" F R I N G E S "

S P A C E   I N T E R F E R O M E T R Y   M I S S I O N   N E W S L E T T E R

                        Number 17,  September 9, 2001


SIM TECHNOLOGY MILESTONES

Most readers of this Newsletter know that SIM is technologically
challenging.  But what specifically are the challenges?  The External Review
Board (ERB) appointed by NASA to review the Shared Baseline design
recommended that specific technology demonstrations be defined.  These would
serve as as `gates' for the Project to pass through in order to proceed with
development.  The Project worked with NASA to define a set of five specific
technology milestones (listed below).  

The first four milestones must be passed for the project to enter Phase B,
and the fifth is a necessary step for entering Phase C/D.  The metrology
beam launcher (Milestone 1) is SIM's last outstanding *component* technology.
The remaining technology milestones pertain to demonstrations of performance
at the subsystem or system level.

These milestones trace back to the fundamental science requirements for SIM.
Wide-angle performance is defined in terms of the all-sky Astrometric Grid.
Narrow-angle performance is defined in terms of (a) a `Broad Survey' to
study planetary system demographics; and (b) a more-demanding `Deep Search'
of nearby stars for planets down to a few Earth masses.  Note that most of
the gates below are considered passed when performance consistent with the
astrometric grid and Broad Survey is achieved.  We will of course strive to
improve performance to enable the Deep Search as well, but the Project will
enter Phase B even if Deep Search performance is still outstanding.  The
Non-Advocate Review (NAR) will determine if the Project has made sufficient
progress toward the Deep Search requirements to allow transition to Phase
C/D development.

We have accomplished the first of these milestones.  Four more to go! 

MILESTONE 1

Demonstrate that the brassboard metrology beam launcher is capable of
performance that meets both narrow angle and global astrometry requirements.
Performance by the date below is sufficient to enable the Broad Survey of
planetary systems and the Astrometric Grid.  

Due date: August 2001

Status:   We have recently accomplished this milestone with brassboards of
both our internal and external beam launchers.  The internal launcher
achieved under 100 pm of `cyclic' error (i.e., error with amplitude under
100 pm and sinusoidal with fiducial motion with a period of 1.3 um, the
wavelength of the metrology laser) and thermal sensitivity of about 4 pm/mK.
The external launcher demonstrated slightly better cyclic error (about 80
pm) and less than 8 pm/mK thermal sensitivity.  Although performance must
improve somewhat to enable the Deep Search planet finding science, the basic
technology is now well in hand and engineering enhancements are expected
provide launchers that will meet all SIM science goals.  No further
technical breakthroughs are needed.

MILESTONE 2

Demonstrate the ability of SIM to stabilize fringes of faint science stars
on the science interferometer of System Testbed-3 (STB-3).  This will
demonstrate that SIM can provide the nanometer control required to enable
the Broad Survey and Deep Search for planets, and the Astrometric Grid.
With this demonstration the nanometer control technology activity will be
successfully concluded.

Due date: December 2001
Status:   We are on track to accomplish this milestone.

MILESTONE 3

Demonstrate on the Microarcsecond Metrology (MAM-1) Testbed that `angles'
(viz., fringe phase delays) between pseudo star sources can be measured at a
level consistent with the narrow angle astrometry requirement. This
demonstrates, at the level of a complete stellar interferometer, SIM's
ability to meet its requirements for planet finding. The level of
performance by the date below will be sufficient to perform the
Broad Survey of planetary systems.

Due Date: July 2002
Status:   We are on track to accomplish this milestone.

MILESTONE 4

Demonstrate that multiple laser metrology gauges can be integrated into an
optical truss representative of SIM's external metrology system.
Performance by the date below will be sufficient to demonstrate the ability
to meet the scientific mission goals for the Broad Survey and the
astrometric Grid.

Due Date: July 2002
Status:   We are on track to accomplish this milestone.

MILESTONE 5

Demonstrate the feed forward of information from a guide interferometer to a
science interferometer on a two baseline testbed.  This demonstrates, at the
system level, the astrometry requirement.  The level of performance by the
date below will be sufficient for the Broad Survey science, for the study of
planetary formation around young stars, and for the global astrometry
science.

Due Date: December 2004
Status:   We are just beginning the design of a major testbed to meet
this milestone.


Future Newsletters will bring updates on our progress against these critical
technology gates.

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Steve Unwin, Editor                              stephen.unwin@jpl.nasa.gov

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