The Pathfinder Mission
O
n 2 December 1996 a new probe was launched
towards Mars: Pathfinder, which carried a small
‘rover’, Sojourner. This time there was to be no gentle,
controlled landing. Pathfinder was encased in tough
airbags, and was designed to land at high speed, bouncing
several times before coming to rest. It would enter the
Martian atmosphere at a speed of 26,700 kilometres
(16,600 miles) per hour; the touchdown would undoubtedly
be violent, so that everything depended upon the airbags.
Once Pathfinder had settled down and assumed an upright
position, its ‘petals’ would open, so that Sojourner could
crawl down a ramp on to the Martian surface. It was an
ambitious project by any standards, and Sojourner itself
was also unusual; it was about the size of a householdmicrowave, but it was in fact a highly sophisticated
probe, capable of carrying out on-the-spot analyses of the
Martian rocks.
Data began to come through even during the descent.
The Martian atmosphere was both clearer and colder
than it had been during the Viking landings of 1976; at a
height of 80 kilometres (50 miles) above the ground the
temperature was 160 degrees C. But everything went
according to plan, and on 4 July 1997 – America’s
Independence Day – Pathfinder landed safely on the
old flood plain at the end of Ares Vallis. After a journey
of over 480 million kilometres (300 million miles),
Pathfinder came down within 20 kilometres (12 miles)
of the planned impact point. It was then 190 million
kilometres (120 million miles) from Earth; the ground
temperature reached a maximum noon value of
13 degrees C, though it plummeted to well below
75 degrees C during the night.
Almost at once, the main station used its camera to
transmit a panorama of the entire scene; it seemed more
interesting than Chryse or Utopia had been. The site, at the
end of Ares Vallis, had been carefully chosen. The valley
had once been a raging torrent of water, bringing down
rocks of all kinds on to the flood plain, and this did indeed
prove to be the case. Next, Sojourner emerged and began
its work (there had been a minor delay because one of the
airbags had not deflated completely after landing, and had
to be manoeuvred away from the ramp). The rocks around
were given distinctive nicknames, such as Barnacle Bill,
Yogi and Soufflé (the main station itself had already been
named in honour of Carl Sagan, the American planetary
astronomer who had died not long before the mission).
Sojourner’s track marks showed that the Martian ‘soil’
might be compared with the very fine-grained silt found
in places such as Nebraska, USA; the grains were less
than 50 microns in diameter, which is finer than talcum
powder.
Sedimentary rocks were identified, and this was not
surprising, since water had once covered the region. There
was, however, one major surprise. Though the rocks were
essentially basaltic, andesite was also found. This occurs
on Earth at the edges of tectonic plates, for instance round
the border of the Pacific, but there is no evidence that plate
tectonics also applied to Mars, so that the origin of the
andesite remains a puzzle.ATLAS OF THE UNIVERSE
Pathfinder on Mars.
The lander was renamed the
Sagan Memorial Station in
honour of Carl Sagan. This
panoramic view was
obtained by the Mars
Pathfinder Imager. Lander
petals, ramps and deflated
air bags are clearly visible, as
is the Sojourner rover, which
is using its Alpha Proton
X-Ray Spectrometer (APXS)
to analyse the rock at right. Pathfinder landing site.
The hills in the distance are
about 30 m (100 feet) high
and approximately 1 km (less
than a mile) away.C Atl of Univ Phil'03stp 2/4/03 3:07 pm Page 88