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Progress on Starshade Alignment, Stability

We’re on the cusp of exciting developments in exoplanet detection, as yesterday’s post about the Near Earths in the AlphaCen Region (NEAR) effort makes clear. Adapting and extending the VISIR instrument at the European Southern Observatory’s Very Large Telescope in Chile, NEAR has seen first light and wrapped up its first observing run of Centauri A and B. What it finds should have interesting ramifications, for its infrared detection capabilities won’t find anything smaller than twice the size of Earth, meaning a habitable zone discovery might rule out a smaller, more Earth-like world, while a null result leaves that possibility open.

The NEAR effort relies on a coronagraph that screens out as much as possible of the light of individual stars while looking for the thermal signature of a planet. An internal coronagraph is one way to block out starlight (the upcoming WFIRST — Wide Field Infrared Survey Telescope — mission will carry a coronagraph within the telescope), but starshade concepts are also in play for the future. Here we separate the space telescope from the large, flat shade making up a separate spacecraft.

Image: Shown here as a potential mission for pairing with the James Webb Space Telescope but likewise applicable to WFIRST, a starshade is a separate spacecraft that blocks out light from the parent star, allowing the exoplanet under scrutiny to be revealed. Credit: University of Colorado/Northrup Grumman.

I’ve been fascinated with starshades ever since learning of the concept through Webster Cash’s work at the University of Colorado Boulder, and discussing with him the possibility of actually imaging distant exoplanets sharply enough to make out weather patterns and continents. But before we get to anything that ambitious, we have to clear the early hurdles, which are numerous. One of them, a big one, is the problem of distance and spacecraft orientation.

Consider: What NASA is looking at right now through its Exoplanet Exploration Program (ExEP) in an effort known as S5 is a pair of spacecraft separated by 20,000 to 40,000 kilometers, using a shade 26 meters in diameter. These numbers aren’t chosen arbitrarily — they mesh with the WFIRST telescope and its 2.4-meter diameter primary mirror, to be launched in the mid-2020s, although a recent report notes that the work is ‘relevant for any roughly 2.4-m space telescope operating at L2.’ As I mentioned above, WFIRST will carry its own coronagraph, but because a starshade is a separate spacecraft, one could join WFIRST in space by the end of the 2020s.

Ashley Baldwin has written extensively about starshades for Centauri Dreams, as a search in the archives will reveal (but start with WFIRST: The Starshade Option). Any consideration of starshades notes the problems to be solved here, as JPL engineer Michael Bottom explains in terms of his work on starshade feasibility for ExEP:

“The distances we’re talking about for the starshade technology are kind of hard to imagine. If the starshade were scaled down to the size of a drink coaster, the telescope would be the size of a pencil eraser and they’d be separated by about 60 miles [100 kilometers]. Now imagine those two objects are free-floating in space. They’re both experiencing these little tugs and nudges from gravity and other forces, and over that distance we’re trying to keep them both precisely aligned to within about 2 millimeters.”

Image: Three views of a starshade. Credit: NASA / Exoplanet Exploration Program.

The S5 team has been working on the technology gaps that have to be closed to allow such a mission to fly given the demands of formation sensing and control. Bottom has come up with a computer program that addresses the issue of spacecraft drifting out of alignment. As discussed in the recent ExoTAC Report on Starshade S5 Milestone #4 Review (‘Milestone #4’ refers to lateral formation sensing and control of the starshade position), a telescope modeled on WFIRST would see the pattern of starlight as it bent around the starshade, a subtle pattern of light and dark that would flag drift down to an inch and less at these distances.

Using algorithms developed by JPL colleague Thibault Flinois, Bottom’s program can sense when the firing of starshade thrusters is needed to return to proper alignment, making this delicate formation flying feasible through automated sensors and thruster controls. It’s also heartening to learn that Bottom and Flinois can demonstrate meeting the alignment demands of larger starshades for future missions, positioned fully 74,000 kilometers from the telescope.

NASA’s starshade technologies became more tightly focused starting in 2016 through a proposal from the ExEP, which anticipated bringing the concept to Technical Readiness Level 5; this is the S5 effort. To put that in context, here is NASA’s overview of what TRL 5 means:

Once the proof-of-concept technology is ready, the technology advances to TRL 4. During TRL 4, multiple component pieces are tested with one another. TRL 5 is a continuation of TRL 4, however, a technology that is at 5 is identified as a breadboard technology and must undergo more rigorous testing than technology that is only at TRL 4. Simulations should be run in environments that are as close to realistic as possible. Once the testing of TRL 5 is complete, a technology may advance to TRL 6. A TRL 6 technology has a fully functional prototype or representational model.

Image: Starshade technology gaps. Credit: NASA / Exoplanet Exploration Program.

There is so much to be analyzed in the starshade concept, from optimal starlight suppression to the stability of the starshade shape and its accuracy in deployment and necessary maneuvering. All of that has to take place within the framework of the above formation sensing and control issues. But Bottom and Flinois’ work has clearly moved the ball. From the report:

Overall, the ExoTAC believes that Milestone #4 has been fully met and congratulates the entire team on their excellent efforts to advance the technology readiness levels of the elements in the S5 activity. Precision lateral control over thousands of kilometers is an unprecedented requirement, and essential for starshade operation. Achieving this first of fifteen S5 Milestones serves as a confidence builder for the entire S5 activity.

We also note that by virtue of the successful achievement of Milestone #4, the Exoplanet Exploration Program’s Technology Gap List item S-3 on “Lateral Formation Sensing” is Retired.

For more on the NASA starshade work, see Starshade Technology Development.

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{ 11 comments… add one }
  • Gary Wilson June 12, 2019, 16:33

    Doesn’t another Kepler type mission but with a bigger telescope make more sense? The star shade concept seems to add a level of complexity that may not be worthwhile. Is the payoff great enough to accept the added risk of failure?

  • Michael Fidler June 12, 2019, 19:24

    What about the new atomic clocks and
    laser transmitters on all the tips to get phase level optical interferometry distances to the 1/8 wavelength of light. Add some quantum memory and ion thrusters for stability.

  • Gary Wilson June 12, 2019, 21:29

    Thank you Paul. That clarifies the situation a great deal. The requirement for an accuracy of 2mm for the starshade placement drove home the requirement for extreme accuracy. I can see the huge benefit of being able to capture spectra of earth-like exoplanets however. That would be a tremendous leap forward.

    • Paul Gilster June 13, 2019, 7:21

      It’s extraordinary to me that the kind of accuracy called for by starshades is possible, but this new work shows it’s feasible. What an accomplishment this kind of observation would be!

  • Roger June 13, 2019, 9:36

    “Consider: What NASA is looking at right now through its Exoplanet Exploration Program (ExEP) in an effort known as S5 is a pair of spacecraft separated by 20,000 to 40,000 kilometers, using a shade 26 meters in diameter. These numbers aren’t chosen arbitrarily — they mesh with the WFIRST telescope and its 2.4-meter diameter primary mirror…”
    “The distances we’re talking about for the starshade technology are kind of hard to imagine. If the starshade were scaled down to the size of a drink coaster, the telescope would be the size of a pencil eraser and they’d be separated by about 60 miles [100 kilometers]. Now imagine those two objects are free-floating in space. They’re both experiencing these little tugs and nudges from gravity and other forces, and over that distance we’re trying to keep them both precisely aligned to within about 2 millimeters.”

    So alignment at full scale is to within 40-80cm?

  • Harry R Ray June 13, 2019, 10:31

    I am extremely pleased to see the progress being made on Starshade. With LUVOIR in jeapordy, Starshade may be the only hope to detect biosignatures in the atmospheres of Earth-like planets orbiting Sun-like stars in the near future. However, I am always looking for the NEXT BIG THING! Enter Nautilus. ArXiv 1906.05079 “A Thousand Earths: A Very Large Aperture Ultralight Telescope Array for Atmospheric Biosignature Surveys.” by Daniel Apai, Tom D Minster, Dae Wok Kim, Alex Bixel, Glenn Sheridan, Rongvong Liang, Jonathan Armstrong. KEY QUOTE FROM THE ABSTRACT: “The mirrors typical to current space telescopes are replaces by MODE lenses with a 10 times lighter areal density that are 100 times less sensitive to misalignments…”. The Planet Finder on steroids! Paul Gilster: The PDF is a must read!

    • ljk June 14, 2019, 11:39

      https://arxiv.org/abs/1906.05079v1

      A Thousand Earths: A Very Large Aperture, Ultralight Space Telescope Array for Atmospheric Biosignature Surveys

      Daniel Apai, Tom D. Milster, Dae Wook Kim, Alex Bixel, Glenn Schneider, Ronguang Liang (University of Arizona), Jonathan Arenberg (Northrop-Grumman Aerospace Systems)

      (Submitted on 12 Jun 2019)

      An outstanding, multi-disciplinary goal of modern science is the study of the diversity of potentially Earth-like planets and the search for life in them. This goal requires a bold new generation of space telescopes, but even the most ambitious designs yet hope to characterize several dozen potentially habitable planets. Such a sample may be too small to truly understand the complexity of exo-earths.

      We describe here a notional concept for a novel space observatory designed to characterize 1,000 transiting exo-earth candidates. The Nautilus concept is based on an array of inflatable spacecraft carrying very large diameter (8.5m), very low-weight, multi-order diffractive optical elements (MODE lenses) as light-collecting elements.

      The mirrors typical to current space telescopes are replaced by MODE lenses with a 10 times lighter areal density that are 100 times less sensitive to misalignments, enabling light-weight structure. MODE lenses can be cost-effectively replicated through molding.

      The Nautilus mission concept has a potential to greatly reduce fabrication and launch costs, and mission risks compared to the current space telescope paradigm through replicated components and identical, light-weight unit telescopes.

      Nautilus is designed to survey transiting exo-earths for biosignatures up to a distance of 300 pc, enabling a rigorous statistical exploration of the frequency and properties of life-bearing planets and the diversity of exo-earths.

      Comments: Accepted in the Astronomical Journal. 28 pages, 12 figures. More info on the project website: http://nautilus-array.space

      Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)

      Cite as: arXiv:1906.05079 [astro-ph.IM]
      (or arXiv:1906.05079v1 [astro-ph.IM] for this version)

      Submission history

      From: Daniel Apai Dr [view email]

      [v1] Wed, 12 Jun 2019 12:17:27 UTC (12,973 KB)

      https://arxiv.org/pdf/1906.05079v1.pdf

  • Brad in Dallas June 13, 2019, 13:46

    I may be missing something obvious, but with such distances and such precision, the telescope-starshade pair couldn’t be pointed to targets, right? They’d have to wait for targets to pass their field of view. With Alpha Centauri well south of the galactic plane, a pair aimed there couldn’t easily also pick up Sirius, Barnard’s star, Epsilon Eridani, etc., right? At best it would require lengthy periods of maneuver, realignment and nulling out vibrations between observations. Not a show stopper, but costly it would seem.

    • James Webb (no, not THAT one) June 13, 2019, 17:18

      I was wondering about that myself. Otherwise it’s a single-target telescope. I assume some form of tracking will be needed; which of the pair will move or will it be both? I hope they have a good supply of reaction wheels!

      • Ashley Baldwin) June 14, 2019, 13:46

        There are various proposals by which either the Starshade or the telescope can manoeuvre . On the balance though , the shade moving makes more sense as this can happen whilst the telescope continues to operate minimising downtime . The shade can also have an efficient ion thruster propulsion system powered by solar arrays attached to its extensive surface, rather than as “wings” on a telescope – which could impact on pointing stability . Something that has to be ultra precise for direct exoplanet imaging. A powered shade can also be sent up separately at a later date to operate with an established telescope ( with the required modifications to the telescope itself only minimal and cheap) . Thus staggering any costs. Such is the proposal for WFIRST. A 30-40m Starshade has been costed out around $0.75 billion – less than that of a NASA “Probe” class mission.

        NASA’s Decadel HaBEX telescope / Starshade concept allows for up to a hundred individual exoplanet observations utilising a propulsive Starshade component. With the telescope conducting general astrophysics science in between . Or utilising an internal coronagrogh to identify suitable target exoplanets for the Starshade , so allowing relatively high res spectroscopic analysis thanks to its near 90 % light throughput – versus say just 30 % for the telescope’s coronagraph . A huge difference that matters a lot for a 4m aperture scope.

        The “formation flying” technology required to allow observations between telescope sn occulter separated by tens of thousands of Kms is now quite mature ( laser metrology assisted ) , with the biggest risks arising from diffraction at the occulter’s petal shaped edges and the fact that they cannot be trialled other than in space. There are even proposals for a low budget cubesat version with a 30cm scope in conjunction with a 5m shade prototype, operating in low earth orbit over a few Kms separation distance.

        Personally I don’t think that we should be worrying about starshades just yet . It’s getting the parent telescope up to use it that is the stumbling block at present . What with JWST’s countless delays , manned soaceflight distraction and uncertainty even over WFIRST – let alone any future LUVOIR telescope.

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