The Space Interferometry Mission, or SIM, also known as SIM Lite (formerly known as SIM PlanetQuest), was a planned space telescope proposed by the U.S. National Aeronautics and Space Administration (NASA), in conjunction with contractor Northrop Grumman. One of the main goals of the mission was the hunt for Earth-sized planets orbiting in the habitable zones of nearby stars other than the Sun. SIM was postponed several times and finally cancelled in 2010. In addition to detecting extrasolar planets, SIM would have helped astronomers construct a map of the Milky Way galaxy. Other important tasks would have included collecting data to help pinpoint stellar masses for specific types of stars, assisting in the determination of the spatial distribution of dark matter in the Milky Way and in the local group of galaxies and using the gravitational microlensing effect to measure the mass of stars. The spacecraft would have used optical interferometry to accomplish these and other scientific goals.

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 proposed for a 2005 launch, aboard an Evolved Expendable Launch Vehicle (EELV). As a result of continued budget cuts, the launch date was pushed back at least five times. NASA had set a preliminary launch date for 2015. 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 dollars for SIM.

In 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. In 2009 the project continued its risk reduction work while waiting for the findings and recommendations of the Astronomy and Astrophysics Decadal Survey, Astro2010, performed by the National Academy of Sciences, which would determine the project's future.

In 2010, the Astro2010 Decadal Report was released and did not recommend that NASA continue the development of the SIM Lite Astrometric Observatory. This prompted NASA Astronomy and Physics Director, Jon Morse, to issue a letter on 24 September 2010 to the SIM Lite project manager, informing him that NASA was discontinuing its sponsorship of the SIM Lite mission and directing the project to discontinue Phase B activities immediately or as soon as practical. Accordingly, all SIM Lite activities were closed down by the end of calendar year 2010.

Mission

SIM Lite would have operated in an Earth-trailing heliocentric orbit, drifting away from Earth at the rate of 0.1 AU per year ultimately reaching a distance of 82 million km from Earth. This would have taken approximately  years. The Sun would have continuously shone on the spacecraft, allowing it to avoid the occultations of target stars and eclipses of the Sun that would occur in an Earth orbit."SIM PlanetQuest ", Northrop Grumman, official site, 2004–2006. Retrieved 24 April 2007."The Mission ", NASA, SIM Planetquest, Jet Propulsion Laboratory. Retrieved 24 April 2007.

Had it been launched, SIM would have performed scientific research for five years.

Planet hunting

SIM Lite would have been the most powerful extrasolar planet hunting space telescope ever built.Davidson, John et al., Ed., SIM Lite Astrometric Observatory , Executive Summary , p. ix et seq., Jet Propulsion Laboratory 400–1360, 2009. Retrieved 8 March 2010. Through the technique of interferometry the spacecraft would be able to detect Earth-sized planets. SIM Lite was to 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 would have accomplished 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 was intended to search approximately 60 nearby stars for terrestrial planets (like Earth and Venus) in the habitable zone (where liquid water can exist throughout a full revolution (one "year") of the planet around its star). The Deep Search was to be the most demanding in terms of astrometric accuracy, hence the name, Deep Search. This program would have used the full capability of the SIM Lite spacecraft to make its measurements.

A flexible search strategy tunes SIM Lite's mass sensitivity at each star to a desired level in the habitable planet search. The value of ηEarth (Eta_Earth), the fraction of stars carrying Earth-analog planets, will be estimated by the Kepler Mission some time before SIM Lite launches. One strategy for a habitable planet search is to do a 'deeper' search (i.e. to lower mass sensitivity in the habitable zone) of a smaller number of targets if Earth analogs are common. A 'shallower' search of a larger number of targets could have been done if Earth analogs are rarer. For example, assuming that 40% of mission time is allocated for the planet search, SIM Lite could have surveyed:

65 stars for planets down to one Earth mass, in scaled 1 AU orbits, OR 149 stars for planets down to two Earth masses, in scaled 1 AU orbits, OR 239 stars for planets down to three Earth masses, in scaled 1 AU orbits.

Aside from searching for Earth-sized planets SIM Lite was scheduled to perform what has been dubbed the "Broad Survey". The Broad Survey would have looked at approximately 1,500 stars to help determine the abundance of Neptune-mass and larger planets around all star-types in Earth's sector of the Milky Way.

A third part of the planet finding mission was the search for Jupiter-mass planets around young stars. The survey would have helped scientists understand more about solar system formation, including the occurrence of hot Jupiters.Davidson, John et al., Ed., ., SIM Lite Astrometric Observatory , Chapter 2 , Jet Propulsion Laboratory 400–1360, 2009. Retrieved 8 March 2010. This portion of the planet hunt was designed to study systems with one or more Jupiter mass planets before the system has reached long term equilibrium. Planet hunting techniques using a star's radial velocity cannot measure the regular, tiny to-and-fro wobble motions induced by planets against the strong atmospheric activity of a youthful star. It is through the techniques pioneered by Albert A. Michelson that the SIM would have been able to execute its three primary planet-finding missions.

The mission's planet finding component was set up to serve as an important complement to the future missions designed to image and measure terrestrial and other exoplanets. SIM Lite was to perform an important task that these missions will not be capable of: determining planet masses.Davidson, John et al., Ed., SIM Lite Astrometric Observatory , Chapter 3 , Jet Propulsion Laboratory 400–1360, 2009. Retrieved 8 March 2010. Another task that the SIM was envisioned to perform for the future missions will include providing the orbital characteristics of the planets. With this knowledge other missions can estimate the optimal times and projected star–planet separation angles for them to observe the terrestrial (and other) planets SIM has detected.

Stellar mass

Another key aspect of SIM Lite's mission was determining the upper and lower limits of star's masses. Today, scientists understand that there are limits to 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.Stars, Neutron Stars, & Black Holes , NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.Stars, Neutron Stars, & SIM Lite , NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.

Part of the SIM's mission was 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 stars, 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 in excess of 60 times the mass of the Sun. The entire study was to focus on binary star systems, stars coupled through a mutual gravitational attraction.

Galactic mapping

Interferometric measurements of stellar positions over the course of the mission would have permitted SIM to precisely measure the distances between stars throughout the Milky Way. This would have allowed astronomers to create a "roadmap" of the Galaxy, answering many questions about its shape and size.The Milky Way and Dark Matter , NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.SIM Lite and Dark Matter , NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.

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 those galaxies. Steven Majewski and his team planned 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.Silberg, Bob. "Building a better guide to the Galaxy ", NASA, SIM PlanetQuest, 14 February 2006, Jet Propulsion Laboratory. Retrieved 24 April 2007.

SIM Lite measurements of Milky Way stars were to yield data to understand four topics: fundamental galactic parameters, the Oort Limit, disk mass potential, and mass of the Galaxy to large radii."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. The first, fundamental galactic parameters, was aimed at answering key questions about the size, shape and the rotation rate of the Milky Way."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. The team hoped to more accurately determine the distance from the Sun to the Galactic Center. The second topic, the Oort Limit, would have attempted to determine the mass of the galactic disk."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.