Cheap balloon-borne telescopes aim to rival space observatories

No instrument has revealed more about the heavens than NASA’s Hubble Space Telescope, which cost nearly $5 billion to build and launch. Yet a team of researchers thinks it can match some of Hubble’s capabilities with a telescope costing a mere $2 million, hoisted to the edge of space on a balloon. On a September 2019 flight, the Superpressure Balloon-borne Imaging Telescope (SuperBIT) demonstrated the ability to hold steady and image distant stars with exquisite resolution approaching Hubble’s, the team is now reporting.

“Like, this actually works,” says Barth Netterfield, a SuperBIT member and an astronomer at the University of Toronto. “That’s pretty awesome.” Richard Ellis, an astronomer at University College London who is not involved in the project, says low-cost, quick-turnaround balloon telescopes open new opportunities. “If somebody comes up with a brilliant idea, a balloon can go for it right away,” he says.

Orbiting above Earth’s obscuring atmosphere, Hubble sees with a clarity limited only by the size of its 2.4-meter mirror. A balloon-borne telescope floating several tens of kilometers up, above 99% of the atmosphere, would enjoy a similar vantage—if scientists could keep it stably pointed while the balloon drifts and turns. The SuperBIT team, split mainly between the University of Toronto and Princeton University, has conquered that problem, it reports in a paper in press at the Review of Scientific Instruments.

To fix on a spot on the sky, the telescope must slowly swivel on all three axes as it dangles from the balloon. SuperBIT employs state-of-the-art bearings and brushless electric motors to make that motion smooth. To orient itself, it refers to gyroscopes, images of the sky, and guide stars. During its 18-hour flight in northern Canada in September 2019, SuperBIT flew so stably that it achieved a resolution of about 260 milliarcseconds, only a factor of five less sharp than Hubble and at the theoretical limit for its 0.5-meter mirror.

To compete with space telescopes, however, a balloon-borne telescope must also stay aloft for many nights. That’s where new superpressure balloons come in. To avoid rupturing, an ordinary research balloon must vent helium during the heat of the day, as it expands and rises, before sinking at night. After a few days, the bobbing balloon has too little helium to stay aloft. A sturdier superpressure balloon maintains a steady volume, doesn’t vent helium, and cruises at constant altitude for weeks. In 2009, a NASA superpressure balloon recorded a 54-day flight, says Thomas Hams, program scientist for NASA’s balloon program. “We are on the cusp where we will soon see these flights more routinely,” he says.

NASA plans to launch one superpressure balloon per year, and SuperBIT researchers hope to ride on one in 2021. That flight would trace the distribution of mysterious dark matter in the universe, says Richard Massey, a SuperBIT co-leader and a cosmologist at Durham University in the United Kingdom. Galaxies are thought to reside within vast clumps and strands of dark matter, whose gravity distorts the images of more distant galaxies, creating correlations among the orientations of the tiny ellipses. “It’s a bit like looking through a textured bathroom window at the street lights,” Massey says. Known as weak lensing, such distortions can reveal the distribution of dark matter and help trace the universe’s evolution.

Massey has done similar work with Hubble. But with its wider-field camera, SuperBIT can cover more sky than Hubble. It could also fly before space telescopes with similar goals, such as the European Space Agency’s Euclid, which could launch in 2022.

Balloon-based telescopes present their own challenges. In Antarctica, NASA only launches balloons during the summer, when the weather is milder, but the constant daylight rules out optical astronomy. So Super-BIT’s scientific flight would have to launch from midlatitudes, probably from a NASA facility in New Zealand, where it would spend most of its time over the ocean. That makes it difficult to download data to cell towers, so the SuperBIT team has tested a system for dropping hard drives when the payload is over land. A balloon-borne telescope also takes a beating when it lands. “In the flight we just had, it got dragged through a forest and took out 50 trees,” Netterfield says.

Balloon-borne telescopes may be limited to niche applications, Ellis says, but the SuperBIT team has grander ambitions. It is working on a 1.5-meter successor to launch after the 2021 flight. For roughly $90 million, it could serve as a partial replacement for Hubble, which launched in 1990 and likely won’t last more than another decade, says William Jones, a SuperBIT member and an astrophysicist at Princeton. “A lot of people want the capabilities that Hubble has,” he says. “Once you’ve demonstrated that this works it would make sense to fly it at every opportunity.”