Author Topic: Starshades: occulters for space telescopes  (Read 6610 times)

Offline Archibald

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Starshades: occulters for space telescopes
« on: 09/06/2018 06:58 pm »
As the title says.

The basic concept is simple: shut down the light of a star to unmask the dim exoplanets around it. Execution, of course, is far, far more complicated !

A lot of work has been done by Webster C. Cash

In the 90's it was called BOSS
In the 2000 it was called either Umbras or New Worlds Observer

There is a dated but extensive website
http://www.umbras.org/

I link a whole bunch of papers I collected over the years.

Han shot first and Gwynne Shotwell !

Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #1 on: 09/06/2018 07:04 pm »
Here is the paper from CNES, in 1985 that got the right shape for the starshade: a sunflower !
Han shot first and Gwynne Shotwell !

Offline jbenton

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Re: Starshades: occulters for space telescopes
« Reply #2 on: 09/06/2018 07:29 pm »
So glad to see a thread dedicated to this concept.

Here are some videos to help you visualize it, if you're not familiar with the concept:







Thanks Archibald for the papers. I always thought that this was a new concept, it's somehow satisfying to know that I was wrong. Obviously, no one can tell when exactly this will fly, but when it does, there's just something that feels special about seeing the launch of something that scientists have wanted for decades.

Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #3 on: 09/07/2018 06:01 am »
I always enjoyed that concept for the sheer beauty and poetry of it. Masking a star with a sunflower to unveil the planets - this has some definite Saint Exupéry vibe (The little prince would love the idea)
« Last Edit: 09/07/2018 07:36 am by Archibald »
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Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #4 on: 09/07/2018 07:16 am »
BOSS - Big Occulting Steerable Satellite - was a pretty interesting idea. That occulter could work with JWST, both being at SEL-2.

There was also a pretty cool alternative. The BOSS occulter would be send into a carefully calculated high Earth orbit. That orbit would allow the BOSS starshade to work with any ground based telescope - all the way from Keck to amateur astronomy telescopes !

So, JWST can have a starshade, and even Keck can have one, so why not Hubble, which stands right between these two, in LEO ?

From what I could dug (mostly from Google books fragments here and there) a Hubble starshade would be pretty hard or even near impossible.

Yet the concept of a Hubble starshade was so exciting, it was nonetheless discussed decades before the space telescope was in orbit.

The reason why it was discussed in the first place related to Hubble mirror shrinking from 120 inch to 94 inch, in 1975. From the inception of the Space Telescope project in the 60's, a lot of astronomers wondered "how good will Hubble be when chasing exoplanets ?"

The answer was that direct imaging with Hubble was impossible, the mirror was way too small - even at 120 inch. From memory, it would take, even out of Earth atmosphere, a 7 m to 20 m+ mirror for direct imaging. Of course, if the light from the star could be masked or dimmed, then even Hubble could do the job. Hence discussions of a starshade.

What was found was the tricky thing relates to the distance between Earth and the closest stars. It seems to work that way:
to unmask an exoplanet at 5 light years with a 100 m wide starshade and Hubble too small mirror of 94-inch, then the starshade must be between 50 000 km to 100 000 km above Hubble.Hubble is in LEO and orbit Earth very, very fast, and the starshade in its (much slower) high Earth orbit simply can't keep up with Hubble. That's the big hassle that explains why a Hubble starshade was only sparcely discussed.

It seems that Nancy Roman and Lyman Spitzer had already done the math decades before Hubble was in orbit (in the 60's, starshades were already discussed)  so they proposed an even more fascinating idea: to use the Moon as a giant occulter, for free ! Once again however, the orbits are not really compatible.

So they started from Hubble and tried different locations for it. By the mid-70's Marshall was flying Stratoscope, a telescope-mounted balloon. The general consensus is that, in order to use the Moon as occulter, or to get an artificial starshade, Hubble would have either to fly lower / slower (suspended to a balloon !) or higher than LEO (high Earth orbit or cislunar space).

Then when you think about it... these two options, are, well, the two BOSS concepts discussed at the beginning of this post. Back to square one ! Hubble-on-a-balloon is essentially the Keck starshade; and a cislunar Hubble is, well, closer from JWST. And that bring us back to the Hubble starshade main issue: in LEO, it doesn't work.
« Last Edit: 09/07/2018 07:30 am by Archibald »
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Online jebbo

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Re: Starshades: occulters for space telescopes
« Reply #5 on: 09/07/2018 07:19 am »
Glad you made a thread for this - a good topic.

I think wide-separation starshades really highlight the need for us to know where to look - manoeuvring and coordination with telescope pointing is non-trivial.

--- Tony

Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #6 on: 09/07/2018 07:33 am »
You nailed it well. It seems to be a very tricky thing unless both are in similar halo orbits around a libration point, either an Earth-Moon or a Sun-Earth one.
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Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #7 on: 09/07/2018 10:07 am »
Starshade costs may be capped by the cost of using an actual spacecraft to do it.
If you can purchase for $150M the right to use a BFS for most of the time, a thirty meter starshade is a lot easier to make if you can fold it twice, roll it up, and make it out of quarter inch aluminium and position it by a bolt to the airlock.

If you could leverage StarLink, or other projects with inexpensive satellite busses, deploying a comparatively large number in one bulk launch where they sit over thirty targets of interest 'near' the scope and do not manoever would also be an interesting possibility.

Offline TrevorMonty

Re: Starshades: occulters for space telescopes
« Reply #8 on: 09/07/2018 05:33 pm »
Starshades are prime candidates for inspace manufacturing. NASA and private companies are working on technology to 3D print and assemble structures like radio disk antenna, sunshades, large solar arrays and telescopes. Starshades are one of easier structures to test this technology on as they are  just light weight framing with plastic cover.


See Made In Space and Tethers Unlimited websites for more info.
« Last Edit: 09/07/2018 05:35 pm by TrevorMonty »

Offline Nomadd

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Re: Starshades: occulters for space telescopes
« Reply #9 on: 09/07/2018 07:50 pm »
Starshade costs may be capped by the cost of using an actual spacecraft to do it.
If you can...

If you could...


Uh, look, no. If you take two low probability numbers and multiply them by each other, you get a lower number, not a higher one. So "if" times "if" makes something less likely. Take the BFS magical thinking to the threads where it already flourishes.


Out of 37,000 odd members in this forum there are none that have more credibility than you. But when it comes to anything SpaceX, I often can't even understand what you're saying. (Although, I do agree a BFS starshade doesn't make much sense)
« Last Edit: 09/07/2018 08:05 pm by Nomadd »
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Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #10 on: 09/07/2018 08:02 pm »
Uh, look, no. If you take two low probability numbers and multiply them by each other, you get a lower number, not a higher one. So "if" times "if" makes something less likely. Take the BFS magical thinking to the threads where it already flourishes.
If we are talking about starshades out in the late 2020s/early 2030s, do you really believe it's very unlikely we will have anything better than todays launchers?

Offline ChrisWilson68

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Re: Starshades: occulters for space telescopes
« Reply #11 on: 09/07/2018 08:17 pm »
Starshade costs may be capped by the cost of using an actual spacecraft to do it.
If you can...

If you could...


Uh, look, no. If you take two low probability numbers and multiply them by each other, you get a lower number, not a higher one. So "if" times "if" makes something less likely. Take the BFS magical thinking to the threads where it already flourishes.

The idea that BFS and Starlink are so unlikely to exist that you feel the need to rudely chide someone for even mentioning it seems to me more worthy of the term "magical thinking".

Offline Blackstar

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Re: Starshades: occulters for space telescopes
« Reply #12 on: 09/07/2018 08:20 pm »
Starshade costs may be capped by the cost of using an actual spacecraft to do it.
If you can...

If you could...


Uh, look, no. If you take two low probability numbers and multiply them by each other, you get a lower number, not a higher one. So "if" times "if" makes something less likely. Take the BFS magical thinking to the threads where it already flourishes.


Out of 37,000 odd members in this forum there are none that have more credibility than you. But when it comes to anything SpaceX, I often can't even understand what you're saying. (Although, I do agree a BFS starshade doesn't make much sense)

I'll put it very simply: when talking about a complicated piece of technology, claiming that it will be "made easier" by another piece of promised future technology is not credible. If you want something to work, you have to either directly develop the technology to make it work, or you have to base it on existing proven technologies. So "unicorn times pumpkin carriage" doesn't get you to the ball, but "horse times cart" does.

And anybody who puts their faith in SpaceX ought to be a little unsettled by the current meltdown of its founder and driving force, who seems to be heading into Howard Hughes territory now.

Offline matthewkantar

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Re: Starshades: occulters for space telescopes
« Reply #13 on: 09/07/2018 09:29 pm »
You mean legendary aerospace executive who amasses multigenerational fortune?

Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #14 on: 09/07/2018 10:03 pm »
when talking about a complicated piece of technology, claiming that it will be "made easier" by another piece of promised future technology is not credible.
A starshade, positioned by something else, is not actually a hugely complicated bit of technology.
It is a very opaque thing, with no particular requirements other than the edges being reasonably not blunt (100um or so), of approximately the right form and diameter. The standoff distance makes this enormously easier than on-instrument coronagraphs or shades.

Adding requirements  makes it hard.
For example, if it's got to fold into a space a hundredth of its area, and unfold multiply and weigh 200kg as you've spent the rest of the mass budget on an ion engine and bus to get it and keep it there, then it's hard.


Offline Lar

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Re: Starshades: occulters for space telescopes
« Reply #15 on: 09/08/2018 02:46 am »
when talking about a complicated piece of technology, claiming that it will be "made easier" by another piece of promised future technology is not credible.
A starshade, positioned by something else, is not actually a hugely complicated bit of technology.
It is a very opaque thing, with no particular requirements other than the edges being reasonably not blunt (100um or so), of approximately the right form and diameter. The standoff distance makes this enormously easier than on-instrument coronagraphs or shades.

Adding requirements  makes it hard.
For example, if it's got to fold into a space a hundredth of its area, and unfold multiply and weigh 200kg as you've spent the rest of the mass budget on an ion engine and bus to get it and keep it there, then it's hard.



But increasing the mass budget for the same launch cost? ...won't reduce costs one cent. Nor will it decrease the complexity at all. Everyone knows that. To think otherwise is magical thinking.

</sarcasm>

« Last Edit: 09/08/2018 02:47 am by Lar »
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Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #16 on: 09/08/2018 01:31 pm »
The mods are under way...
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Offline orbitjunkie

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Re: Starshades: occulters for space telescopes
« Reply #17 on: 09/08/2018 01:54 pm »
As an aerospace professional who has personally done some analysis on this concept in the past, it is an engineering challenge but absolutely a solvable one. There is serious work being done right now at NASA Goddard among other places on the possibility of adding this to WFIRST. Precursor tech demos are already happening. https://www.nasa.gov/feature/goddard/2016/nasa-engineer-awaits-launch-of-cubesat-mission-demonstrating-virtual-telescope-tech

The delta-V required for an occulter in a libration orbit can be high (but manageable) and ion engines or solar sailing can help. In my opinion this is now a when, not if, question. Of course the if depends on adequate prioritization and funding...

Offline Nomadd

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Re: Starshades: occulters for space telescopes
« Reply #18 on: 09/08/2018 02:18 pm »
 I'm probably missing something embarrassingly obvious here, but won't the sun lighting up one of these pretty, golden starshades be worse than the star it's blocking?
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Offline orbitjunkie

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Re: Starshades: occulters for space telescopes
« Reply #19 on: 09/08/2018 03:19 pm »
I'm not an optics expert but I think that can be mitigated to some extent with materials, coatings, spectral filters, etc...

But it's surely got some impact, as does the solar radiation pressure on this huge, lightweight structure when you are trying to maintain extremely precise alignment with a telescope thousands of kilometres away. Not all observation geometries will be favorable or feasible at all times.

What does that tell you about the operational constrains of such a system?

Offline Archibald

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Re: Starshades: occulters for space telescopes
« Reply #20 on: 09/08/2018 04:10 pm »
As an aerospace professional who has personally done some analysis on this concept in the past, it is an engineering challenge but absolutely a solvable one. There is serious work being done right now at NASA Goddard among other places on the possibility of adding this to WFIRST. Precursor tech demos are already happening. https://www.nasa.gov/feature/goddard/2016/nasa-engineer-awaits-launch-of-cubesat-mission-demonstrating-virtual-telescope-tech

The delta-V required for an occulter in a libration orbit can be high (but manageable) and ion engines or solar sailing can help. In my opinion this is now a when, not if, question. Of course the if depends on adequate prioritization and funding...

Provided JWST does not eat WFIRST budget for supper. Nice to see the latest development in the concept.

Quote
as does the solar radiation pressure on this huge, lightweight structure when you are trying to maintain extremely precise alignment with a telescope thousands of kilometres away. Not all observation geometries will be favorable or feasible at all times.

The flip side of solar sails... no, I'm not a solar sail, I'm a starshade, please don't push me away...
« Last Edit: 09/08/2018 04:11 pm by Archibald »
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Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #21 on: 09/08/2018 07:35 pm »
I'm probably missing something embarrassingly obvious here, but won't the sun lighting up one of these pretty, golden starshades be worse than the star it's blocking?
The sun does not light the telescope-side up unless it is pointing away from the sun, meaning you have a hemisphere you can observe in,with the keep-out zone of pointing at the sun in addition of course.

Making the surface reflective means that 'all' of the suns light goes into a 3 degree or so cone, meaning that you can observe over most of the sky.
An additional shade pointed almost at the telescope but canted sunwards would let you remove illumination if this was desired.

Offline jbenton

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Re: Starshades: occulters for space telescopes
« Reply #22 on: 09/10/2018 06:22 am »
Is there any advantage of using a starshade with a telescope that has a coronagraph? (I mean to say using the internal coronagraph and the external starshade together)

I think I read mention of that awhile back ago, but it seems to me that having a starshade would defeat the purpose of having a coronagraph.
« Last Edit: 01/23/2020 10:38 pm by jbenton »

Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #23 on: 09/10/2018 09:49 am »
Is there any advantage of using a starshade with a telescope that has a coronagraph? (I mean to say using the internal coronagraph and the external starshade together.

I think I read mention of that awhile back ago, but it seems to me that having a starshade would defeat the purpose of having a coronagraph.
Yes.
Coronographs are terrible performers in comparison to starshades. (for comparable effort in design).
But, starshades are very hard to position, and you're not going to be doing twenty observations a day of different targets with them.

An internal coronograph means you can do limited amounts of the same sort of science without spending the fuel on the starshade.
The only thing that springs to mind that the two together can usefully do is observation of planets around close binaries, or ones with a bright background star very nearby.

Xray star-shields seem like an interesting idea that have no hope of being launched by conventional scopes.

Offline ncb1397

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Re: Starshades: occulters for space telescopes
« Reply #24 on: 09/10/2018 03:15 pm »
Is there any advantage of using a starshade with a telescope that has a coronagraph? (I mean to say using the internal coronagraph and the external starshade together.

I think I read mention of that awhile back ago, but it seems to me that having a starshade would defeat the purpose of having a coronagraph.

For HabEx, the coronograph will do a survey to pick starshade targets. You can detect planets with the coronograph and then choose habitable zone terrestrial planets or whatever else you are interested in for follow up.

Offline jbenton

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Re: Starshades: occulters for space telescopes
« Reply #25 on: 09/16/2018 08:48 am »
Stupid question:

Why are standard chronographs disk-shaped, rather than sunflower-shaped?

Offline speedevil

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Re: Starshades: occulters for space telescopes
« Reply #26 on: 09/16/2018 10:20 am »
Stupid question:

Why are standard chronographs disk-shaped, rather than sunflower-shaped?
The scattered light that in a starshade you don't care about as it all misses the telescope now all hits the inside of your telescope and you have to absorb it.
The diffraction pattern is also more complex if you're near to the disk, and will require more complex adaptive optics and cancellation.

Offline Star One

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Re: Starshades: occulters for space telescopes
« Reply #27 on: 06/14/2019 05:42 pm »
Starshade Would Take Formation Flying to Extremes

Anyone who's ever seen aircraft engaged in formation flying can appreciate the feat of staying highly synchronized while airborne. In work sponsored by NASA's Exoplanet Exploration Program (ExEP), engineers at the Jet Propulsion Laboratory in Pasadena, California, are taking formation flying to a new extreme.

Their work marks an important milestone within a larger program to test the feasibility of a technology called a starshade. Although starshades have never flown in space, they hold the potential to enable groundbreaking observations of planets beyond our solar system, including pictures of planets as small as Earth.

A future starshade mission would involve two spacecraft. One would be a space telescope on the hunt for planets orbiting stars outside of our solar system. The other spacecraft would fly some 25,000 miles (40,000 kilometers) in front of it, carrying a large, flat shade. The shade would unfurl like a blooming flower — complete with "petals" — and block the light from a star, allowing the telescope to get a clearer glimpse of any orbiting planets. But it would work only if the two spacecraft were to stay, despite the great distance between them, aligned to within 3 feet (1 meter) of each other. Any more, and starlight would leak around the starshade into the telescope's view and overwhelm faint exoplanets.

"The distances we're talking about for the starshade technology are kind of hard to imagine," said JPL engineer Michael Bottom. "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."

Researchers have found thousands of exoplanets without the use of a starshade, but in most instances scientists have discovered these worlds indirectly. The transit method, for example, detects the presence of a planet as it passes in front of its parent star and causes a temporary drop in the star's brightness. Only in relatively few cases have scientists taken direct images of exoplanets.

Blocking out starlight is key to performing more direct imaging and, eventually, to carrying out in-depth studies of planetary atmospheres or finding hints about the surface features of rocky worlds. Such studies have the potential to reveal signs of life beyond Earth for the first time.

Seeking Shade

The idea of using a space-based starshade to study exoplanets was initially proposed in the 1960s, four decades before the discovery of the first exoplanets. And while the ability to point a single spacecraft steadily at a distant object is not new, either, keeping two spacecraft aligned with each other toward a background object represents a different kind of challenge.

Researchers working on ExEP's Starshade Technology Development, known as S5, have been tasked by NASA with developing starshade technology for possible future space telescope missions. The S5 team is addressing three technology gaps that would need to be closed before a starshade mission could be ready to go to space.

The work done by Bottom and fellow JPL engineer Thibault Flinois closes one of those gaps by confirming that engineers could realistically produce a starshade mission that met these stringent "formation sensing and control" requirements. Their results are described in the S5 Milestone 4 report, available on the ExEP website.

Get Into Formation

The specifics of a particular starshade mission — including the exact distance between the two spacecraft and the size of the shade — would depend on the size of the telescope. The S5 Milestone 4 report looked primarily at a separation range of between 12,500 to 25,000 miles (20,000 to 40,000 kilometers), with a shade 85 feet (26 meters) in diameter. These parameters would work for a mission the size of NASA's Wide Field Infrared Survey Telescope (WFIRST), a telescope with a 2.4-meter-diameter primary mirror set to launch in the mid-2020s.

WFIRST will carry a different starlight-blocking technology, called a coronagraph, that sits inside the telescope and offers its own unique strengths in the study of exoplanets. This technology demonstration will be the first high-contrast stellar coronagraph to go into space, enabling WFIRST to directly image giant exoplanets similar to Neptune and Jupiter.

Starshade and coronagraph technologies work separately, but Bottom tested a technique by which WFIRST could detect when a hypothetical starshade drifted subtly out of alignment. A small amount of starlight would inevitably bend around the starshade and form a light-and-dark pattern on the front of the telescope. The telescope would see the pattern by using a pupil camera, which can image the front of the telescope from inside — akin to photographing a windshield from inside a car.

Previous starshade investigations had considered this approach, but Bottom made it a reality by building a computer program that could recognize when the light-and-dark pattern was centered on the telescope and when it had drifted off-center. Bottom found that the technique works extremely well as a way to detect the starshade's movement.

"We can sense a change in the position of the starshade down to an inch, even over these huge distances," Bottom said.

But detecting when the starshade is out of alignment is an entirely different proposition from actually keeping it aligned. To that end, Flinois and his colleagues developed a set of algorithms that use information provided by Bottom's program to determine when the starshade thrusters should fire to nudge it back into position. The algorithms were created to autonomously keep the starshade aligned with the telescope for days at a time.

Combined with Bottom's work, this shows that keeping the two spacecraft aligned is feasible using automated sensors and thruster controls. In fact, the work by Bottom and Flinois demonstrates that engineers could reasonably meet the alignment demands of an even larger starshade (in conjunction with a larger telescope), positioned up to 46,000 miles (74,000 kilometers) from the telescope.

"With such an unusually large range of scales at play here, it can be very counterintuitive that this would be possible at first glance," Flinois said.

A starshade project has not yet been approved for flight, but one could potentially join WFIRST in space in the late 2020s. Meeting the formation-flying requirement is just one step toward demonstrating that the project is feasible.

"This to me is a fine example of how space technology becomes ever more extraordinary by building upon its prior successes," said Phil Willems, manager of NASA's Starshade Technology Development activity. "We use formation flying in space every time a capsule docks at the International Space Station. But Michael and Thibault have gone far beyond that, and shown a way to maintain formation over scales larger than Earth itself."

JPL manages ExEP for NASA's Astrophysics Division.

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