Satellites and Space Probes
ATLAS OF THE UNIVERSE
I
f an artificial satellite is to be put into a closed path
round the Earth, it must attain ‘orbital velocity’, which
means that it must be launched by a powerful rocket; the
main American launching ground is at Cape Canaveral
in Florida, while most of the Russian launches have been
from Baikonur in Kazakhstan. If the satellite remains suf-
ficiently high above the main part of the atmosphere it
will be permanent, and will behave in the same way as a
natural astronomical body, obeying Kepler’s Laws; but if
any part of its orbit brings it into the denser air, it will
eventually fall back and burn away by friction. This was
the fate of the first satellite, Sputnik 1, which decayed dur-
ing the first week of January 1958. However, many other
satellites will never come down – for example Telstar, the
first communications vehicle, which was launched in 1962
and is presumably still orbiting, silent and unseen, at an
altitude of up to 5000 kilometres (3000 miles).
Communications satellites are invaluable in the
modern world. Without them, there could be no direct
television links between the continents. Purely scientific
satellites are of many kinds, and are used for many differ-
ent programmes; thus the International Ultra-violet
Explorer (IUE) has surveyed the entire sky at ultra-violet
wavelengths and operated until 1997, while the Infra-Red
Astronomical Satellite (IRAS) carried out a full infra-red
survey during 1983. There are X-ray satellites, cosmic-ray
vehicles and long-wavelength vehicles, but there are also
many satellites designed for military purposes – some-
thing which true scientists profoundly regret.
To leave the Earth permanently a probe must reach
the escape velocity of 11.2 kilometres per second (7 miles
per second). Obviously the first target had to be the
Moon, because it is so close, and the first successful
attempts were made by the Russians in 1959. Lunik 1
bypassed the Moon, Lunik 2 crash-landed there, and
Lunik 3 went on a ‘round trip’ sending back the first
pictures of the far side of the Moon which can never be
seen from Earth because it is always turned away from
us. During the 1960s controlled landings were made by
both Russian and American vehicles, and the United
States Orbiters circled the Moon, sending back detailed
photographs of the entire surface and paving the way for
the manned landings in 1969.
Contacting the planets is much more of a problem,
because of the increased distances involved and because
the planets do not stay conveniently close to us. The first
successful interplanetary vehicle was Mariner 2, which
bypassed Venus in 1962; three years later Mariner 4 sent
back the first close-range photographs of Mars. During the
1970s controlled landings were made on Mars and Venus,▲ Sputnik 1.Launched
on 4 October 1957, by the
Russians; this was the first
artificial satellite, and marked
the opening of the Space
Age. It orbited the Earth
until January 1958, when
it burned up.▲ Lunik 1 (or Luna 1).
This was the first space
probe to pass by the Moon.
It was launched by the
Russians on 2 January 1959,
and bypassed the Moon
at a range of 5955 km
(3700 miles) on 4 January.▼ ROSAT– the Röntgen
satellite. It provided a link
between studies of the sky
in X-radiation and in EUV
(Extreme Ultra-Violet);
it carried a German X-ray
telescope and also a British
wide-field camera.▲ Satellitescan orbit the
Earth in the plane of the
equator (1) or in inclined
orbits (2). Polar orbiting
satellites (3) require lesspowerful rockets than those
in geostationary orbits (4),
which need to be much
higher at 36,000 km (22,500
miles) above the Earth.1
2
3
4
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