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Science with the Space Interferometry Mission . Outline . Introduce SIM and the current program of SIM science Summarize the plan for future opportunities to propose for SIM observations Describe how SIM works. 1. What is SIM ?. SIM is the Space Interferometry Mission

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Presentation Transcript
  • Introduce SIM and the current program of SIM science
  • Summarize the plan for future opportunities to propose for SIM observations
  • Describe how SIM works
1 what is sim
1. What is SIM ?
  • SIM is the Space Interferometry Mission
    • One of key missions in NASA’s Origins Program
  • Precision astrometry on stars to V=20
  • Optical interferometer on a 9-m structure
    • One science interferometer
    • Two guide interferometers to stabilize the fringes
  • Launch date: in the coming decade
    • Astrometry requires patience !
  • Global astrometric accuracy: 4 microarcseconds (µas)
    • At end of 5-year mission lifetime
  • Narrow-field astrometric accuracy: 1 µas, in a single measurement
    • Current state of the art is HST/FGS at ~500 µas
    • Ground-based differential astrometry will reach ~20 µas
  • Typical observations take about 1 minute; ~ 5 million observations in 5 years
sim configuration
SIM Configuration

Guide Interferometer Fields of View

Science Interferometer Field of Regard

External Metrology



Corner Cube



Optical Delay Lines (ODLs)

Astrometric Beam Combiners (ABCs)

External MET

Beam Launchers



sim science objectives
SIM Science Objectives
  • Broad program
    • Searches for low-mass planets
    • Study of planetary systems
    • Stellar astrophysics
    • Galactic structure
    • Dynamics of the Galaxy and stellar systems
    • Ages of stars and the Galaxy
    • Structure and dynamics of Active Galactic Nuclei
  • More information on SIM at:
  • See “Science with SIM” on our website
    • Contains excellent 2-3 page summaries of each Key Project

SIM Science Team

Key Science Projects

Dr. Geoffrey Marcy U. California, Berkeley Planetary Systems

Dr. Michael Shao NASA/JPL Extrasolar Planets

Dr. Charles Beichman MSC/Caltech Young Planetary Systems and Stars

Dr. Andrew Gould Ohio State University Astrometric Micro-Lensing

Dr. Edward Shaya Univ. of Maryland Dynamic Observations of Galaxies

Dr. Kenneth Johnston U.S. Naval Observatory Reference Frame-Tie Objects

Dr. Brian Chaboyer Dartmouth College Population II Distances & Globular Clusters Ages

Dr. Todd Henry Georgia State University Stellar Mass-Luminosity Relation

Dr. Steven Majewski University of Virginia Measuring the Milky Way

Dr. Ann Wehrle MSC/Caltech Active Galactic Nuclei

Mission Scientists

Dr. Guy Worthey Washington State University Education & Public Outreach Scientist

Dr. Andreas Quirrenbach Leiden University Data Scientist

Dr. Stuart Shaklan NASA/JPL Instrument Scientist

Dr. Shrinivas Kulkarni MSC/Caltech Interdisciplinary Scientist

Dr. Ronald Allen Space Telescope Science Inst. Synthesis Imaging Scientist

Only Principal Investigators listed. Including co-investigators the SIM Science Team has 86 members.

planet detection with sim
Planet Detection with SIM
  • Deep search for terrestrial planets
  • Broad survey of planetary system architectures
  • Planetary systems Around young stars
knowledge and ignorance of extrasolar planets
Knowledge and Ignorance of Extrasolar Planets
  • What we do know
    • Giant planet occurrence is high: ~7%
    • Mass distribution extends below Saturn mass
    • Eccentric orbits are common: scattering?
    • Many multiple systems of giant planets are known
  • What we don’t know
    • Existence of terrestrial planets
      • Are there low-mass planets in ‘habitable zone’ ?
    • Planetary system architecture
      • Coplanarity of orbits
    • Mass distribution of planets is incomplete and has strong selection effects
      • What about spectral type?
      • Stellar age?
      • Evolutionary state?
accurate masses are important
Accurate masses are important
  • Mass is a fundamental astrophysical quantity
    • along with radius, density, temperature, chemical composition
  • Accurate masses are notoriously difficult to measure
    • Spiral galaxy mass from luminous matter vs. rotation curves ?
      • dynamical masses preferred

 radial velocities and astrometry

  • SIM will measure the mass of every planet it detects
    • Accuracy depends only on the performance of the instrument
      • not on models or assumptions
  • Accurate masses are complementary
    • Combine with transit data or direct detection to measure density of the planet
towards a planetary census
Towards a Planetary Census
  • Radial velocity studies have identified gas giants around 7~10 % of nearby stars on orbits within ~1-3 AU
  • Transits will determine incidence of Earths in habitable zone around hundreds of stars
  • Next decade will yield a census of planets down to a few Mearth
  • Astrometric interferometry will detect and characterize gas giants around 2,000 stars and rocky planets around 200 stars:

 Target list for Terrestrial Planet Finder (TPF)

deep search for terrestrial planets
Deep Search for Terrestrial Planets
  • Are there Earth-like (rocky) planets orbiting the nearest stars?
  • Sample of ~250 of the nearest stars
    • Focus on F, G, K stars within 10 pc
    • Concentrate on the habitable zone
  • Sensitivity limit is ~3 ME in a 1 AU orbit, at 10 pc (~5  detection)
    • Requires 1 µas single-measurement accuracy
    • 25 measurements in each axis
deep search for terrestrial planets12
Deep Search for Terrestrial Planets

Masses of 104 known planets

  • Ground-based radial velocity technique detects planets above a Saturn mass
  • SIM will detect planets down to a few Earth masses and measure their masses







astrometry at 1 m as precision
Astrometry at 1 mas precision

± 3 µas

± 5 µas

± 7 µas

Performance worth waiting for: dynamical masses of terrestrial planets

Error bars are 1 µas

Simulation of detection of terrestrial planets around stars at 5 pc: Data are positions of solar-mass parent star’s photocenter during 5-year mission

broad survey of planetary systems
Broad Survey of Planetary Systems

Out of all planetary systems discovered to-date, only one resembles our solar system

We ask:

  • Is our solar system normal or unusual? E.g. gas giants
  • Are planets more common around sun-like stars? Contrast with A, B type stars
  • What are the ‘architectures’ of other planetary systems? E.g., coplanar?
investigate coplanarity of doppler detected multiple systems
Investigate Coplanarity of Doppler Detected Multiple Systems

We’ve assumed they are coplanar.

We have theoretical and simulation results supporting this assumption.

But are they really coplanar?

planets around young stars
Planets around Young Stars
  • Questions:
    • How do systems evolve?
    • Is the evolution conducive to the formation of Earth-like planets in stable orbits?
    • Do multiple Jupiters form and only a few (or none) survive?
  • Search for Jupiter mass planets around young stars to understand formation and evolution of planetary systems
    • Study ~150 stars with ages from ~ 1- 70 Myr Distances from 50 to 150 pc; V ~ 11-12
    • A Jupiter at 1 AU around 0.8 Mo star produces 8 µas signal at 140 pc
  • Determine physical properties of young stars through precise measurements of distances and orbits of young stars in multiple systems
    • Masses, ages, evolutionary tracks of stars < 1 Mo are poorly known
beyond planet detection sim covers the entire galaxy
Beyond Planet Detection: SIM Covers the Entire Galaxy

Global astrometric precision to 4 µas (microarcseconds)


Faint targets down to 20th mag

The combination of these two capabilities is not matched by any other instrument or mission


25 kpc

(10 %)


2.5 kpc

(1 %)

You are here


100 pc

What is a parsec ?

“Parallax of one arcsecond”

At 1 pc Earth-Sun subtends 1 arcsec

1 parsec = 3.26 light-years

~ distance to closest stars

stellar evolution and the distance scale
Stellar Evolution and the Distance Scale
  • Distances in the Universe are uncertain because we don’t know the distances to “standard candle” stars
    • SIM will measure accurate distances
  • Masses of most stars are very poorly known
    • SIM will measure accurate masses (to 1 %) by using binary orbits
  • Stellar evolution models can’t be further tested without accurate masses for ‘exotic objects’
    • SIM will measure the masses of OB (massive) stars, supergiants, brown dwarfs
taking measure of the milky way
Taking Measure of the Milky Way

Cover page from S. Majewski

Key Project proposal

  • SIM will probe the structure of our Galaxy:
  • Fundamental measurements of:
    • Total mass of the Galaxy
    • Distribution of mass in the Galaxy
    • Rotation of the Galactic disk
  • How?
    • By observing samples of stars throughput the Galaxy
    • By sampling different star populations
dark halo of our galaxy
Dark Halo of our Galaxy
  • ‘Dwarf spheroidal’ galaxy orbits the Milky Way
  • Gravitational forces pull out ‘tidal tails’ of stars
  • The orbits of these tails trace the past history of the dwarf
  • They also trace the mass distribution of the Milky Way
  • SIM provides:
    • Astrometric motions of stars out to 20 kpc
  • Why SIM?
    • Need astrometric accuracy
    • and sensitivity
dynamics of galaxy groups within 5 mpc
Dynamics of Galaxy Groups within 5 Mpc

You are here

Simulated ‘time-lapse’ photo of

30 galaxies closest to our Milky Way

(1-billion year exposure)


  • Simulated 3-D motions projected onto a plane
  • ‘Smeared’ tracks show the simulated motions of galaxies
  • Circles show current positions

SIM will test this model

  • SIM will measure current 2-D velocities across the sky
quasar astrophysics using astrometry
Quasar Astrophysics Using Astrometry
  • Quasars are the most powerful objects in the universe
  • Many quasars emit twin jets of relativistic plasma
  • Optical observations average the entire region
    • Accretion disk, hot corona, jets
  • Jets have been studied by VLBI (radio) at ~100 µas scales
  • SIM will measure:
    • position shifts due to variability
    • color-dependent relative positions of the emission
  • These measurements will open up a research area only studied with VLBI
3 how sim works
3. How SIM Works
  • SIM “sees” 15 degrees in its field of regard, of which any 2 arcseconds can be observed with the science interferometer (one baseline orientation).
  • Interferometer observes objects sequentially within a 15 degree “tile”, including reference grid stars (K giants) and science targets. Bright stars take about 60 seconds of observing time, including siderostat movement. A “tile” takes about an hour.
  • Spacecraft then slews to the next tile and observes some of the same grid stars and new science targets. Continue around celestial sphere.
  • Spacecraft baseline slowly rotates, eventually capturing perpendicular baseline orientation.
  • SIM will execute about 5 million observations in 5 years which is a non-trivial scheduling challenge.
planet search observing scenario
Planet Search Observing Scenario

~1º field

15º Field

of Regard

Grid star

Science target

Reference star

sky coverage of astrometric grid stars
Sky Coverage of Astrometric Grid Stars
  • ~1300 stars
  • Magnitude ~12
  • Stable to ~1 µas



  • Phase 1 complete: candidate star identification
  • Phase 2 ongoing: precision radial-velocity monitoring

----- Celestial equator -----



  • SIM’s currently selected science program includes planetary searches, main-sequence and “exotic star” astronomy, Galactic dynamics, Local Group motions, and AGN astrophysics
  • Astronomers will propose for additional science programs, including Science Team Key projects and Guest Observer projects.
parameter space for planetary companions
Parameter space for planetary companions


Young Jupiters

Radial velocity

10 years

3 m/s

SIM 5 years

1 µas @ 3 pc