Sky and Telescope - February 2018 - 17

STR ATOSPHERE: MONTANA STATE UNIVERSIT Y; VICTOR HESS: WIKICOMMONS / PD - OLD -70

p THE EDGE OF SPACE Although the stratosphere is far from the official boundary of space (defined as 100 km, or 62 miles, above Earth's surface),
stratospheric balloons nevertheless climb above more than 99% of the atmosphere. The Pi camera that took this image was attached to a Montana
State University high-altitude balloon, which soared to a grand view of the Tetons on August 21, 2017.

overhead, the launch vehicle operator hits the gas, driving the
gondola-holding truck to match the balloon's speed and drift,
so that the telescope remains directly underneath it. As the
balloon struggles against this tether, the operator releases the
gondola, and the whole contraption takes off.
After two or three hours the balloon reaches a stable
altitude of 35 to 42 km. By now it has expanded by a factor of
100 in the stratosphere's lower-pressure environment. Large,
fully inﬂated scientiﬁc balloons extend up to 140 meters in
diameter at their widest point - they wouldn't ﬁt within a
football ﬁeld.
The balloon stays visible for several hours after launch,
like a perfectly round, small cloud - indeed, its kind is often
mistaken for UFOs - until stratospheric winds push it over the
horizon. During this extremely busy period, the scientists left
behind on the ground can communicate directly with the balloon gondola. We turn on the motors that point the telescope,
focus the optics, test the star-tracking cameras that let the
onboard computer know where we are looking, and make sure
that the solar panels give us enough power to run everything.
Every system on the telescope needs to be working before
the balloon crosses the horizon. After that, we can only
monitor the progress of the telescope by the data streamed via
satellite. We can send occasional commands to the telescope,
but these satellite links are intermittent and can have a lag of
up to several minutes, so the telescope has to be able to operate completely free of human intervention. It's essentially a
stratospheric robot.
The balloon and the equipment it carries must adjust to
their surroundings in the stratosphere. The balloon bobs
up and down over the course of a day, rising and sinking as

it warms in the sunlight and cools at night. Venting gas or
dropping ballast can help maintain a stable altitude, but it
may still ﬂuctuate by several hundred to thousands of meters.
In addition, energetic particles from space called cosmic rays
can cause problems in the electronics. If they hit one of the
control computers in just the wrong place, causing it to crash,
circuits monitoring computer status must initiate a reboot.

Observatories on the Go
Balloon telescopes are inherently mobile - they could theoretically launch from anywhere on Earth. Usually, they're
ﬂown from airports or designated ballooning facilities. In
most cases, after the ﬂight is over they can be recovered,
upgraded, and ﬂown again.
In the U.S., short-duration balloon ﬂights generally last less
than 24 hours and are launched from either the Columbia
Scientiﬁc Balloon Facility headquarters in Palestine, Texas, or
from Fort Sumner, New Mexico. These ﬂights are limited by
the ﬂight trajectory: Stratospheric winds tend to blow westward, toward southern California, with its millions of inhabitants, and toward the balmy coastline. It's generally inadvisable
to ﬂy conventional balloons over the Paciﬁc Ocean - astrono-

HISTORIC DISCOVERY
º Victor Hess (right) detected
cosmic rays - energetic
particles speeding through space
- when he accompanied his
equipment aboard balloon ﬂights
launched in 1911 and 1912.
sk yandtelescope.com * FE B RUA RY 2 018

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Sky and Telescope - February 2018

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