The Stellar Imager (SI) is a space-based, UV/Optical Interferometer (UVOI) designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the Universe in general. It will also probe via asteroseismology flows and structures in stellar interiors. SI will enable the development and testing of a predictive dynamo model for the Sun, by observing patterns of surface activity and imaging of the structure and differential rotation of stellar interiors in a population study of Sun-like stars to determine the dependence of dynamo action on mass, internal structure and flows, and time. SI's science focuses on the role of magnetism in the Universe and will revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magnetohydrodynamically controlled processes in the Universe. SI is a "LandmarklDiscovery Mission" in the 2005 Heliophysics Roadmap, an implementation of the UVOI in the 2006 Astrophysics Strategic Plan, and a NASA Vision Mission ("NASA Space Science Vision Missions" (2008), ed. M. Allen). We present here the science goals of the SI Mission, a mission architecture that could meet those goals, and the technology development needed to enable this missio

Carpenter, Kenneth G.

Christensen-Dalsgaard, Jorgen

Karovska, Margarita

Schrijver, Carolus J.

NASA Technical Reports Server

GONG-SoHO 24: A new era of seismology of the sun and solar-like stars lOP Publishing 
JournaJ of Physics: Conference Series 271 (2011) 012085 dol: 1 0.1 088/ 1742-6596/27 I/l/OI2085 
The Stellar Imager (SI) - A Mission to Resolve Stellar 
Surfaces, Interiors, and Magnetic Activity 
Jllrgen Christensen.DaIsgaard', Kenneth G Carpenter', Carolus J Schrijver', 
Margarita Korovska' ,and the SI Team 
'Department of Physics and Astronomy, Aarhus University, Denmark 
'Code 667 NASA-GSFC, Greenbelt , MD 20771 USA 
' LMA TC 3251 Hanover St. , Bldg. 252, Palo Alto, CA 94304, USA 
'60 Garden St., Cambridge, MA 02138, USA 
Emails: jcd@pbysau.dk. Kenneth.G Camepter@nasa.gov . schryver@lmsa1.coID, 
karovska@head cfa.harvard.edu 
Abstract. The Stellar Imager (SI) is a space-based, UV/Optica! Interferometer (UVOI) 
designed to enable 0 .1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the 
Universe in general. It will also probe via asteroseismology flows and sbuctures in stellar 
interiors. SI will enable the development and testing of a predictive dynamo model for the Sun, 
by observing patterns of surface activity and imaging of the structure and differentia] rotation 
of stellar interiors in a population study of Sun-like stars to detennine the dependence of 
dynamo action on mass, internaJ stroctwe and flows, and time. Sl's science focuses on the role 
of magnetism in the Universe and will revolutionize our understanding of the formation of 
planetary systems, of the habitability and climatology of distant planets, and of many magneto-
hydrodynamically controlled processes in the Universe. 51 is a "LandmarklDiscovery Mission" 
. in the 2005 Heliophysics Roadmap, an implementation of the UVOI in the 2006 Astrophysics 
Strategic Plan, and a NASA Vision Mission ("NASA Space Science Vision Missions" (2008), 
ed. M. Allen). We present here the science goals of the SI Mission, a mission architecture that 
could meet those goals, and the technology development needed to enable this mission. 
Additional information on SI can be found at: h t t p: //hires . g sfc. nasa. gov/ si / . 
1. Science goals of the mission 
The Stellar Imager (S1) is a NASA Vision Mission (VM), developed by an international team, to study 
I) solar and stellar magnetic activity and their impact on space weather, planetary climates, and life 
and 2) magnetic processes in genera! and the roles they play in the origin and evolution of its structure 
and the transport of matter throughout the Universe 
S1's primary science goal will be addressed by observing and measuring spatial and temporal stellar 
surface magnetic activity patterns through ultra-high angular resolution (sub-milliarcsec) UV imaging, 
and by measuring via disk-resolved asteroseismology the internal structure and flows that produce it , 
in a sample of stan; covering a broad range of masses , radii, and activity levels . These observations 
will lead to an improved understanding of the underlying dynamo process(es) and thus enable 
improved forecasting of solar (and stellar) activity on time scales of days to centuries. This, in turn, 
Published under licence by lOP Publishing Ltd 
https://ntrs.nasa.gov/search.jsp?R=20120013442 2019-08-30T21:54:58+00:00Z
GONG-SoHO 24: A new era of seismology of the sun and solar-like stars lOP Publishing 
Journal of Physics: Conference Series 271 (201 1) 012085 doi: 10.1088/1 742-6596/27 1111012085 
will facilitate an improved understanding of the impact of stellar magnetic activity on life on earth and 
on exo-planets found around more distant stars. 
1.1. Key Questions 
There are a number of important questions that SI needs to address in order to achieve its goal of 
understanding dynamos and magnetic activity, including: 
• what do the internal structure and dynamics of magnetically active stars look like? 
• what sets the dynamo strength and pattern in individual stars, from dwarfs to supergiants? 
• how can active stars form polar spots? 
• what can we expect next from the Sun, on time scales from hours to centuries? 
• why do 2 in 3 Sun-like stars show no cycles? 
• what causes solar-type "Maunder minima" or "grand maxima"? 
• how does stellar activity drive all aspects of "space weather" and 
affect planetary climates and life around solar-type and evolved stars? 
• how do dynamos evolve with time? 
• how do dynamos differ in dwarf vs. giant stars? 
Only with the answers to such questions will it be possible more fully to constrain theoretical 
dynamo models and enable true forecasts of future solar and stellar magnetic activity. These questions 
will be addressed by spatially resolving stellar disks to map evolving atmospheric activity as a tracer 
of dynamo patterns and by asteroseismic probing (to at least degrees of order 60) of internal stellar 
structure and flows in stars of various masses, radii, and activity levels. Such a "population study" 
will provide answers far more rapidly than by continuing our close-up observations of the Sun over 
many decades as we observe it moving through the multiple and different activity cycles that are 
needed to obtain a full set of observational constraints - and some of these data would never be 
obtainable from the Sun alone, as it is only one example of how dynamos operate and magnetic 
activity is produced. 
2. Proposed mission architecture 
2.1. Design requirements 
Sl's science goals require it to have the following capabilities: 
• Wavelength coverage: 1200-6600 A 
• access to UV emission lines from Ly-alpha 1216 A to Mg II 2800 A for stellar surface imaging 
o Important diagnostics of most abundant elements 
o much higher contrast between magnetic structures and background 
o smaller baselines (UV saves a factor 2-4 vs. optical, active regions 5 times larger) 
o -10 A UV pass-bands, to isolate, e.g., C IV (formed at 100,000 K) & Mg IT h&k 
(IO,OOOK) 
• broadband, near-UV or optical (corresponding to temperatures of 3,000-10,000 K) for high 
temporal resolution, spatially-resolved asteroseismology to resolve internal structure 
angular resolution of 50 micro-arcsec at 1200 A (120 mas at 2800 A) 
o resolution of -1000 pixels over the surface of nearby (-4 pc) dwarf stars and over 
the surface of the many giant and supergiant stars within -2 kpc 
energy resolution/spectroscopy possible for detected structures 
a long-term (-10 year) mission to study stellar activity cycles: individual telescopeslhub(s) 
must be able to be refurbished or replaced 
2 
GONG-SoHO 24: A new era of seismology of the sun and solar-like stars lOP Publishing 
Journal of Physics: Conference Series 271 (2011) 012085 doi: 10.1088/1742-6596/27111/012085 
2.2. "Strawman" design 
The VM Study developed a baseline mission design that satisfies all of the above requirements. This 
design is for a space-based, UV-Optical Fizeau Interferometer with 20-30 one-meter primary mirrors, 
mounted on formation-flying "mirrorsats" distributed over a parabolic virtual surface whose diameter 
can be varied from 100 m up to as much as 1000 m, depending on the angular size of the target to be 
observed. The individual mirrors are fabricated as ultra-smooth, UV-quality flats and are actuated to 
produce the extremely gentle curvature needed to focus light on the beam-combining hub that is 
located at the prime focus from I - 10 kin distant. The focal length scales linearly with the diameter of 
the primary array, Le., a 100 m diameter array corresponds to a focal length of I kin and a 1000 m 
array to a focal length of 10 kin. The typical configuration has a 500 m array diameter and 5 kin focal 
length. A one-meter primary mirror size wes chosen to ensure that the prunary stellar activity targets 
caD be well observed with good signal/noise. The mirrorsats fly in formation with a bearn-combining 
hub in a Lissajous orbit around the Sun-Earth L2 point. The satellites are controlled to mm-micron 
radial precision relative to the hub and the mirror surfaces to 5 om radial precision, rather than using 
optical delay lines inside the hub for fine tuning the optical path lengths. A second hub is strongly 
recommended to provide critical-path redundancy and major observing efficiency enhancements. The 
observatory may also include a "reference craft" to perform metrology on the formation, depending on 
which metrology desigu option is chosen. The VM Study identified two launch concepts that are quite 
feasible, assuming 1m diameter primary mirrors, with current vehicles. Depending on the number of 
hubs to be launched initially, one or two Delta IV launches will suffice to lift the entire observatory to 
Sun-Earth L2. 
Additional details on the architectural concept can be found in [I] and in the complete VM report at 
http://nires.gsfc.nasa.gov/si/. 
3. Technology development needed to enable the mission 
The major technology challenges to building SI are: 
• formation-flying of -30 spacecraft 
o deployment and initial positioning of elements in large formations 
o real-time correction and control offormation elements 
o staged~ontrol system (kin -+ cm -+ om) 
o aspect sensing and control to lO's of micro-arcsec 
o positioning mirror surfaces to 5 nm 
o variable, non-condensing, continuous micro-Newton thrusters 
• precision metrology over multi-km baselines 
o 2nm if used alone for path length control (no wavefront sensing) 
o 0.5 microns if hand-off to wavefront sensing & control for om-level positioning 
o multiple modes to cover wide dynamic range 
• wavefront sensing and real-time, autonomous analysis and control 
• methodologies for ground-based validation of distributed systems 
• additional challenges (perceived as easier than the above) 
Q mass-production of "mirrorsat" spacecraft: cost-effective, high-volume fabrication, 
integration, and test · 
o long mission lifetime requirement 
o light-weight UV quality mirrors with kin-long IlIdii of curvature, using active 
deformation of flats 
o larger format (6 K x 6 K) energy resolving detectors with finer energy resolution 
(R=l00) 
3 
GONG-SoHO 24: A new era of seismology of the sun and solar-like stars lOP Publishing 
Journal of Physics: Conference Series 271 (2011) 012085 doi: 10. 108811742-6596/271111012085 
The major challenges in this list are being attacked via a number of ground-based testbeds [2] to 
develop and assess precision (to the cm level) fonnation flying algorithms and closed-loop optical 
control of tip, tilt, and piston of the individual mirrors in a sparse array, based on feedback from 
wavefront analysis of the science data stream. The GSFC Fizeau Interferometer Testbed (FIT) is 
developing closed-loop optical control of a many-element sparse array, with 7-elements in Phase 1, 
and 18-elements in Phase 2. GSFC, MIT, and MSFC are collaborating on an experiment, the 
Synthetic Imaging Fonnation Flying Testbed (SIFFf), utilizing the MIT SPHERES hardware on the 
MSFC Flat Floor facility to test cm-level fonnation flying algorithms. The GSFC Formation Flying 
Testbed (FFTB) is a software simulation facility that has been used to develop deployment of array 
spacecraft and the multi-stage acquisition of target light from the individual mirrors by the beam-
combiner. In addition, there are relevant high precision metrology development efforts at SAO [3] 
and JPL [4]. The ultimate goal of all these efforts is to demonstrate staged-control methodologies 
covering over 12 orders of magnitude, from km down to nm scales. We are also stndying alternative 
optical designs for SI to optimize its imaging and spectral energy resolution capabilities [5]. 
The results from these testbeds and stndies will be combined with experienoe from existing ground-
based interferometers, to enable a small, space-based UV/Optical Pathfinder mission, which will use a 
small number of elements (3-5) with smaller baselines (20-50m) and frequent array reconfigurations 
(to fill in the Fourier uv-plane and enable high quality imaging) to both accomplish important new 
science and demonstrate in space the technologies needed for the full-up SI. One or more such 
Pathfinder missions (others are possible in the IR and X-ray as pathfinders for MAXIMlBlack Hole 
Imager (BID) and the Sub-millimeter Probe of the Evolution of Cosmic Strocture (SPECS) will lay the 
ground-work for the long-baseline, Strategic "Vision" Missions that will do troe high angular 
resolution interferometric imaging, including SI, Life Finder, and Planet Imager. 
Acknowledgments 
Contributions are gratefully acknowledged from the wide range of scienoe and technology 
investigators and collaborators on \he Stellar Imager Vision Mission Team (see [1]). This work was 
supported, in part, by Vision Mission Stndy grants from NASA HQ to NASA-GSFC and from GSFC 
to Smithsonian Astrophysical Observatory, Seabrook Engineering, SUNY/Stonybrook, 
U. ColoradolBoulder, and STScI. Substantial complementary internal institutional support is 
gratefully acknowledged from all of the participating institutions. 
References 
[1] Carpenter K G, Schrijver C J, Karovska M and the SI Vision Mission Team 2006 "The Stellar 
Imager (SI) Vision Mission", in Advances in Stellar Interferometry, eds J D Monnier, M 
Scholler and W C Danchi, Proceedings of the SPIE, 6268, pp. 626821 
[2] Carpenter K G, Lyon R G, Liu A, Dogoda P, Petrone P, Mozurkewich D, Miller D, Mohan S 
and Stahl P 2006 "Steps Toward a UV IOpticai Interferometer in Space: FIT, SIFFT, FFTB", 
in BAAS, 38, P 1129 (available from http: Ihires. gsfc .nasa. govl sif) 
[3] Phillips J D and Reasenberg R D 2005 "Tracking Frequency Laser Distance Gauge," Review of 
Scientific Instruments, 76, 064501 
[4] Lay 0 P, Dubovitsky S, Peters R D, Burger J P, Ahn SoW, Steier W H, Fettennan H R and 
Chang Y 2003 "MSTAR: a submicrometer absolute metrology system", Opt. Lett. 28, 890 
[5] Mozurkewich D, Carpenter K G and Lyon R G 2007 "Beam Combiner for Stellar Imager and its 
Application to Full Aperture Imaging", in UVIOpticallIR Space Telescopes: Innovative 
Technologies and Concepts III, Proceedings of the SPIE, 6687, 66870K 
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