http://www.skyandtelescope.com.au 29STRATOSPHERE: MONTANA STATE UNIVERSITY; VICTOR HESS: WIKICOMMONS / PD-OLD-70
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 metres in
diameter at their widest point — about as wide as the MCG.
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 metres.
In addition, energetic particles from space called cosmic rayscan 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 — astronomers don’t like having to dive to retrieve their
equipment — and it’s definitely unwise to fly a several-ton
telescope over a dense urban area.
Conventional balloons can take longer flights from Arctic
or Antarctic sites. In these regions the Sun never sets in the
summer, so balloons’ altitudes stay relatively stable and
flights have to drop less ballast to stay afloat. (Of course,HISTORIC DISCOVERY
Victor Hess (right) detected
cosmic rays — energetic
particles speeding through space
— when he accompanied his
equipment aboard balloon flights
launched in 1911 and 1912.STHE EDGE OF SPACE Although the stratosphere is far from the official boundary of space (defined as 100 km above Earth’s surface),
stratospheric balloons nevertheless climb above more than 99% of the atmosphere.