Philips Atlas of the Universe

THE SOLAR SYSTEM

Phobos is spiralling slowly downwards at the rate of about

18 metres (60 feet) per century, in which case it may crash

on to Mars in about 40 million years from now.

In July 1988 the Soviet Union launched two probes

towards Mars, the main aim being to land on Phobos and

examine its surface (in fact, the gravitational pull of the

tiny satellite is so slight that an encounter would be more

in the nature of a docking operation). Unfortunately, both

missions failed. Phobos 1 was lost during the outward

journey because of a faulty command sent out by the

controllers. Phobos 2 was scheduled to touch down on

Phobos, ‘hook’ on to the surface and then use an ingenious

mechanism to hop around the satellite, but contact was

lost before Phobos was reached, though some images were

obtained. The Russians’ ill-fortune with Mars continued,

though the failure of America’s sophisticated Mars

Observer probe in 1993 was even more of a loss. New

probes to Mars were launched in 1996 and 1997.

Deimos is even smaller than Phobos, with a longest

diameter of no more than 15 kilometres (9 miles). Its

regolith is deeper, so that the surface is more subdued;

craters and pits are seen. The apparent diameter of Deimos

as seen from Mars would be only about twice the maxi-

mum apparent diameter of Venus as seen from Earth, and

with the naked eye the phases would be none too easy to

see. Deimos would remain above the Martian horizon for

two and a half ‘sols’ consecutively; it would transit the

Sun about 130 times a Martian year, each passage taking

1 minute 48 seconds, and of course Deimos too would often

be eclipsed by the shadow of Mars. It would be invisible to

a Martian observer at a latitude higher than 82° North or

South. The orbit, unlike that of Phobos, seems to be stable.

The Martian satellites are quite unlike our massive

Moon, and it is very probably that they are ex-asteroids

which were captured by Mars long ago. This idea is

supported by the fact that the first two asteroids surveyed

from close range by a spacecraft (Gaspra and Ida) seem to

be very similar to Phobos, and are of much the same size.

Photographs show similar irregular shapes and cratered

surfaces. All in all, Phobos and Deimos are interesting

little bodies – and one day they will no doubt be pressed

into service as natural space stations.

 Map of Deimos. This

satellite shows fewer

well-marked features than

Phobos; the sub-Mars point

lies some way south of

Swift, one of the two named

craters.

 Deimos. The outer of

the two satellites, Deimos

is smaller and more irregular

in shape. Neither of the

satellites is massive enough

to become spherical; both

have synchronous rotations,

always keeping the same

face to their parent planet.

▲Stickney. The largest

crater on Phobos, it is 10 km

(6 miles) across. Boulders

visible near Stickney are

assumed to have been

produced by the impact that

formed the crater.

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Voltaire

Swift

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