SIM-Lite: Progress Report

James C. Marr-IV[^1], Michael Shao, Renaud Goullioud

NASA Jet Propulsion Laboratory, California Institute of Technology

Abstract

SIM, a micro-arcsecond astrometry space mission, has been impacted by
significant changes in NASA priorities over the last two years,
resulting in the mission being indefinitely delayed. The project team
has responded by investigating alternative mission concepts based upon
completed SIM technology. Several alternative mission concepts have been
identified, ranging from a planets-only concept, to versions of SIM,
called SIM-Lite, that still address the full breadth of science
envisioned by two previous National Academy Astrophysics Decadal Surveys
but with lower precision and reduced throughput. These mission concepts
are significantly more affordable and may fit into a nearer-term future
scenario than the full SIM PlanetQuest^17^ would. This paper describes
the current state of the project, including its design and technology,
and the alternative mission concepts for the use of these designs and
technology.

Keywords: Interferometer, Interferometry, astrometry, SIM, SIM-Lite,
exoplanet, planet-finding

1\. Introduction

![](media/image1.pct){width="3.3645833333333335in"
height="2.589583333333333in"}

SIM resulted from the 1991 recommendation of the U.S. National Research
Council's (NRC) Astronomy and Astrophysics Survey Committee (AASC)^1^
that the U.S. National Aeronautics and Space Administration (NASA)
undertake an Astrometric Interferometry Mission (AIM) that would employ
the then recently developed, and still evolving, techniques for
ground-based, optical, long-baseline interferometers in a space mission
that would extend the performance into the few micro-arcsecond (µas)
regime by eliminating the effects of Earth's atmospheric disturbance.
The NRC envisioned a broad array of scientific objectives, including a
broad astrophysics program and characterization of extra-solar planetary
systems, and the demonstration of technologies needed to enable future
very large space-based telescopes by breaking the linkage between
resolving power and aperture size.

The NRC's AIM mission was renamed the Space Interferometry Mission (SIM)
in late 1996 following a NASA selection from competing design concepts
and project management was assigned to the Jet Propulsion Laboratory
(JPL) in Pasadena, California. SIM was later renamed SIM-PlanetQuest in
2005 to emphasize SIM's role in exo-solar planetary system
characterization. Lower cost SIM options that address the full breadth
of the NRC's recommended science capability are now being called
SIM-Lite to distinguish them from SIM PlanetQuest.

The NRC's AASC again endorsed the mission in its 2001 Decadal report^2^,
refining the objectives slightly and shifting additional focus to the
characterization of extra-solar planetary systems due to the
increasingly frequent discovery of large planets using ground-based
radial-velocity, photometric, and other techniques. SIM's scientific
capabilities cited were: *"...\[enabling\] the discovery of planets much
more similar to Earth in mass and orbit than those detectable now, and
... \[permitting\] astronomers to survey the Milky Way Galaxy 1,000
times more accurately than is possible now*." The report also emphasized
the dual capability of SIM, noting that this capability would enable
*"...both narrow-angle astrometry for detecting planets and wide-angle
astrometry for mapping the structure of the Milky Way and other nearby
galaxies.*"

The NRC AASC's Panel on Ultraviolet, Optical and Infrared Astronomy
(UVIOR Panel)^3^ stressed that "the primary scientific objective of the
SIM mission is ultrahigh-accuracy astrometry." SIM's capabilities for
making distance measurements of "*1 percent accuracy to distances of
several kiloparsecs and of 10 percent accuracy throughout the Galaxy*"
would address the astrophysical goals of "*providing a firm foundation
for the understanding of stellar astrophysics ... \[making\] luminosity
determinations for key classes of stars ... \[to\] reduce the
calibration uncertainties in the cosmological distance scale ... \[and
refining\] our knowledge of the distribution of dark matter*." The UVOIR
Panel stated that *"\[s\]earching for planets near stars in the solar
neighborhood is the most ambitious of SIM's goals ... \[because the
search\] should generate a preliminary survey of the local planetary
population and a more extensive survey of the Jovian-mass planets ...
and ... \[thereby\] not leave any ambiguity about the masses of
extrasolar planets*."

The SIM project completed an ambitious 10-year technology program in
2005, perhaps one of the most externally reviewed technology programs in
history. This program demonstrated that SIM would be able to achieve
roughly 40% better performance than even the Goal levels of performance
hoped for by the NRC AASC Decadal survey committees. A later section of
this paper reviews the technology program and ongoing developments to
demonstrate flight-qualified hardware.

In 2007, in response to redirection of NASA priorities, SIM was
indefinitely delayed, funding was reduced, and the project directed to
enter an Engineering Risk Reduction phase. In 2008, the project was
directed to prepare for the upcoming NRC AASC Decadal survey, slated for
2009/2010.

Given the exceptional performance of the completed technology, the
project team undertook to identify the lowest cost mission that could be
accomplished using this technology yet still provide significant
scientific results. This resulted in a planets-only concept with the
potential for greater than 50% cost savings over that required to
complete the full SIM-PlanetQuest mission. Further studies have examined
variations of the resulting design, called SIM-Lite, that restore
additional capability to this minimum design in order to develop an
array of options with differing cost/benefit ratios to carry forward to
the upcoming Decadal survey. These SIM-Lite studies are discussed
further in a later section of this paper.

Meanwhile, the U.S. Congressionally appointed Astronomy and Astrophysics
Advisory Committee (AAAC) with the charter to jointly advise NASA, NSF
and the DOE regarding their astronomy and astrophysics programs. The
AAAC appointed an Exoplanet Task Force with the charter "*to recommend a
15-year strategy to detect and characterize exoplanets and planetary
systems, and their formation and evolution, including specifically the
identification of nearby candidate Earth-like planets and study of their
habitability*." This task force final report^11^ concluded that "*The
only technique appropriate to survey the nearest hundred or so bright
sun-like stars in the mid-term is space-based astrometry, and this is
one cornerstone of the Task Force recommendations*." The report
recommended that: "*Preparations should begin for a space-borne
astrometric mission capable of surveying between 60 and 100 nearby main
sequence stars with the goal of finding planets down to the mass of the
Earth orbiting their parent star within the habitable zone - i.e.,
approximately 0.8 to 1.6 AU, scaled appropriately for stellar
luminosity. Achieving this goal will require the capability to measure
convincingly wobble semi-amplitudes down to 0.2 microarcseconds ("µas")
integrated over the mission lifetime. Space-borne astrometry is
currently the only technique that can distinguish masses in the range of
1-10 earth mass*." Any of the SIM-Lite options currently under study
would be capable of, or nearly capable of, achieving these objectives.

2\. Astrometric Measurement Perormance

SIM-Lite measures the position and proper motions of stars in one of two
observing modes: Narrow Angle (NA) or Wide Angle (WA).

The NA mode is probably the easiest to understand in that in this mode
SIM-Lite measures the position of a target star relative to nearby
reference stars (four or more) that are within about 1° of the target
star by 'chopping' between them to measure their relative positions. The
NA mode is the more accurate of the two modes enabling accuracy to about
1 µas from a single measurement and accuracies as low as 0.2 µas
following many hundreds of measurements.

Chopping removes instrument drifts, enabling the most accurate
performance from the SIM-Lite instrument. A single set of
target/reference-star measurements (1,100 second sequence involving 12
chops between the target and the set of typically four reference stars)
is called a single NA observation and represents a stand-alone
one-dimensional (i.e., baseline orientation) measurement. Accuracy
achieved will depend upon star brightness. For a 6^th^ magnitude target
star and 9^th^ magnitude reference stars, 1 µas single measurement
accuracy can be obtained. Multiple single NA observations of the same
object over the duration of the mission average down as the square root
of the number of observations, providing the ability to detect
one-Earth-mass planets in the habitable zone. The instrument noise floor
has been demonstrated in the laboratory to be below 0.035 µas obtainable
after 1600 NA observations for a 6-meter instrument baseline. Detecting
a one-Earth-mass planet at mid-habitable zone around a solar mass star
at 10 parsec distance requires \~800 NA observations to detect the 0.3
µas signature of the planet with a signal to noise ratio of 6.0.

![](media/image2.pct){width="3.0368055555555555in"
height="2.2805555555555554in"}

Overall accuracy obtainable depends upon instrument errors and
astrophysical errors. Instrument errors consist of thermal drift, field
dependence (e.g. beamwalk), and spectral dependent errors. Astrophysical
errors include contributions from reference star companions and star
spots.

Instrument thermal drift is largely measured by the metrology system and
largely removed by chopping in the Narrow Angle mode where extreme
accuracy is required for planet finding. The effects of instrument
thermal drift have been measured in laboratory testbeds with no floor
being observed on the longest data sets taken to date (Figure-2).
Modeling of flight instrument performance on-orbit predicts that flight
instrument thermal stability will be better than that of the testbeds,
suggesting better on orbit performance. Field dependent errors for NA
measurements are minimized in the SIM-Lite design by using the target
star for the Guide-1 star, resulting in only the central portion of the
siderostat field of regard (FOR) being used for NA measurements,
minimizing field dependent errors and allowing those that remain to be
well calibrated. Color dependent errors, those that result from chopping
between target and reference stars of different spectral properties, has
been studied in the Spectral Calibration Development Unit (SCDU) testbed
where it has been shown that these can be calibrated and compensated
for^12,13,14^.

Astrophysical noise from reference star companions falls into three
categories: 1. Large companions that can be pre-screened using radial
velocity from the ground; 2. Too small to be of consequence; and 3. Of
an intermediate size such that they must be solved for along with the
target star companions. The effects of all of these are minimized by
choosing reference stars which are sufficiently distant (e.g., K giants
at \~ 1kpc). Astrophysical noise from star spots has been shown by
analysis to not significantly affect astrometric measurements but do
significantly affect radial velocity measurements^15^.

The WA mode measures the position of stars ultimately relative to a set
of distant quasars. This is done by collecting the positions of target
and grid stars measured relative to each other within each of about
1,300 overlapping 15° tiles that cover the entire sky, with each tile
being visited repeatedly over the full 5-year mission. The positions of
all of the stars and some instrument parameters are then solved for all
at once, producing stellar positions accurate to between 4 µas and 10
µas, depending upon the star brightness and the length of integration
time on that object. A subset of the observed WA stars referred to as
Grid Stars, are of magnitude 10.6 or brighter and are mostly K-giants
which were carefully selected to facilitate closure of the tiles into a
reference grid. Grid star positions will be determined to an accuracy of
approximately 4 µas and will serve as a full-sky optical reference grid
that will be tied to the International Celestial Reference Frame (ICRF).

WA observations are made over the instrument's entire fifteen-degree
field-of-regard (FOR) and consist of a single measurement of each star's
position. Since measurements of stars within the FOR (up to 50 of them)
could require more than an hour of observing time and be distributed
over the whole FOR, there are larger errors in WA measurements than for
NA measurements.

SIM's astrometric measurement accuracy specified at two levels:
Baseline/Full (the required performance from NASA's perspective), and
Goal (the desired capability of the system, from which the project's
current design point is derived). Table 1 shows the astrometric
performance of three mission options versus the recommended performance
from the prior three NRC Decadal surveys. The astrometric performance of
the SIM-Lite and planets-only options results from a number of design
changes discussed later in this paper but most significantly the length
of the science interferometer baseline of 6-meters (versus the former
9-meters).![](media/image3.pct){width="6.535416666666666in"
height="3.379861111111111in"}

3\. Science: PLANET FINDING

The NASA's Science and Mission Directorate (SMD) has been developing a
number of missions that will contribute to characterization of
proto-planetary discs, planetary systems and planets themselves. None of
these missions tell the whole story by themselves. Each mission provides
pieces of the puzzle that will enable assembling solid understanding of
how planetary systems form, their distribution, their characteristics
and whether any harbor Earth-like planets (considered to be rocky
planets in the range 0.3 to 10 Earth masses) in their habitable zones
(where liquid water can exist, considered to be 0.85 to 1.6 AU^11^ but
scaled for stellar brightness, see Catanzarite, el al^8^), that are
about the same density as our rocky planets so that they have a solid
surface (giving diameters of about 0.5 to 2 times the diameter of the
Earth), that are far enough away from their parent star to avoid tidal
lock, and that actually exhibit characteristics that we would recognize
as indicating the presence of biological activity. Additionally, we
would like to know that there are gas giants in these same systems much
further away (ala Jupiter) that can sweep out comets sufficiently to
allow life to evolve on the Earth-like planet(s) in these systems.

The Kepler^7^ 4-year mission, currently slated for launch in February
2009, will characterize the frequency of occurrence of rocky planets in
inner orbits around about 100,000 14^th^ magnitude or brighter dwarf
stars lying in a 12 degree field of view pointed in the direction of the
Cygnus constellation. Kepler uses the transit method whereby the light
intensity from a star dips during the transit of a planet (requiring, of
course, that the planetary system by oriented in such a way that the
planets cross the line of sight between the star and Kepler, estimated
to occur \~0.5% to \~1% of the time). Most of the stars that Kepler will
observe are very distant (out to about 3000 light years, ly) compared to
the stars that SIM will characterize (out to \~100 ly). If such planets
are very common, Kepler should find something like a few hundred rocky
planets. SIM can assist in determining the diameter of planets that
Kepler finds by accurately measuring the distance to the star to better
than 1%.

What Kepler primarily does is to answer the currently unknown
fundamental question as to the relative frequency of occurrence of rocky
planets around dwarf stars (like our Sun). Kepler's results will be used
to tailor SIM's planet observing program. Should Kepler show that rocky
planets be rare, SIM-Lite could use its planetary discovery time to
survey a greater number of nearby stars (\~250) for larger rocky planets
in habitable zones. Should rocky planets be common, SIM-Lite could
survey fewer stars (in the range 60 to 100) searching deeper for smaller
planets. SIM-Lite will survey the best candidates for planetary systems
within 30 pc and will be able to detect planets of all sizes around all
of those surveyed, returning orbital parameters and mass^8^.

In addition for looking for close-by inner orbit rocky planets, SIM-Lite
will characterize the planetary architecture of some 1,500 stars by
looking for planets in the range from Uranus (14 Earth masses) to
Jupiters (318 Earth masses).

Unlike Kepler, SIM-Lite will be able to detect planets around every star
that has them, regardless of the orientation of the planetary disk. Also
unlike Kepler, SIM-Lite will determine the mass of any planets
discovered and, in fact, is the only NASA mission capable of determining
planetary mass, the most fundamental property of a planet and one that
is key to the existence and characteristics of the planet's atmosphere.
Also unlike Kepler, all planets discovered by SIM-Lite will be close
enough that they will be candidates for direct planet observation by
direct detection missions such as coronagraphs or nulling
interferometers.

A third investigation will search \~50 young stars (\<100 Myrs) for
large planets to help characterize how large planets form and migrate
during the early life of a planetary system. Astrometry is the only way
to assess planets around these types of stars due to the substantial
protoplanetary disk's obscuration.

As a facility instrument, the allocation of SIM-Lite observing time
between exoplanet finding and general astrophysics will be determined by
the science community as the mission nears launch. While three of
SIM-Lite's key projects address parts of the three planet finding
investigations described above, responding to the recommendations of the
Exoplanet Task Force^11^ will require substantially more time than has
been allocated thus far. Such allocation could be handled by additional
key projects or through a general observer program.

4\. Science: Astrophysics Program

A significant portion of SIM-Lite science mission time is used for
astrophysics objectives that can be accomplished with SIM-Lite's
precision astrometric capabilities for dim stars (resulting from
SIM-Lite being a pointed instrument as opposed to a scanning instrument
such as Gaia), the primary reason for the original NRC recommendation
that NASA undertake the precision astrometry mission that has become
SIM-Lite. While astrometry is one of the oldest of sciences, the
precision offered from a space advantage opens vast new horizons for
science based upon accurate measurements of stellar size, mass, distance
and motion. SIM-Lite's science program aims to take advantage of
SIM-Lite's micro-arcsecond class measurements for stars as dim as visual
20^th^ magnitude to open these vistas using both of SIM-Lite's observing
modes (WA and NA).

Some of SIM-Lite's astrophysics science observing time has been
allocated to a set of seven astrophysics Key-Projects and the observing
programs of five Mission Scientists, all of whom were competitively
selected by NASA. The details of each of these programs can be found on
the SIM website^10^ where both summaries of their programs and their
full proposals can be found. More recent descriptions of these
investigations can be found in a recent refereed PASP paper^16^. These
programs were selected early in the development of SIM to enable the
teams to accomplish preparatory science that includes radial velocity
pre-screening of candidate objects and to perform fundamental research
that will enable teams to better plan their observing and data reduction
programs.

The projects of the current science team will contribute to areas such
as: How did the galaxy form and how is it evolving now?; What are the
masses and mass distributions of nearby galaxies?; What is the nature of
dark galactic matter?; How old are the oldest stars in the galaxy?; What
are the masses of the stars?; What is the rotation of our local apparent
standard of rest?; How do optical and radio images register at the
smallest angular scale?; What is the origin, structure, and evolution of
the 'central engines' in the nuclei of quasars and active galaxies
(AGNs)?.

Future solicitations will address the allocation of the remaining
SIM-Lite facility instrument time.

Outside of the science observation time allocations, SIM-Lite will also
undertake WA observations of stars from a set of approximately 1300 Grid
stars (mostly K-giant at distances of \~1kpc) distributed roughly
uniformly over the entire sky that will be used to develop a
quasar-anchored reference grid of stars whose positions are known to an
accuracy of better than roughly 4 µas. These will be related to the
International Celestial Reference Frame (ICRF), a primarily radio
frequency frame, for use by future astrophysics missions.

Further information about SIM-LIte's research opportunities is available
on the SIM web site^10^.

5\. INTERFEROMETER TUTORIAL

Interferometers are a form of sparse aperture telescope and can combine
light from multiple telescopes in either an image plane (Fizeau),
forming an image, or in a pupil plane (Michelson), forming (for point
sources like stars) a white light interference fringe. SIM-Lite is a
Michelson interferometer operating in the visible portion of the
spectrum (400 to 1000 nm). The advantage of a pupil plane interferometer
over a filled-aperture telescope is higher resolution but this is at the
expense of field of view and fewer photons collected per unit time
(which isn't a problem if looking at relatively bright objects).

![](media/image4.pct){width="2.907638888888889in"
height="1.979861111111111in"}

Figure 3 shows a single interferometer "sensor", consisting of two
collector telescopes separated by a baseline, B, a delay line and a beam
combiner. Light from a star arrives at the further collector telescope
slightly later than its arrival time at the nearer collector telescope
resulting in an external (or geometric) optical path delay (OPD). Optics
route the light from the two collector telescopes back to the beam
combiner, inserting in the nearer telescope leg an internal optical
delay equal to the external delay in the other leg, resulting in the
optical path lengths being matched to within a few nanometers. Internal
laser metrology is used to measure the inserted internal optical delay
as an analog for the external geometric optical delay.

Assuming that one can determine the length and orientation of the
interferometer baseline, B, then one can solve for the angle of the
incoming stellar wavefront. Much of SIM-Lite's complexity revolves
around determining these parameters. In practice on SIM-Lite, angles are
measured between closely spaced stars and solving for their positions as
a set.

6\. lower cost System Concept exploration

When it became apparent in 2007 that NASA's Science Mission Directorate
(SMD) was not going to be able to afford the SIM-PlanetQuest mission
described in our 2006 SPIE paper^17^, the Project team undertook to
study lower cost options that might still preserve a significant science
program. Constraints for the study included using only already developed
technology and designs that were just "good enough" to do the job. The
sequence of mission concepts studied and some of their more significant
characteristics is shown in Figure-4.

The first concept was for a planets-only (PH) mission that returned to
low Earth orbit (LEO). Since in planet finding both the target and
reference stars are bright (\<10^th^ magnitude) this allowed eliminating
all of the hardware and software required to achieve the ability to
integrate on dim target stars. Further, one of the two Guide
interferometers (guide interferometers serve as a µas two-axis star
tracker, about a million times more accurate than a typical spacecraft
star tracker) could be bore sighted with the science interferometer
leaving the other Guide at 90 degrees with much smaller effect on the
overall instrument error. This allowed replacing the Guide-2
interferometer with a 30 cm telescope built from a few of the components
from the Guide-1 interferometer. Since the NA observations for planet
finding involved chopping between target and reference stars, thermal
stability could be significantly relaxed allowing the mission to fly in
LEO where radiation from the Earth results in a much less stable thermal
environment than the deep space of the Earth-trailing Solar orbit (ETSO)
of SIM-PlanetQuest. A planets-only mission also eliminates much of the
science observation planning and science data reduction system. One
penalty of LEO is, of course, that about 50% of the mission time must be
spent ensuring that the Earth is never in the instrument field of
view.![](media/image5.pct){width="3.35in" height="2.8375in"}

The resulting mission concept was quite attractive both scientifically
and economically, with an estimated cost significantly below that of the
unaffordable SIM-PlanetQuest. The mission was estimated to be able to
search \~65 nearby Sun-like FGK stars for planets as small as one
Earth-mass in the mid habitable zone during a five-year mission.

The team then looked at an even less capable mission, called PH-Lite,
from which some good design ideas emerged but ended up with the ability
to search fewer than fifteen nearby FGK stars for Earth-mass planets.
The design ideas from this design were applied to the original
planets-only design, resulting in additional estimated cost savings.

Encourage by the success of the planets-only approaches, the team then
asked what the minimum changes might be to restore some of the
astrophysics program. These trades resulted in the SIM-Lite concepts
which return the mission to an Earth-trailing Solar orbit (ETSO) and
restore the science interferometer siderostat field of regard (FOR) to
15° (from the 4° in the planets-only concepts). These concepts also
required the restoration of much of the science planning and data
processing that was eliminated in the planets-only options and required
a larger launch vehicle to reach ETSO. Other changes to the spacecraft
resulting from moving from LEO to ETSO were estimated to result in no
significant cost difference.

Three versions of SIM-Lite are currently under study. These are: (1) 30
cm science siderostats with all time pre-allocated pre-launch in order
to simplify the ground operations; (2) 30 cm science siderostats but
with an active guest observer program during mission operations,
resulting in higher cost mission operations; and (3) 50 cm science
siderostats with a guest observer program. Independent cost estimates of
these options will be performed in late calendar year 2008, prior to the
upcoming astrophysics Decadal survey.

7\. SIM-LIte Flight and Mission System

The SIM-Lite flight segment consists of one large facility-class
instrument, a 3-axis stabilized spacecraft, a launch vehicle and a
launch vehicle adapter. The ground segment consists of the Deep Space
Network, a Mission Operations System and a Science Operations System
(the launch vehicle ground control system is considered part of the
launch vehicle).

The SIM-Lite instrument consists of two visible-wavelength Michelson
(pupil plane) stellar interferometer sensors (each like Figure 2), one
30 cm Guide telescope, and one external metrology sensor, all supported
by a Precision Structure subsystem (PSS) and controlled by the Real-Time
Control (RTC) subsystem. Instrument layout is shown in Figure-5.

One of the two Michelson stellar interferometers has a baseline of 6
meters and serves as the Science interferometer. The other
interferometer has a somewhat shorter baseline (4.2 m) and serves as the
Guide-1 interferometer co-bore sighted with the center of the science
interferometer field of regard (FOR). A Guide-2 Telescope is oriented at
90° to the look direction of the science and Guide-1 interferometers and
is contained on the same optical bench as one end of the Guide-1
interferometer. The Guides track relative bright stars, accurately
estimating the interferometric baseline orientation change in inertial
space and generating stabilizing feed forward control information for
the science interferometer (to enable making science measurements on dim
stars down to visual magnitude 19 or 20 in
brightness).![](media/image6.pct){width="3.0319444444444446in"
height="2.1395833333333334in"}

The External Metrology sensor shown in Figures-5 is a laser truss that
measures the baseline length and relative orientation of the
interferometers. The truss consists of ten laser beams launched by ten
metrology gauges that are each capable of measuring the distance between
optical corner cube fiducials to an accuracy of a few picometers. This
truss is used in non-real-time to reconstruct on the ground where the
optical elements were located relative to each other as a function of
time, correlated to science measurements.

The Precision Structure subsystem (PSS) is a composite truss structure
that supports the components of the interferometer as shown in Figure-6.
Instrument optical elements (collector telescopes and astrometric beam
combiners) are connected to the PSS via support struts. Thermal
radiators are mounted on the top surface of the PSS that is always
facing away from the sun (the top surface of the PSS is a sun exclusion
zone).

The instrument electronics and metrology laser source are mounted in two
enclosures located on each side of the
PSS.![](media/image7.pct){width="2.89375in"
height="1.8131944444444446in"}

The SIM-Lite spacecraft is a generic spacecraft suitable for a deep
space mission and is mounted to one end of the PSS. The spacecraft
provides an X-band telecom system with fixed high gain antenna, 5-kw
GaAs solar array, 512 Gbit solid state recorder, four vibration-isolated
reaction wheels, a monoprop momentum unloading system, and thermal
control. The Project is currently studying an option to place the
spacecraft on the side of the instrument to shorten the flight segment
length for easier handling during system integration and test.

The full spacecraft-instrument configuration is shown undeployed (and
without the solar array) in figure-6.

SIM-Lite will launch on intermediate class launch vehicle similar to an
Atlas V 531 from the Kennedy space center, Cape Canaveral, Florida, USA.
Selection of the particular vehicle will be made at the start of
full-scale development.

SIM-Lite will utilize the NASA Deep Space Network (DSN) and Advanced
Multi-Mission Operations System (AMMOS) services as the base for the
project mission operations system. The SIM-Lite mission operations will
be conducted from the Jet Propulsion Laboratory (JPL) in Pasadena, CA.

Science operations will be coordinated by the SIM-Lite Science
Operations System (SOS) located at the California Institute of
Technology (CIT) in Pasadena, CA, USA. The SOS will provide the primary
interface to the science user community and will provide uplink
observation planning and downlink science data processing and
distribution functions. Additionally, the SOS will coordinate science
user proposal calls and selections.

The instrument system subsystems are assembled at the Jet Propulsion
Laboratory (JPL) in Pasadena, California, USA, where functional testing
in air will be completed. The spacecraft will be separately functionally
and performance tested at NGST. The instrument will then mate with the
spacecraft for full flight segment functional and environmental testing
prior to ship to Cape Canaveral, Florida, USA for launch operations.

8\. TECHNOLOGY COMPLETion

The SIM technology development program has been completed after the
better part of a decade^18^. Eight technology Gates (Figure-7), taken
from the full SIM technology development plan, were established in 2001
by NASA representing significant milestones in the project's technology
development, completion of which would signify the project's readiness
to enter full scale development (Phase C/D in NASA vernacular). Each of
these technology gates had specific performance criteria, due dates, and
completion evaluation criteria, and were carefully reviewed and
evaluated by a NASA Headquarters appointed external independent review
team. Completion of these Gates constitutes demonstration that a NASA
Technology Readiness Level (TRL) of six (out of nine levels) has been
reached, signifying readiness to proceed to flight hardware
development.![](media/image8.pct){width="2.5229166666666667in"
height="1.3909722222222223in"}

These eight Technology Gates spanned the range from component
technologies to system level technologies that demonstrate not only
system performance in relevant environments but also demonstrates the
methodology that will be used to test and demonstrate performance of the
flight system.

All eight of these Technology Gates have been completed, with the last
of the eight gates being completed in July 2005. All eight Gates either
met or exceeded their performance criteria and were completed on
schedule, a significant accomplishment for an advanced technology
development program.

The last of these Technology Gates was key in that it demonstrated the
integration of specific test data into an overall model of the
instrument and mission to predict overall mission performance,
demonstrating the methodology to be followed during the full flight
system integration and test. Work in support of this technology gate
also demonstrated that the instrument 'sensors' (the interferometer
sensors or the external metrology sensor) can be performance tested
separately (i.e., they do not interfere with each other) and the results
combined via models, verifying the viability of this approach for flight
instrument performance verification.

Completion of the full SIM technology development program was formally
signed-off by NASA in March 2006 following extensive review of all
technology program objectives and their status by three external review
teams.

Following completion of the SIM technology development program, a new
sequence of engineering milestones were established to further retire
engineering risk by developing brassboard hardware instrument assemblies
for full environmental and performance testing to determine whether
standard flight hardware development practices were sufficient to
transform the technology development hardware into flyable hardware.
Metrology hardware was the first to be so demonstrated with the result
that the brassboard hardware, even after exposure to vibration and
thermal qualification level environments worked better than the
technology development hardware. Development of other instrument
brassboards is continuing.

8\. SCIENCE OPERATIONS

SIM-Lite will be launched into an Earth-Trailing Solar Orbit (ETSO) that
takes the system away from the time varying thermal influence of the
Earth. The orbit selected is slightly more elliptical than Earth's orbit
resulting in a gradually increasing distance (\~0.1 AU/year) between SIM
and the Earth. This increasing distance from Earth with mission time
results in increasing coverage requirements from the NASA Deep Space
Network (DSN) as the mission progresses, being driven during the early
mission by velocity determination requirements but later by the demands
of maintaining telecom downlink volumes as the Earth-spacecraft distance
increases during the mission.

SIM-Lite observes the sky in 15° 'field of regard' (FOR) 'tiles' (about
1,300 of them) that are obtained by positioning the spacecraft
orientation until the two Guides have acquired their guide stars and
then using the science interferometer's siderostat (moveable telescope
mirror) to observe targets anywhere in the full 15° FOR. During each
tile, various types of targets might be observed, including Grid stars,
WA targets and NA targets. A typical tile takes about an hour.
Subsequent tiles overlap previous tiles by roughly half so that one or
more identical Grid stars can be observed for each tile orientation,
providing the means to tie the tiles together. Over a relatively short
time, each target within a tile needs to be observed with the instrument
in roughly orthogonal orientations in order to obtain a two dimensional
position for the object.

9\. PROJECT STATUS and near term activities

The SIM-Lite mission, while officially in Phase B, since much of the
original team has been dissipated due to funding reductions, will need
to go through an abbreviated full Formulation Phase prior to entering
full-scale development. Whether or not that happens will depend upon the
recommendations from the upcoming astrophysics Decadal survey.
Technology development is complete and the development team continues to
retire engineering risk through the development of flight qualifiable
instrument hardware elements. SIM-Lite's launch readiness date will be
primarily determined by the Decadal survey ranking and the funding
profile that NASA can support given its current priorities relative to
the U.S. Space Exploration Policy.

The project is currently conducting a double-blind planet finding
capability study to characterize SIM-Lite's capability to detect
Earth-mass planets in the habitable zone of nearby FGK stars. This study
involves five planetary system modeling teams, one data simulation team,
five data analysis teams, one results synthesis team, and an external
oversight body appointed by NASA. The initial phase of this study will
report to NASA in August 2008. More detailed follow-on work is
anticipated.

The project plans to award up to twenty science study contracts for
one-year studies aimed at encouraging a broad science community to look
at new ways to apply SIM's capabilities. By the time this paper is
published, proposals will have been received (due June 13, 2008) and
awards made (August 2008).

NASA independent cost estimates (ICEs) of the SIM-Lite options are
expected to begin in October 2008. These ICEs are aimed at gaining
further confidence in the mission concept cost estimates performed by
the project team and JPL's Team-X.

10\. SUMMARY

The SIM-Llite mission is a key element in the search for nearby
habitable planets around Sun-like stars. SIM-Lite is the only mission in
NASA's future mission suite that can unambiguously determine the masses
of planets in these nearby planetary systems, a key parameter in the
identification of planets suitable for life as we know it. SIM-Lite also
supports a broad astrophysics program that will enhance our knowledge of
our Milky Way and other galaxies as well as establishing a new stellar
reference frame and significantly improving our calibration of the
distance scales for measuring the universe.

SIM-LIte has completed the technology development required to
demonstrate readiness to proceed to full-scale development but will need
to reconstitute the team to accomplish this development should yet a
fourth NRC Decadal survey recommend a space optical interferometry
mission with µas capability. The project could support a launch in mid
next decade but the actual launch readiness date will be determined by
the funding profile that NASA's SMD is able to support.

ACKNOWLEDGMENTS

The contents of this paper represent the combined efforts of the
scientists, engineers and support personnel from the Navigator
Program^9^ and SIM-Lite project team, whose superb efforts have
generated the technology results and these concepts and designs which
hold so much promise of future scientific discovery.

The research described in this paper was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.

Reference herein to any specific commercial product, process, or service
by trade name, trademark, manufacturer, or otherwise, does not
constitute or imply its endorsement by the United States Government or
the Jet Propulsion Laboratory, California Institute of Technology.

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[^1]: ^\*^ <james.c.marr-iv@jpl.nasa.gov>, ph 818-393-1528;
    <Michael.shao@jpl.nasa.gov>, ph 818-354-7834;
    <renaud.goullioud@jpl.nasa.gov>, ph 818-354-7908. For all: fax
    818-393-5239; NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive
    MS 301-486, Pasadena, CA, USA 91109-8099