A PALE blue dot. That's what Earth looked like from the Voyager 1 space probe in 1990. But if Earth has a twin out there, it has yet to show up on any photograph – even as a dot. That could change now that a plan to use a space umbrella to shade a telescope from the light of an alien sun is back on the table.
Called Starshade, the mission is an old idea that US researchers are now reviving. Most recently, they unfurled a half-size mock-up of the shade, as shown in the video above.
"This is the first time we've actually seen this thing deploy," says team member Jeremy Kasdin of Princeton University. "That was a huge accomplishment."
One way to look for exoplanets, used by NASA's Kepler space telescope, is to look for the dip in starlight as a planet passes in front of its star. That can reveal its size but not whether it hosts chemicals friendly to life, such as water. The light from larger planets can be seen directly, but those as small as Earth cannot.
Firefly in a searchlight
"This is a challenging problem," says exoplanet hunter Sara Seager of the Massachusetts Institute of Technology, another Starshade team member. "It's like us here in Boston looking for a firefly next to a searchlight in San Francisco."
Enter project Starshade, which, more than a decade ago, was part of a planned planet-hunter that never was. The Terrestrial Planet Finder was supposed to launch between 2012 and 2015 to directly image exoplanets the same size and mass of the Earth, orbiting in the habitable zones of sun-like stars.
It included a large telescope and a shade to block starlight and allow the tiny planet to shine alone, but in 2005, it was postponed due to budget restrictions.
Then in January, NASA revived the idea, convening a team to get going on a smaller, cheaper version of the original Starshade concept.
Last month, on 28 August, this culminated in the unfurling demo shown above, which took place at a Northrop Grumman Corporation laboratory in Goleta, California, where the mock-up was built.
Petal avoidance
The shade is shaped like a flower because of the wave nature of light. Light waves would bend around a circle, say, like ripples around a rock in a stream, and could create bright spots brighter than the planets astronomers hoped to detect. The petals avoid this.
The team is working towards having a mission concept that costs no more than $1 billion by 2015, when it will compete with another exoplanet mission, an X-ray telescope and a dark energy telescope for NASA cash.
That price tag means the mission probably won't buy a big enough telescope to find many Earths. "We're going to have to be lucky," Seager says. But at least it's a proof of concept.
She points out that alien life might produce or consume the same chemicals as life on Earth, so another strand to the mission is working out what to look for. "The question is, how are we going to look for biosignature gases when we have no idea what we're doing?" she says. "We're so terracentric right now, it's kind of limiting."
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