Cubesats are potential guide stars for next-gen telescopes

08 Jan 2019

MIT researchers develop mini satellites with lasers to act as reference light for investigation of exoplanets.

Engineers at NASA are developing designs for next-generation space telescopes specifically for the investigation of planets beyond our solar system, known as exoplanets, of which more than 3,900 have so far been confirmed. Most of these planets have been detected because of their transits across their stars, momentarily blocking the starlight.

Such telescope designs include segmented telescopes with multiple small mirrors that could be assembled or unfurled to form one very large telescope once launched into space. For example, NASA’s forthcoming James Webb Space Telescope is a segmented primary mirror, with a diameter of 6.5m and 18 hexagonal segments. Next-generation space telescopes are expected to be as large as 15m across, with more than 100 mirror segments.

One challenge for segmented space telescopes is keeping the mirror segments stable and pointing collectively toward a target exoplanetary system. Such telescopes would need be equipped with coronagraphs — instruments sensitive enough to differentiate between the light given off by a star and the weaker light emitted by an orbiting planet. But the slightest shift in any of the telescope’s parts can throw off a coronagraph’s measurements and disrupt measurements of oxygen, water, or other planetary features.

Satellite as guide star

Now MIT engineers have proposed that a second, shoebox-sized spacecraft equipped with a simple laser could travel near the large space telescope and act as a guide star, providing a steady, bright light near the target system that the telescope could use as a reference point in space to maintain stability. The work has been just published in the Astronomical Journal.

The design of such a laser guide star would be feasible using today’s existing technology. The researchers say that “using the laser light from the second spacecraft to stabilize the system relaxes the demand for precision in a large segmented telescope, saving time and money, and allowing for more flexible telescope designs.”

“In the future, we might be able to build a telescope that’s a little floppier, a little less intrinsically stable, but could use a bright source as a reference to maintain its stability,” commented Ewan Douglas, a postdoc in MIT’s Department of Aeronautics & Astronautics and a lead author on the paper. The workgroup also includes Kerri Cahoy, associate professor of aeronautics and astronautics at MIT, along with graduate students James Clark and Weston Marlow at MIT, and Jared Males, Olivier Guyon, and Jennifer Lumbres from the University of Arizona.

“If imperfections in the telescope motor or gears were causing your telescope to track slightly faster or slower, you could watch your guide star on a crosshairs by eye, and slowly keep it centered while you took a long exposure,” said Douglas.

In the 1990s, scientists started using lasers on the ground as artificial guide stars by exciting sodium in the upper atmosphere, pointing the lasers into the sky to create a point of light some 40 miles from the ground. Astronomers could then stabilize a telescope using this light source, which could be generated anywhere the astronomer wanted to point the telescope. Douglas added, “Now we’re extending that idea, but rather than pointing a laser from the ground into space, we’re shining it from space, onto a telescope in space.”

Ground telescopes need guide stars to counter atmospheric effects, but space telescopes for exoplanet imaging have to counter minute changes in the system temperature and any disturbances due to motion. The space-based laser guide star idea arose out of a project that was funded by NASA.

Maximizing stability

“The reason this is relevant now is that NASA has to decide in the next couple years whether these large space telescopes will be our priority in the next few decades,” Douglas continued. “That decision-making is happening now, just like the decision-making for the Hubble Space Telescope happened in the 1960s, but it didn’t launch until the 1990s.”

Cahoy’s lab has been developing laser communications for use in CubeSats, which are shoebox-sized satellites that can be built and launched into space at a fraction of the cost of conventional spacecraft. For the latest study, the researchers looked at whether a laser, integrated into a CubeSat or slightly larger SmallSat, could be used to maintain the stability of a large, segmented space telescope modeled after NASA’s LUVOIR (Large UV Optical Infrared Surveyor), a conceptual design that includes multiple mirrors that would be assembled in space.

The researchers have estimated that such a telescope would have to remain perfectly still, within 10 picometers, in order for an onboard coronagraph to take accurate measurements of a planet’s light, apart from its star.

Douglas says a small fleet of guide stars could be deployed, affordably, and spaced across the sky, to help stabilize a telescope as it surveys multiple exoplanetary systems. Cahoy points out that the recent success of NASA’s MARCO CubeSats, which supported the Mars Insight lander as a communications relay, demonstrates that CubeSats with propulsion systems can work in interplanetary space, for longer durations and at large distances.