Science 28Feb2020

SCIENCE sciencemag.org 28 FEBRUARY 2020 • VOL 367 ISSUE 6481 967

N

o 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 re-

searchers 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 as-

tronomer 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 atmo-

sphere, Hubble sees with a clarity limited only

by the size of its 2.4-meter mirror. A balloon-

borne telescope floating several tens of kilo-

meters up, above 99% of the atmosphere,

would enjoy a similar vantage—if scientists

could keep it stably pointed while the bal-

loon drifts and turns. The SuperBIT team,

split mainly between the University of To-

ronto and Princeton University, has con-

quered 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 em-

ploys state-of-the-art bearings and brush-

less electric motors to make that motion

smooth. To orient itself, it refers to gyro-

scopes, images of the sky, and guide stars.

During its 18-hour flight in northern Canada

in September 2019, SuperBIT flew so sta-

bly 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 super-

pressure balloons come in. To avoid ruptur-

ing, an ordinary research balloon must vent

helium during the heat of the day, as it ex-

pands and rises, before sinking at night. Af-

ter a few days, the bobbing balloon has too

little helium to stay aloft. A sturdier super-

pressure balloon maintains a steady volume,

doesn’t vent helium, and cruises at constant

altitude for weeks (Science, 23 June 2017,

p. 1227). In 2009, a NASA superpressure

balloon recorded a 54-day flight, says

Thomas Hams, program scientist for NA-

SA’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 ori-

entations of the tiny ellipses. “It’s a bit like

looking through a textured bathroom win-

dow 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 Hub-

ble. But with its wider-field camera, Super-

BIT 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 day-

light rules out optical astronomy. So Super-

BIT’s scientific flight would have to launch

from midlatitudes, probably from a NASA fa-

cility 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 mil-

lion, 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.” j

Cheap balloon-borne telescopes

aim to rival space observatories

Stable pointing system enables SuperBIT to deliver images

almost as sharp as those from the Hubble Space Telescope

ASTRONOMY

Last year, in an 18-hour balloon flight over Canada,

SuperBIT held steady and took sharp pictures.

By Adrian Cho

PHOTO: SUPERBIT COLLABORATION

Published by AAAS