Smallsats as star guides for huge space telescopes



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There are more than 3,900 confirmed planets beyond our solar system. Most of them were detected because of their "transits" – instances in which a planet crosses its star, momentarily blocking its light.

These starlight dives can tell astronomers a little about the size of a planet and its distance from its star.

But knowing more about the planet, including whether it contains oxygen, water and other signs of life, requires much more powerful tools. Ideally, these would be much larger telescopes in space, with light-gathering mirrors as wide as those of the largest terrestrial observatories. NASA engineers are now developing designs for these next-generation space telescopes, including "segmented" telescopes with multiple small mirrors that could be mounted or unfolded to form a very large telescope once launched into space.

The next NASA James Webb Space Telescope is an example of a segmented primary mirror with a diameter of 6.5 meters and 18 hexagonal segments. The next-generation space telescopes are expected to be as large as 15 meters, with more than 100 mirror segments.

One challenge for segmented space telescopes is how to keep the mirrored segments stable and collectively pointing to an exoplanet system. Such telescopes would be equipped with coronagraphs – instruments that are sensitive enough to discern between the light emitted by a star and the considerably weaker light emitted by an orbiting planet. But the slightest change in any part of the telescope could cause the measurements of a coronagraph to be interrupted and discontinue measurements of oxygen, water, or other planetary characteristics.

Now engineers at MIT propose that a second space ship the size of a shoebox equipped with a simple laser could fly at a distance from the large space telescope and act as a "star guide", providing a constant and bright light near the system target. use as reference point in the space to stay stable.

In an article published today in the Astronomical Journal, researchers show that the design of a star laser guide would be feasible with current technology. Researchers say 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.

"This article suggests that in the future we may build a somewhat flexible, somewhat less intrinsically stable telescope, but use a bright source as a benchmark to maintain its stability," says Ewan Douglas, a postdoctoral fellow in the Department of Aeronautics and Astronautics at MIT and lead author of the article.

The article also includes Kerri Cahoy, associate professor of aeronautics and astronautics at MIT, along with graduate students James Clark and Weston Marlow of MIT, and Jared Males, Olivier Guyon and Jennifer Lumbres of the University of Arizona.

In sight

For more than a century, astronomers have used real stars as "guides" to stabilize terrestrial telescopes.

"If the engine's imperfections or telescope gears were making your telescope crawl a little faster or slower, you could observe your star guide in an eye-to-eye, and slowly keep it centered while you take a long exposure," says Douglas.

In the 1990s, scientists began using lasers in the soil as artificial guide stars, exciting the sodium in the upper atmosphere, pointing the lasers up into the sky to create a spot of light about 40 miles (60 kilometers) from the ground. Astronomers could then stabilize a telescope using this light source, which could be generated anywhere the astronomer wanted to aim the telescope.

"Now we are extending that idea, but instead of pointing a laser from the ground to the space, we are shining from space to a space telescope," says Douglas. Terrestrial telescopes need guide stars to combat atmospheric effects, but space telescopes for exoplanet images must contain minimal changes in system temperature and any disturbances caused by movement.

The idea of ​​the space-based laser guide star came from a project that was funded by NASA. The agency has been studying projects for large, space-segmented telescopes and has instructed researchers to find ways to reduce the cost of huge observatories.

"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," says Douglas. "This decision-making is happening now, just as Hubble Space Telescope decision-making took place in the 1960s, but it was not released until the 1990s."

Star Fleet

Cahoy's lab has been developing laser communications for use in CubeSats, which are shoebox-sized satellites that can be built and launched in space at a fraction of the cost of conventional spacecraft.

For this new study, the researchers looked at whether a laser, integrated into a slightly larger CubeSat or SmallSat, could be used to maintain the stability of a large segmented space telescope modeled after NASA's LUVOIR (for Large UV Optical Infrared Surveyor) a conceptual design that includes several mirrors that would be mounted in space.

Researchers have estimated that such a telescope would have to remain perfectly still, within 10 picometers – about a quarter of the diameter of a hydrogen atom – so that a coronagraph on board can make accurate measurements of the light of a planet as well as its star.

"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," Douglas says. "If you have disturbances greater than about 10 picometers, you begin to see a change in the pattern of starlight within the telescope, and the changes mean that you can not subtract perfectly the light from the stars to see the reflected light from the planet. "

The team came up with a general design for a laser star guide that would be far enough away from a telescope to be seen as a fixed star – some tens of thousands of miles away – and that would point back and send its light to the mirrors of the telescope, each of which would reflect the laser light toward an onboard camera. This camera measures the phase of this reflected light over time. Any change of 10 mph or more would signal a compromise in telescope stability, which the on-board actuators could then correct quickly.

To see if such a laser guide star design would be feasible with today's laser technology, Douglas and Cahoy worked with colleagues at the University of Arizona to find different sources of brightness, to find, for example, the brightness of a laser provide a certain amount of information about the position of a telescope, or provide stability using segment stability models from large space telescopes. They then designed a set of existing laser transmitters and calculated how stable, strong and distant each laser would have to be from the telescope to act as a reliable guiding star.

In general, they found that laser guide star designs are feasible with existing technologies and that the system could fit entirely within a SmallSat the size of a cubic foot. Douglas says that a single star guide could follow the "look" of a telescope, traveling from one star to the other while the telescope changes its observation targets. However, that would require the smaller spacecraft to travel hundreds of thousands of miles paired with the telescope at a distance, while the telescope repositions itself to observe different stars.

Instead, Douglas says a small fleet of guiding stars could be deployed, accessible and spaced across the sky, to help stabilize a telescope while examining various exoplanet systems. Cahoy points out that the recent success of NASA's MarCO CubeSats, which supported the Mars Insight probe as a communications relay, demonstrates that CubeSats with propulsion systems can work in interplanetary space for longer periods of time and over great distances.

"We are now looking at existing propulsion systems and figuring out the best way to do that, and how many spacecraft we want to outpace each other in space," says Douglas. "Ultimately, we think this is a way to reduce the cost of these large, segmented space telescopes."

Reference: "Laser Star Guide for Large Segmented Aperture Space Telescopes I. Implications for Terrestrial Detection of Exoplanets and Observatory Stability", E. S. Douglas et al., 2019 January 4, Astronomical Review [http://iopscience.iop.org/article/10.3847/1538-3881/aaf385, preprint: https://arxiv.org/abs/1811.05309].

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