Philips Atlas of the Universe

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The Magellan Mission

ATLAS OF THE UNIVERSE

▲Magellan.Released from

the cargo bay of the space

Shuttle Atlantison 4 May

1989, the spacecraft reached

Venus on 10 August 1990.

After 37 silent minutes while

it swung around the back

of the planet, it emerged

in a perfect orbit. Its radar

mapping programme was

completed by September

1992, and it began a cycle of

gravity mapping. Magellan

has set a record for the

amount of data gathered

by a space mission.

T

he various American and Russian spacecraft to Venus,

launched between 1961 and 1984, had provided a great

deal of information about the planet, but there was still

need for better radar coverage. This was the purpose of

the Magellan probe, launched from the Shuttle Atlantison

4 May 1989. It was hoped that the resolution would be far

better than anything achieved by the earlier missions, and

so it proved. Radar mapping began in September 1990,

and by 1993 over 98 per cent of the planet’s surface had

been covered. Cycle 4 ended on 24 May of that year. One

cycle is 243 Earth-days, during which Venus rotates com-

pletely beneath the spacecraft’s orbital plane. When

Magellan first went into orbit round Venus, the period was

3.2 hours, and the minimum distance was 289 kilometres

(180 miles), though in September 1992 this was reduced

to 184 kilometres (115 miles).

Magellan could resolve features down to 120 metres

(400 feet). The main dish, 3.7 metres (12 feet) across,

sent down a pulse at an oblique angle to the spacecraft,

striking the surface below much as a beam of sunlight will

do on Earth. The surface rocks modify the pulse before

it is reflected back to the antenna; rough areas are radar-

bright, smooth areas are radar-dark. A smaller antenna

sends down a vertical pulse, and the time-lapse between

transmission and return gives the altitude of the surface

below to an accuracy of 10 metres. Magellan has shown

fine details on the volcanic surface. There are for example

multiple lava flows, with varying radar reflectivity indi-

cating rocky and smoother areas. There are flows which

have clearly been due to very liquid lava, and even show

river-like meandering. The features known as tesserae are

high, rugged tracts extending for several thousands of

kilometres; one of these is Alpha Regio, shown on the

facing page. Magellan showed many ‘coronae’, caused by

plumes of hot material rising from below the surface.

Arachnoids, so far found only on Venus, are so named

because of their superficial resemblance to spiders’ webs;

they are circular to ovoid in shape, with concentric rings

and intricate outward-extending features. They are similar

in form to the coronae – circular volcanic structures

surrounded by ridges, grooves and radial lines. There are

strange-looking objects which have been nicknamed

‘pancakes’; these too are of volcanic origin. There are

strong indications of explosive vulcanism here and there.

The crater Cleopatra on the eastern slopes of the Maxwell

Montes is about 100 kilometres (62 miles) in diameter.

The scale and colours of the images shown here are

products of the computer processing; for example, the

vertical scale in the image of Gula Mons seen at top right

in the main picture has been deliberately increased to

accentuate its features. The colours are not as they would

be seen by an observer on the planet – assuming that he or

she could get there. For example, the bright patches repre-

senting lava flows would not appear so to the naked eye.

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