Sky and Telescope - February 2018 - 16

Balloon Astronomy

p FIRST FLIGHT The Montgolfier balloon was lavishly decorated with
the signs of the zodiac and illustrations of King Louis XVI, as shown here,
but inside Jean-François Pilâtre de Rozier and François Laurent, Marquis
d'Arlandes, had to work hard to keep the balloon afloat.

To the Stratosphere

p STRATOSCOPE The first uncrewed, balloon-borne astronomical
telescope, Stratoscope I, flew aboard the Skyhook in the 1950s. It took
detailed images of turbulence and granulation in the Sun's photosphere.

Balloon astronomy has changed a great deal in the intervening
decades. Balloons themselves have grown larger and lighter,
enabling them to climb into the upper stratosphere. Rather
than using paper or rubberized silk, we now build them from
polyethylene ﬁlm, the same material used to make plastic bags
- although the stuff used for balloons is only half as thick
as a typical sandwich bag. Improvements in electric motors,
microcontrollers, computers, and satellite communications
mean that astronomers are (thankfully) no longer expected to
accompany their telescopes to these extreme heights.
How we launch a balloon has changed quite a bit, too. We
start with it mostly empty - helium ﬁlls less than a percent. This creates a small, tightly secured bubble with a long,
uninﬂated tail. Laid out on the ground with the tail are a
parachute (for the payload's return to Earth) and the gondola
that contains the telescope. The gondola is suspended from a
motorized crane on a truck. We align the balloon-parachutegondola sequence with the direction of the wind that's blowing about 1,000 feet above the ground. That way, once the
balloon launches, it will lift the payload as it passes overhead.
After we release the balloon, it shoots up into the air
at several meters per second, catching the wind. As it ﬂies

FE B RUA RY 2 018 * SK Y & TELESCOPE

STR ATOSCOPE: U.S. N AV Y; M ON TGOLFIER BA LLOON: LIBR A RY OF CONG RESS LC -DIG -PPMSCA- 02447

Less than two weeks later, the physicist Jacques Charles
made the ﬁrst ascent in a hydrogen balloon, which launched
before a crowd of 400,000 people in Tuileries Gardens, Paris.
This balloon was much easier to control than the Montgolﬁer: Charles had bags of ballast to drop and a valve to release
hydrogen gas as needed. After landing 40 km away, his
assistant, Nicolas-Louis Robert, climbed out of the wicker
passenger basket. With the balloon's weight reduced, Charles
shot up to an astonishing 3,000 meters (10,000 feet) in only
10 minutes. He measured the temperature and atmospheric
pressure during his ascent, declaring himself the ﬁrst man to
see the Sun set twice on the same day.
One of the ﬁrst balloons' most important contributions was
the new and unique viewpoint they offered humankind, one
that we perhaps take for granted in this age of airplanes and
spaceﬂight. From a balloon, one could look down on clouds,
see how roads and rivers wound through vast mountain ranges
dotted with villages, and observe the whole curved disk of the
Earth beneath you "like a beautifully coloured map or carpet,"
as John Jeffries wrote after a ﬂight in November 1784.
But balloons offer us more than a new view of our planet.
They also give us access to the sky.
The aeronauts Joseph Croce-Spinelli and Théodore Sivel
wielded a spectroscope from a balloon in 1874, but it wasn't
until the 20th century that a complete astronomical telescope made the journey. In 1954 the father-and-son team
Charles and Audouin Dollfus took a 28-cm Cassegrain to
7,000 meters to look for water vapor in Mars's atmosphere. A
few years later saw the ﬁrst uncrewed astronomy ﬂight with
Stratoscope I, a 12-inch reﬂector launched to study turbulence on the Sun (S&T: Jan. 1958, p. 112).

Sky and Telescope - February 2018

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