Starshade Could Help Photograph Distant Planets
As a planet orbits near its sun, the bright stellar light interferes with observations made from Earth. To block the excess light, NASA scientists want to launch into space a large instrument called a starshade that will allow a space telescope to directly image an Earth-sized planet in an Earth-like orbit.
“A starshade removes most of the starlight by blocking it outside the telescope. It creates a shadow much like the Moon does during a solar eclipse,” said Jeremy Kasdin of Princeton University in an email.
Kasdin is the principal investigator of NASA’s starshade project.
“By choosing the size and shape and distance just right, it can block the starlight while letting the planet light pass by.”
A telescope in bloom
Resembling a giant sunflower, a starshade, or external occulter, operates in space separately from the observing telescope. Ideally, the pair would travel in a special area known as the second Lagrangian point, a region 930,000 miles (1.5 million kilometers) from Earth, outside of the orbit of the Moon. As the telescope points toward a distant star, it peers over the sharp edge of the petals, which block the stellar light and allow scientists to directly image distant planets.
Developing the occulter brought a number of engineering challenges. The design had to be created and modified to precisely block the incoming light. This meant developing a material that minimizes the glint from the distant starlight.
Unlike most space-based instruments, the starshade and telescope are two separate pieces that require precise alignment, creating another hurdle.
“We have to design and build a very large deployable starshade that could be controlled to stay in line with the telescope to sub-meter accuracy,” Kasdin said.
At present, engineers are building prototypes of various parts, constantly testing accuracy and deployment. The team recently tested the massive petals to ensure that they lay correctly every time.
When the mission is ready to launch, the 110-foot (34-meter) starshade would fold up small enough to fit into a launch vehicle. Once positioned in an Earth-leading orbit, the 28 massive petals would bloom and the occulter would hold a rigid, unmoving form.
The starshade, which will sit 23,000 miles (37,000 kilometers) from its companion telescope, won’t be able to undergo a simultaneous test of all components while earthbound.
“We can’t do a typical ‘end-to-end’ test. We can’t shine light past it and see a shadow,” Kasdin said. “However, we can do a lot of other tests to confirm it works.”
Such tests include measuring all of the key properties such as the shape, deployment accuracy, and thermal stability. Tests on the individual subsystems will also be performed. According to Kasdin, this approach has been used quite often on other space systems.
“Sometimes you simply can’t fully test something without going into space,” he said.
Seeking the holy grail
NASA’s Kepler Telescope spotted the dips in starlight that occur when a planet moved between Earth and its sun. The slight dimming allowed scientists to calculate a number of properties about the planet, but was not a direct observation. The upcoming James Webb telescope will work similarly.
A telescope linked to a starshade, however, should be able to directly photograph a distant planet, which will lead to more in-depth studies about the distant body.
“We need to find efficient ways to study that very small amount of light, so we can say something about whether there might be water on that planet, carbon dioxide, or other important atmospheric gases,” said Margaret Turnbull, of the Global Science Institute in Wisconsin, in an email.
“When we have that kind of information, we will be able to make statements about whether such a planet could be habitable for life as we know it on Earth.”
She went on to call the starshade “a mission that is seeking the ‘holy grail’ of exo-Earth detection.”
In its first 18 months, the starshade should enable its telescope to study 55 stars. Its precision will allow its telescope to detect Jupiter-sized planets around all the stars, with the possibility of detecting Earth-analog planets around 22 others. The remaining 18 months would allow the pairing to revisit stars and obtain more detailed observations.
The starshade serves as one part of a two-piece mission, with the telescope forming the other crucial half. The instrument can work with any telescope, though high-quality optics will capture the best data. Because the starshade filters out the light before it even reaches the optical device, it can work with simpler telescopes that omit several technical components, as well as instruments smaller than even the Hubble Space Telescope, said Kasdin.
In addition to imaging far-off planets, the telescope flown with the starshade should be able to perform other sorts of astronomical research. As the observing target changes, the starshade will use thrusters to shift its position, a process that can take up to two weeks. Rather than sitting idle, the adjacent telescope could perform research beyond planet searching.
“With the starshade moving from target to target, about half the mission lifetime would be available for general astrophysics science programs,” Turnbull said.
A starshade also has the possibility to improve planned missions, such as NASA’s Wide-Field Infrared Survey Telescope (WFIRST), a proposed observatory that would hunt for and directly image planets outside the Solar System, as well as search for dark matter.
“If a starshade could be flown with, say, WFIRST, then that would completely change the landscape and allow for incredible science both in exoplanets and many, many other areas of astrophysics,” Turnbull said.
The time required to reposition the starshade is one of its biggest drawbacks. The mission would also be restricted by the amount of fuel carried by the occulter.
“We have to design an incredibly efficient path across the sky that intersects the most interesting targets and uses the least amount of fuel,” Turnbull said. “Sometimes that means passing up one interesting target in order not to lose other interesting targets.”
However, Turnbull thinks the science generated by such a mission would reap rewards beyond the scientific data.
“When the public can get excited about a NASA mission seeking something as relatable and tangible as another Earth-like planet, then that benefits the field as a whole, including research efforts that might seem more esoteric to the public,” she said.