Anyone who has ever seen aircraft involved in flight training can appreciate the feat of remaining highly synchronized while in the air. In a job sponsored by NASA's Exoplanet Exploitation Program (EXEP), engineers at the Jet Propulsion Laboratory in Pasadena, Calif., Are taking the training to a new extreme.
His work marks an important milestone within a larger program to test the viability of a technology called starshade. Although starshades have never flown in space, they have the potential to allow for innovative observations of planets beyond our solar system, including images of planets as small as Earth.
A future starshade mission would involve two spacecraft. One would be a space telescope in search of planets orbiting stars outside our solar system. The other spacecraft would fly about 40,000 kilometers ahead, carrying a large, flat shadow. The shadow unfolded like a blossoming flower – complete with "petals" – and blocked the light of a star, allowing the telescope to have a clearer view of any planet in orbit. But it would only work if the two spacecrafts remained, despite the great distance between them, aligned less than a meter apart. Once again, starlight would leap around the star-of-horizon to the telescope's view and overload weak exoplanets.
"The distances we're talking about for the starshade technology are hard to imagine," said JPL engineer Michael Bottom. "If the starshade were reduced to the size of a roller coaster, the telescope would be the size of a pencil eraser and they would be separated by about 100 kilometers. [100 kilometers]. Now imagine these two objects floating freely in space. Both are experiencing these small pulls and pokes of gravity and other forces, and along that distance we are trying to keep the two aligned accurately within about 2 millimeters. "
Researchers have found thousands of exoplanets without the use of a starshade, but in most cases 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 star brightness. Only in relatively few cases have scientists captured direct images of exoplanets.
Blocking starlight is the key to making more direct images and eventually to conducting in-depth studies of planetary atmospheres or to finding clues about the surface characteristics of the rocky worlds. Such studies have the potential to reveal signs of life beyond Earth for the first time.
Looking for Shade
The idea of using a space-based starshade to study exoplanets was first proposed in the 1960s, four decades before the discovery of the first exoplanets. And while the ability to point a spacecraft continuously to a distant object is not new either, keeping two spacecrafts aligned with each other toward a background object represents a different type of challenge.
Researchers working with Starshade Technology Development of ExEP, known as S5, were commissioned by NASA to develop starshade technology for possible future missions of space telescopes. The S5 team is dealing with three technology gaps that would need to be closed before a starshade mission could be ready to go into space.
Bottom's work and JPL's fellow engineer, Thibault Flinois, closes one of those gaps by confirming that engineers could realistically produce a starshade mission that meets these stringent requirements for "training detection and control." Its findings are described in the Milestone 4 S5 report, available on the ExEP website.
The details of a particular starshade mission – including the exact distance between the two spacecraft and the size of the shadow – would depend on the size of the telescope. The S5 Milestone 4 report mainly considered a separation range of between 12,000 to 25,000 miles (20,000 to 40,000 kilometers), with a shadow of 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 set of primary mirrors of 2.4 meters in diameter to be launched in the mid-2020s.
WFIRST will carry a different star-blocking technology, called coronagraph, which lies inside the telescope and offers its own unique forces in the study of exoplanets. This technology demonstration will be the first high-contrast stellar coronagraph to go into space, allowing WFIRST the direct image of giant exoplanets similar to Neptune and Jupiter.
The starshade and coronagraph technologies work separately, but Bottom has tested a technique by which WFIRST could detect when a hypothetical star star has subtly deviated from alignment. A small amount of starlight would inevitably bend around the star and form a light, dark pattern in front of the telescope. The telescope would see the pattern using a pupil camera, which can view the front of the telescope from within – similar to photographing a windshield inside a car.
Earlier research on starshade had considered this approach, but Bottom made it a reality by building a computer program that could recognize when the pattern of light and dark was centered on the telescope and when it moved away from the center. The background found that the technique works very well as a way to detect the movement of the starshade.
"We can feel a shift in the starshade position up to an inch, even over those huge distances," Bottom said.
But detecting when the starshade is out of alignment is a completely different proposition than actually keeping it aligned. To that end, Flinois and his colleagues developed a set of algorithms that use information provided by the Bottom program to determine when the starshade impellers should fire to bring it back into position. The algorithms were created to keep the starshade autonomously in line with the telescope for days on end.
Combined with Bottom work, this shows that keeping the two ships aligned is feasible using automated sensors and propulsion controls. In fact, the work of Bottom and Flinois demonstrates that engineers could reasonably meet the alignment demands of an even larger star (along with a larger telescope), positioned up to 46,000 miles (74,000 kilometers) from the telescope.
"With such a wide variety of scales here, it can be very counterintuitive that this is possible at first glance," said Flinois.
A starshade project has not yet been approved for the flight but one of them could join the WFIRST in space in the late 2020s. Meeting the flight training requirement is just one step in demonstrating that the project is feasible.
"This is a good example of how space technology becomes more and more extraordinary, building on its previous successes," said Phil Willems, NASA Starshade Technology Development manager at NASA. "We used space-flying training every time a capsule docks on the International Space Station, but Michael and Thibault went far beyond that, and they showed a way to keep formation on scales larger than Earth itself."
Exoplanet Exploitation Program (ExEP)
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