A flexibly-scheduled astrometric interferometer can be used to address a wide range of problems in astrophysics. We use
NASA's Space Interferometry Mission (SIM) Lite with microarcsecond accuracy astrometry on targets as faint as V=19
to illustrate the opportunities. SIM Lite can be scheduled to efficiently detect Earth-mass planets around nearby stars,
including multiple planet systems, seriously test models of the astrophysics of stars, probe dark matter in our galaxy, and
to track changes in the parsec-scale structure of distant active galactic nuclei.
A space-based optical interferometer enables microarcsecond precision astrometry of stars, for a wide range of
interesting problems in Galactic and stellar astronomy, including planet detection and characterization. The Space
Interferometry Mission Lite will be the first space-based Michelson optical interferometer for precision astrometry. In
this paper, we briefly summarize the many science applications of this flexibly-scheduled instrument. Details of the
design and operation of SIM Lite are covered in other papers in this conference.
One of the most important science areas for SIM Lite is the detection and characterization of planets orbiting other stars
via the well-known astrometric wobble. With a precision of smaller than one microarcsecond in a single observation,
SIM Lite has the capability to detect Earth-like planets around at least 60 nearby stars. This ability to sensitively survey
our local stellar neighborhood is a unique opportunity. SIM Lite will be able to characterize multiple-planet systems,
which are now known to exist, studying their dynamical properties including long-term stability. Detailed follow-up of
the most interesting (perhaps Sun-like) systems is an exciting prospect. Astrometry is complementary to other
techniques such as radial velocity, which has already yielded many new planets, because it enables measurement of
planetary masses rather than mass lower limits. It will detect small planets around young stars (up to 100 Myr old) to
help understand the formation and evolution of planetary systems; these are hard to study other than by astrometry.
Thus astrometry permits the study of the nature and evolution of planetary systems in their full diversity, including age,
by including young (0.5-100 Myr) solar-type stars.
Because it is a pointed instrument, SIM Lite maintains its full astrometric accuracy on targets as faint as V=19, which
opens up a range of rare (and therefore distant) stellar types to be observed. Stellar masses and luminosities can be
measured to accuracies better than 1%, which is currently hard to do, especially for rare types. Its reach extends to
probing dark matter in our Galaxy, and tracking changes in the nuclei of distant active galaxies. SIM Lite will make
astrometric measurements by observing a grid of reference stars covering the sky, and make inertial observations of
distant quasars; in this frame SIM Lite will deliver positions and parallaxes to better than 4 microarcsecond.
SIM Lite uses technologies developed during more than a decade of testbed work and will see application in many future
astrophysics missions, so this mission paves the way to the future technically as well as scientifically. The mission is
currently in NASA Phase B, and is being considered for full-scale development.
SIM is a space astrometric interferometer capable of better than one-microarcsecond ( as) single measurement accuracy,
providing the capability to detect stellar "wobble" resulting from planets in orbit around nearby stars. While a search for
exoplanets can be optimized in a variety of ways, a SIM five-year search optimized to detect Earth analogs (0.3 to 10
Earth masses) in the middle of the habitable zone (HZ) of nearby stars would yield the masses, without M*sin(i)
ambiguity, and three-dimensional orbital parameters for planets around ~70 stars, including those in the HZ and further
away from those same stars. With >200 known planets outside our solar system, astrophysical theorists have built
numerical models of planet formation that match the distribution of Jovian planets discovered to date and those models
predict that the number of terrestrial planets (< 10 M(+) ) would far exceed the number of more massive Jovian planets.
Even so, not every star will have an Earth analog in the middle of its HZ. This paper describes the relationship between
SIM and other planet detection methods, the SIM planet observing program, expected results, and the state of technical
readiness for the SIM mission.
Optical interferometry will open new vistas for astronomy over the next decade. The Space Interferometry Mission
(SIM-PlanetQuest), operating unfettered by the Earth's atmosphere, will offer unprecedented astrometric precision that
promises the discovery of Earth-analog extra-solar planets as well as a wealth of important astrophysics. Results from
SIM will permit the determination of stellar masses to accuracies of 2% or better for objects ranging from brown dwarfs
through main sequence stars to evolved white dwarfs, neutron stars, and black holes. Studies of star clusters will yield
age determinations and internal dynamics. Microlensing measurements will present the mass spectrum of the Milky
Way internal to the Sun while proper motion surveys will show the Sun's orbital radius and speed. Studies of the
Galaxy's halo component and companion dwarf galaxies permit the determination of the Milky Way's mass distribution,
including its Dark Matter component and the mass distribution and Dark Matter component of the Local Group.
Cosmology benefits from precision (1-2%) determination of distances to Cepheid and RR Lyrae standard candles. The
emission mechanism of supermassive black holes will be investigated. Finally, radio and optical celestial reference frames will be tied together by an improvement of two orders of magnitude.
Optical interferometers present severe technological
challenges. The Jet Propulsion Laboratory, with the support of
Lockheed Martin Advanced Technology Center (LM ATC)
and Northrop Grumman Space Technology (NGST), has
addressed these challenges with a technology development
program that is now complete. The requirements for SIM have
been satisfied, based on outside peer review, using a series of
laboratory tests and appropriate computer simulations: laser
metrology systems perform with 10 picometer precision;
mechanical vibrations have been controlled to nanometers,
demonstrating orders of magnitude disturbance rejection; and
knowledge of component positions throughout the whole test
assembly has been demonstrated to the required picometer
level. Technology transfer to the SIM flight team is now well
The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm-1 with a spectral resolution up to 0.5 cm-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm-1 in two spectral bandpasses: 600-1100 cm- 1100-1400 cm(superscript -1 with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.