28 AUSTRALIAN SKY & TELESCOPE April 2018
STRATOSCOPE: U.S. NAVY; MONTGOLFIER BALLOON: LIBRARY OF CONGRESS LC-DIG-PPMSCA-02447Montgolfier: 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 metres (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 metres to look for water vapour in Mars’s atmosphere.
A few years later saw the first uncrewed astronomy flight with
Stratoscope I, a 30-cm reflector launched to study turbulence
on the Sun.To the stratosphere
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 rubberised 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 one
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 motorised crane on a truck. We align the
balloon-parachute-gondola 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 metres per second, catching the wind. As it flies
overhead, the launch vehicle operator hits the accelerator,
driving the gondola-holding truck to match the balloon’sSFIRST 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.SSTRATOSCOPE 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