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.