For several decades, Earth-based astronomers relied on laser-induced artificial "guiding stars" in the Earth's mid-atmosphere to enhance the view of their cosmos telescopes. Now a research team has proposed that a different type of star-shaped laser, mounted on CubeSats flying in formation, could help keep future generations of space-based giant telescopes as they peered into the faint reflected light of distant extrasolar planets (Astron, J., doi: 10.3847 / 1538-3881 / aaf385).
For terrestrial telescopes, laser guide stars – incandescent sodium atoms in the middle atmosphere, or mesosphere, excited by the light of a terrestrial continuous-wave laser – form the crux of adaptive optics techniques. The artificial creation of light at a precise and known point allows on-ground astronomers to use wavefront modeling techniques to subtract computationally the turbulent effects of the atmosphere. These techniques, in turn, allow for a sharper view of the true target: the stars beyond the atmosphere.
Large space telescopes – such as the James Webb Space Telescope under construction (JWST), which will include a 6.5 m wide segmented mirror, or the gigantic Ultraviolet Optical Infrared Surveyor (LUVOIR), a state-of-the-art spatial scope currently in phase of concept, with a potential mirror diameter more than double the JWST – does not have an atmosphere to worry about. But space-based giants face their own technical challenges. And in one aspect of their mission, perception reflects the light of extra-solar planets for spectroscopic analysis; these challenges, in large part, are about keeping the dead telescopes on target.
For example, to filter out the starlight from the distant exoplanet and allow the telescope to directly sample the reflected light from the exoplanet itself, gigantic, next-generation space telescopes will carry delicate instruments called coronagraphs (see "Exoplanets: Getting a Closer Look "For an adequate yield of exoplanet candidates in the" habitable zone "(ie at Earth-like distances of their parent stars), the coronagraphs should be able to dim the light host star for a contrast in the order of 10-11 to the much weaker reflected light of the planet to be measured.
Performance at this level requires the coronagraph to be a master of wavefront errors. For a large aperture instrument such as the LUVOIR – the conceptual drawings for which a 15 m diameter mirror consisting of individual 1 m hex segments is estimated – it is estimated that, for coronographs, a sufficient flux of photons exoplanet reflected light, wavefront errors would need to be limited to less than 10 picometers of jaw dropping.
This exquisite tolerance is easy to overcome when doing business in space, according to Ewan Douglas of the Massachusetts Institute of Technology (MIT), the lead author of the new study. "Any disturbance in the spacecraft, such as a slight change in the angle of the sun, or a piece of electronics on and off and changing the amount of heat dissipated through the spacecraft, will cause a slight expansion or contraction of the structure," he said. said in a press release accompanying the research. Such structural changes, if they exceeded the 10 o'clock limit, could add speckle into the coronary "dark hole", disrupting the light signal from the exoplanet.
CubeSats for the ransom?
Douglas, along with colleagues from MIT and the University of Arizona, proposed a scheme to keep future telescopes like LUVOIR well-behaved and within the stability limit of 10 o'clock in the afternoon. The scheme provides for a small fleet of CubeSats (or slightly larger SmallSats), each equipped with a 980 nm continuous wave laser. The CubeSats would be deployed along with the base telescope, flying over distances of over 40,000 km.
A CubeSat positioned at such a distance, and slightly out of the angle of the host star of the exoplanet being targeted by the space telescope, would shoot a continuous laser beam back into the telescope's mirrors, which would direct the light to a camera on board. By measuring the light phase of that artificial star over time, spacecraft computers could detect changes larger than the 10 o'clock limit. At such a detection, the telescope actuators could adjust the mirrors to correct the error.
In principle, a single CubeSat could do the work by shifting its position as the telescope moved from target to target. But a more efficient and viable approach, according to Douglas, would be a small fleet of guiding stars, implanted in the sky, to stabilize the telescope while examining multiple star systems.
Looking at the 2030s
Taking the LUVOIR mission concept as a test case, the MIT-Arizona team modeled the CubeSat flight range and positioning stability, the laser wavelength and beam divergence characteristics, sensor noise, and other parameters that would need to be met to make the system work. They concluded that such a system could be built using current technology and packaged in a SmallSat in the order of a cubic foot in size. And the recent success of MarCO CubeSats, deployed by NASA to accompany the Mars InSight mission, has reinforced the confidence that these small satellites may ultimately become charge burros not only in Earth orbit but also in interplanetary space .
LUVOIR, of course, is still a mission in the concept phase, which, even if approved, will only be released in the mid-2030s, at the very least. But the researchers behind the new study believe that their work could help defend this ambitious future venture by providing a way to take a thorny technical image problem at a relatively small cost.
"The reason this is pertinent now is that NASA must decide in the next two years whether these large space telescopes will be our priority in the coming decades," Douglas said in a statement. "This decision-making is happening now, just as Hubble Space Telescope decision-making took place in the 1960s but was not released until the 1990s."