Laser CubeSats could become artificial stars to keep space telescopes focused

NASA is poised to send larger and larger telescopes into space over the next couple of decades and a team of researchers at MIT is working out how to use CubeSats to keep them fixed on target. Using lasers, the bread-loaf sized spacecraft will serve as artificial guide stars to act as points of reference and make sure the giant scopes are tracking and focusing properly.

Since it was launched in 1990, NASA's Hubble space telescope has returned an incredible gallery of images, not to mention volumes of data that will keep scientists busy for generations. However, its 2.4-m (8-ft) mirror will be dwarfed by the James Webb Space Telescopes' 6.5-m (21-ft) mirror made of 18 hexagonal segments and the next-generation telescopes that will have 100 segment mirrors measuring up to 15 m (49 ft).

This jump in size will provide researchers with unprecedented power to probe deep into space and to carry out such specialized operations as not only seeking out exoplanets, but also making direct, detailed observations of their atmospheres. However, this magnifying power also requires an extremely high level of precision to make sure the telescope is pointed in the right direction and that it is properly focused. Otherwise, you end up with a multi-billion dollar equivalent of taking a snapshot of your own thumb.

It's a problem that earthbound astronomers, both amateur and professional, have grappled with ever since some clever person figured out how to mount a camera on a telescope. Doing so turned the telescope into a tremendously important instrument, but it also meant that the scope had to be kept very stable and pointed in exactly the right spot for minutes or even hours at a time to get the correct exposure.

The way in which astronomers do this is by using guide stars. That is, bright stars in the vicinity of the subject of study that can be locked onto and tracked either automatically or manually. This allows the telescope to not only eliminate the movement of the heavens by the Earth's rotation, but can also be used to compensate for atmospheric distortion, variations caused by mechanical vibration or the uneven heating and cooling of the telescope and its mounting.

In the 1990s, scientists learned how to create their own guide stars by firing lasers into the upper atmosphere. At an altitude of 40 mi (64 km) these excite sodium atoms, producing a point of light that the telescope can track.

According to the MIT team, as space telescopes become larger, keeping them properly pointed and stable becomes more difficult. To overcome this and allow such giant instruments to be cheaper and more flexible, they propose that the laser technique be turned on its head by using a CubeSat that would be stationed tens of thousands of miles away and fire a laser beam back at the telescope to act as a reference.

Not only would a CubeSat be cheaper to launch than a conventional spacecraft, but the team estimates that the laser would allow the telescope to orient itself within 10 picometers, or one quarter the diameter of a hydrogen atom. This level of precision would allow the telescope to focus a coronagraph on a distant star, eclipsing it so that the telescope can home in directly on any planets orbiting it.

Another advantage of using CubeSats would be that a constellation of them could be sent up to service the telescope, so it would have a laser guide star in any part of the sky without the reference satellite having to travel thousands of miles. Based on models of such lasers and segmented telescope mirrors, the MIT team and the University of Arizona determined that existing technologies could place such a laser within a spacecraft of only a foot (30 cm) on each side

"Now we're analyzing existing propulsion systems and figuring out the optimal way to do this, and how many spacecraft we'd want leapfrogging each other in space," says Ewan Douglas, a postdoc in MIT's Department of Aeronautics and Astronautics. "Ultimately, we think this is a way to bring down the cost of these large, segmented space telescopes."

The research was published in the Astronomical Journal.

Source: MIT