We Should Just Build Giant Telescopes … in Space

In 2021, a rocket is scheduled to lift off from French Guiana carrying the largest space telescope ever made. Known as the James Webb Space Telescope, astronomers will use this beast of an observatory to study everything from habitable exoplanets to the formation of the first galaxies. JWST is the first mega-telescope of its kind to ever launch into space—and it may also be the last. The next behemoth might instead get assembled in space with the help of robots.

The beauty of that approach is it would allow a space agency to launch a telescope in pieces. That would lower the risks of the project significantly. Perhaps more importantly, it would remove the cap on the size of telescopes that can be sent to space. Pre-assembled telescopes are limited by the size of the rocket sending it up. Assembly in space would allow for instruments and exploration that so far haven’t been possible.

This is the conclusion of a recent NASA study that attempted to answer a question that has long plagued the astronomical community: Is building a telescope in space worth it?

From a risk-lowering perspective the answer is a definite yes, says Nick Siegler, the chief technologist of NASA’s exoplanet exploration program and a co-author of the study. With a monster of a telescope like JWST, a mishap as dramatic as a rocket explosion or as slight as a glitch in unfolding the telescope’s mirror could easily turn the telescope into a $10 billion piece of space junk. But if the telescope is launched in pieces and assembled by robot errors become less devastating. If something goes wrong, you can just send up a replacement part on the next rocket heading out.

In the NASA study, Siegler and his colleagues explored the hypothetical assembly of a 20-meter telescope in space. About three times the size of JWST and twice the size of the Gran Telescopio Canarias, the largest optical telescope on Earth, this imaginary instrument could be used to look for exoplanets, which means it has to be incredibly stable and precise. According to Siegler, this was the “hardest case possible.”

First, NASA would send up the telescope parts on multiple rockets. The inaugural batch would carry the main build platform for the telescope, some disassembled trusses for the support structure, and a pair of robotic arms. For a 20-meter telescope, 11 additional launches would deliver the remaining telescope pieces in capsules that dock with the telescope platform. At that point, the robot arms can start putting it all together.

“At first I thought this was science fiction,” says Siegler. “But these are exactly the type of operations that already exist.”

Indeed, the largest artificial object in space—the International Space Station—was assembled in orbit with humans and robots, and the Hubble Space Telescope also had its parts updated during its mission. Robotic arms regularly guide cargo capsules to berth with the ISS and roam the length of the space station to make repairs. Siegler says the JWST team even considered robotic assembly in the early 2000s, but at that point the technology wasn’t mature.

“Now NASA has a new tool in the toolbox,” says Siegler. “Telescope designers can be more creative in their approach. Everything’s on the table now.”

Constructing a massive telescope in space comes with some unique engineering challenges, like how to make sure the build platform doesn’t enter an uncontrolled spin during assembly. But at least, Siegler says, most of the technology needed already exists.

Then there’s the question of whether assembling telescopes in space can lower their cost. Siegler says we’ll only find out once a concrete mission, rather than a hypothetical one, comes about.

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