a US$118.5 million cut over the FY 2006 NASA budget request for the Exoplanet Exploration Program. The budget also laid out projections for the program through the year 2010. Each year will have successive funding cuts, if compared to the 2006 request numbers. Starting with FY 2008, the Exoplanet Exploration Program will receive around $223.9 million less compared to 2006. The following years will have cuts of $155.2 million in 2009 and $172.5 million in 2010, compared to the 2006 request.

When SIM Lite entered what JPL terms "Phase B", or the definition phase of the project, in 2003 Fringes: Space Interferometry Mission Newsletter, called it a most important milestone on the way to a 2009 launch. The delays are budgetary in nature. In 2006, the mission received $117 million, an increase of $8.1 million over the previous year, but 2007 cuts amounted to $47.9 million less for the SIM program. In 2008, $128.7 million of the $223.9 million estimated to be cut from the Exoplanet Program budget will come from the SIM Lite mission. After an additional $51.9 million less in FY 2009, the program should be receiving a significant amount more funding by 2010 if predictions hold true.

By February 2007 many of the budget cuts outlined in the FY 2007 budget were already being felt within the project. Engineers who worked on SIM were forced to find other areas to work in. A February 2007 editorial in the Space Interferometry Mission Newsletter described the situation as, "entirely due to budget pressures and priorities within the Science Mission Directorate at NASA (with) scientific motivation for the mission is as strong as ever." NASA, per the budget cuts, directed the SIM project to refocus its efforts toward engineering risk reduction. As of the February 2007 newsletter the plans for the refocus were in the process of being completed.

Instruments

Optical interferometry

How the Astrometric Interferometer works
Interferometry is a technique pioneered by Albert Michelson in the 19th century. Optical interferometry, which has matured within the last two decades, combines the light of multiple telescopes so they act as a single, much larger telescope. It is the interaction of light waves, called interference, that makes this possible. Interference can be used to cancel out the glare of bright stars or to measure distances and angles accurately. The construction of the word partially illustrates this: interfere + measure = interfer-o-metry. At radio wavelengths of the electromagnetic spectrum, interferometry has been used for more than 50 years to measure the structure of distant galaxies.

The Space Interferometry Mission is being managed by the Jet Propulsion Laboratorymarker and developed and designed in partnership with their principal contractor for the project, Northrop Grumman based in Redondo Beach, Californiamarker. The SIM Lite telescope functions through optical interferometry. Interferometry combines light from two or more telescopes as if they were pieces of a single, large telescope mirror. SIM is composed of one science interferometer and two guide interferometers. The combined collecting area of each of the three instruments is 325 cm².

The SIM Lite design includes the interferometers, the science interferometer has a 6 m baseline and the two guide interferometers have 7.2 m baselines. The spacecraft's operational limiting magnitude goes down to 20 at 20 millionths of an arcsecond (μas) and its planet-finding, astrometric accuracy of 1.12 µas is for single measurements. The accuracy of its global astrometric grid is 3.47 µas. As of 2000, SIM is to consist of two telescopes mounted on opposite ends of a ten-meter boom. The instruments will be able to measure the small wobbles in stars and detect planets of four Earth masses up to 33 light years away. SIM Lite will be able to detect Earth-sized planets orbiting the stars that are closest to the Sun.

Mission

A 2006 artist's impression of SIM PlanetQuest
SIM Lite will operate in an Earth-trailing heliocentric orbit. The SIM will drift away from Earth at the rate of 0.1 AU per year until it reaches a distance of 95 million km. This will take approximately 5½ years. The Sun will continuously shine on the spacecraft, allowing it to avoid the occultations that would occur in an Earth orbit. After achieving orbit, the spacecraft's solar array and high gain antenna will be fully deployed. Systems will be checked and tracking data collected in order to determine the orbit achieved. The Space Interferometry Mission will then take several days to disperse contaminants and, eventually, it will deploy all final instruments. The calibration of the interferometer will take several months. It is after the calibration that the mission truly begins.

For the ensuing five years SIM Lite will perform almost continuous scientific research. The spacecraft will be pointed using its reaction wheel and small thrusters. The SIM's nominal viewing axis can never be within 45 degrees of the Sun or the viewing optics could suffer degradation due to overheating. Because of this danger, pointing the telescope must be precisely performed. In addition, the velocity of the spacecraft must be exactly calculated, to within 20 mm/sec or better, in order to correct for relativistic stellar aberration. Deep Space Network (DSN) ground stations plan to provide doppler data to ensure that SIM PlanetQuest's velocity is accurately calculated. Data obtained by SIM will be stored onboard and returned to Earth several times each week. Controllers will use special "quick turnaround" procedures to use the SIM telescope on any "targets of opportunity that may arise".

Planet hunting

SIM Planetquest, when completed, will be the most powerful extrasolar planet hunting space telescope ever built. Through the technique of interferometry the spacecraft will be able to detect Earth-sized planets. SIM Lite will perform its search for nearby, Earth-like planets by looking for the "wobble" in the parent star's apparent motion as the planet orbits. The spacecraft will accomplish this task to an accuracy of one millionth of an arcsecond, or the thickness of a nickel viewed at the distance from Earth to the Moon. Titled the Deep Search, the planet hunting program is intended to search for terrestrial planets (such as Earth or Mars) around approximately 250 nearby stars. The Deep Search is the most demanding in terms of astrometric accuracy, hence the name, Deep Search. This program will use the full capability of the SIM Lite spacecraft to make its measurements.

Aside from searching for Earth-sized planets SIM Lite will perform what has been dubbed the "Broad Survey". The Broad Survey will look at around 2,000 stars to help determine the abundance of Neptune-mass and larger planets around all star-types in Earth's sector of the Milky Way. The Broad Survey will aim for quantity over quality; this lower accuracy program, when compared to the Deep Search, will survey a much larger 2,000 star selection.
SIM Lite will be able to detect Earth-sized planets, such as in this artist's rendering.
A third part of the planet finding mission will include the search for Jupiter-mass planets around young stars. The survey will help scientists understand more about solar system formation, including the occurrence of hot Jupiters. It is through the techniques pioneered by Albert Michelson that the SIM will be able to execute its three primary planet-finding missions. This portion of the planet hunt is designed to study systems with one or more Jupiter mass planets before the system has reached a long term equilibrium.

The mission's planet finding component will serve as an important complement to the future Terrestrial Planet Finder (TPF) mission. SIM Lite will perform important tasks that the TPF will not be capable of, such as determining planet mass. Other tasks that the SIM will perform for the TPF mission will include providing optimal times for TPF to observe stars where SIM detected terrestrial planets, as well as the orbital characteristics of the planets. In September 2006 a scientific study determined estimates for discovery results to expect from the SIM. A team from NASA's Jet Propulsion Laboratory (JPL) determined that SIM Lite could expect to find planets smaller than Earth around six stars, planets smaller than twice Earth's mass around 24 stars and planets smaller than three times Earth's mass around all 120 stars in their survey group. Three of the stars in the study's group of six stars likely to contain smaller than Earth mass planets include familiar points in the night sky: Sirius, Altair and Alpha Centauri.

Stellar mass

White dwarfs, imaged by NASA's Hubble Space Telescope
Another key aspect of SIM PlanetQuest's future mission is figuring out the upper and lower limits for a star's mass. Today, scientists understand that there are limits to the how small or large a star can be. Objects that are too small lack the internal pressure to initiate thermonuclear fusion, which is what causes a star to shine. These objects are known as brown dwarfs and represent the lower end of the stellar mass scale. Stars that are too large become unstable and explode in a supernova.

Part of the SIM's mission is to provide pinpoint measurements for the two extremes in stellar mass and evolution. The telescope will not be able to measure the mass of every star in the Galaxy, since there are over 200 billion, but instead, it will take a "population census." Through this technique, SIM will be able to output accurate masses for representative examples for nearly every star type, including brown dwarfs, hot white dwarfs, red giant star, and elusive black holes. Current space telescopes, including NASA's Hubble Space Telescope, can accurately measure mass for some types of stars, but not all. Estimates put the range for stellar mass somewhere between 8% the mass of the Sun and 60 times the mass of the Sun. The entire study will be focused on binary star systems, stars coupled through a mutual gravitational attraction.

Galactic mapping

How scientists think the Milky Way is shaped
The mission will use optical interferometry, which combines light from two or more telescopes as if they were one, to precisely measure the distances between stars throughout the Milky Way. This will allow astronomers to create a "roadmap" of the Galaxy which will answer many questions about its shape and size.

A team led by the University of Virginiamarker's Steve Majewski, a principal investigator on the SIM project, plans to use SIM Lite to make the ultra precise measurements necessary to determine the shape and size of the Milky Way. These measurements should reveal more about the nature of the Galaxy than ever before, and, eventually, build the foundation for a map of the Milky Way. Currently, astronomers know little about the shape and size of our galaxy relative to what they know about other galaxies; it is difficult to observe the entire Milky Way from the inside. A good analogy is trying to observe a marching band as a member of the band. Observing other galaxies is much easier because humans are outside of those galaxies. Majewski and his team plan to use SIM Lite to help determine not only the shape and size of the Galaxy but also the distribution of its mass and the motion of its stars.

Majewski's team has established four topics for their "Key Project", "Taking Measure of the Milky Way", during the SIM Lite Mission. The four topics include, fundamental Galactic parameters, the Oort Limit, disk mass potential, and mass of the Galaxy to large radii. The first, fundamental Galactic parameters, is aimed at answering key questions about size, shape and the rotation rate of the Milky Way. The team hopes to more accurately determine the distance from the Sun to the Galactic center. The second topic, the Oort Limit, will attempt to determine the mass of the Galactic disk. The third Key Project topic is disk mass potential. This topic is designed to make measurements of the distances to disk stars as well as their proper motions. The results of the third topic of study will be combined with the results of the fundamental Galactic parameters portion of the study to determine the Sol system's position and velocity in the Galaxy. The final topic for the University of Virginia team deals with dark matter distribution in the Milky Way. The team will be able to create a three-dimensional model of mass distribution in the galaxy, out to a radius of 270 kiloparsecs (kps). They will then use two different tests to determine the Galactic potential at large radii.

Dark matter

The gray portion of this pie graph shows the estimated distribution of dark matter in the universe.
The spatial distribution of dark matter in the universe is largely unknown, SIM PlanetQuest will help scientists determine an answer to this question through another integral part of its mission. Dark matter is the matter in the universe that cannot be seen. Because of the gravitational effect it exerts on stars and galaxies, scientists know that approximately 80% of the matter in the universe is dark matter. The strongest evidence for dark matter comes from galactic motion. Galaxies rotate much faster than the amount of visible matter suggests they should; the gravity from the ordinary matter is not enough to hold the galaxy together. Scientists theorize that the galaxy is held together by huge quantities of dark matter.

SIM PlanetQuest will measure galactic motion in the neighboring galaxies near the Milky Way. Many of these measurements will be the first of their kind. The telescope's measurements will be used in conjunction with other, currently available, data to provide astronomers with the first total mass measurements of individual galaxies. These numbers will enable scientists to estimate the spatial distribution of dark matter throughout the universe.

See also



Notes

  1. McHale, John. " JPL eyes super-accurate star mapper", (EbscoHost), Academic Search Premier, Military & Aerospace Electronics, March 1997, Vol. 8, Issue 3, p.1. Retrieved 26 April 2007.
  2. Platt, Jane. " Contractors chosen for Space Interferometry Mission", (Press Release), NASA, 10 September 1998, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  3. Unwin, Steve. " SIM Project Update", Fringes: Space Interferometry Mission Newsletter, Number 13, 24 August 2000. Retrieved 25 April 2007.
  4. Platt, Jane. " Science team chosen for Space Interferometry Mission", (Press Release), NASA, 28 November 2000, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  5. " Planet Finding Missions: The Big Picture", Missions, NASA, PlanetQuest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  6. " Star- and Planet-Mapping Telescope Mission Ready to Move Forward", (Press Release), Northrup Grumman, 14 November 2006. Retrieved 24 April 2007
  7. " The eight technology milestones", NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  8. " SIM Current Mission Status," NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 25 April 2007
  9. Doody, Dave and Stephan, George. " Chapter 7 - Mission Inception Overview", Basics of Spaceflight, NASA, 1993 and other editions, Jet Propulsion Laboratory. Retrieved 25 April 2007.
  10. Halverson, Peter G., et al. " Progress towards picometer accuracy laser metrology for the Space Interferometry Mission", (PDF), NASA, PlanetQuest, Jet Propulsion Laboratory; paper originally released 17 October 2000 and presented at the International Conference of Space Optics, ICSO 2000, December 5-7, 2000, Toulouse, France. Retrieved 25 April 2007.
  11. " Space Interferometry Mission (SIM)", Missions, Michelson Science Center, California Institute of Technology. Retrieved 24 April 2007.
  12. Project Schedule, NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 25 April 2007.
  13. " Taking Measure of the Universe", NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  14. FY 2007 NASA Budget, (PDF), NASA, pg. 80–81. Retrieved 2 November 2006.
  15. " NASA picks three teams to study space interferometry", (Lexis Nexis), Academic Universe, Aerospace Daily, 14 January 1997, Vol. 181, No. 9, pg. 62. Retrieved 26 April 2007.
  16. " FY 2007 NASA Budget", (PDF), NASA, p. 85. Retrieved 25 April 2007.
  17. " SIM enters Phase B!", Fringes: Space Interferometry Mission Newsletter, 20 June 2003. Retrieved 2 November 2006.
  18. FY 2006 NASA Budget, (PDF), NASA, p. 65. Retrieved 25 April 2007.
  19. FY 2007 NASA Budget, (PDF), NASA, pg. 83. Retrieved 2 November 2006.
  20. Unwin, Steve, ed. " Editorial", Fringes: Space Interferometry Mission Newsletter, No. 40, 27 February 2007. Retrieved 24 April 2007.
  21. " Albert Michelson: The pioneer of interferometry", NASA, PlanetQuest, Jet Propulsion Laboratory. Retrieved 25 April 2007.
  22. " A new window on the universe", NASA, PlanetQuest, Jet Propulsion Laboratory. Retrieved 25 April 2007.
  23. " SIM PlanetQuest", Northrop Grumman, official site, 2004-2006. Retrieved 24 April 2007.
  24. Edberg, Stephen J., et al.; eds. " SIM PlanetQuest White Paper", (PDF), NASA, Jet Propulsion Laboratory, 2 May 2005. Retrieved 25 April 2007.
  25. " A roadmap for planet-hunting", (EbscoHost), Academic Search Premier, Economist, 8 April 200, Vol. 355, Issue 8165, p. 87–89. Retrieved 26 April 2007.
  26. " The Mission", NASA, SIM Planetquest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  27. " Searching for new worlds", NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 25 April 2007.
  28. " Planet detection", NASA, SIM Planetquest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  29. Catanzarite, J.; Tanner, A.; Shao, M. " Space Interferometry Mission (SIM) PlanetQuest's Discovery Space and Potential Synergy with Terrestrial Planet Finder (TPF): I. Detection of Terrestrial Planets in the Habitable Zone", {Abstract), Smithsonian/NASA ADS: Astronomy Abstract Service, American Astronomical Society Meeting 206, #14.04; Bulletin of the American Astronomical Society, May 2005 Vol. 37, p.453. Retrieved 24 April 2007.
  30. Catanzarite, Joseph, Shao, Michael, Tanner, Angelle, Unwin, Stephen , and Yu, Jeffrey. " Astrometric Detection of Terrestrial Planets in the Habitable Zones of Nearby Stars with SIM PlanetQuest", Publications of the Astronomical Society of the Pacific, Vol. 118 September 2006, pages 1319–1339, University of Chicago Press. Retrieved 24 April 2007.
  31. " Planet Hunting by Numbers", (Press Release), NASA, Stars and Galaxies, Jet Propulsion Laboratory, 18 October 2006. Retrieved 24 April 2007.
  32. " How Big Are Stars?", SIM PlanetQuest, NASA, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  33. " Mapping the Milky Way", NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  34. Silberg, Bob. " Building a better guide to the Galaxy", NASA, SIM PlanetQuest, 14 February 2006, Jet Propulsion Laboratory. Retrieved 24 April 2007.
  35. " Main Page", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.
  36. " Fundamental Galactic Parameters", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.
  37. " Oort Limit", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.
  38. " Disk Mass Potential", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.
  39. " Mass of the Galaxy to Large Radii", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.
  40. " Finding Black Holes and Dark Matter", NASA, SIM PlanetQuest, Jet Propulsion Laboratory. Retrieved 24 April 2007.


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