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Magellan, the space probe circling Venus, has lost the use of the transmitter
that should send data back to Earth, as it tries to map the last 5 per cent
of the planet’s surface.

The problem is not with the transmitter itself, says Gordon Pettengill,
a professor of space sciences at the Massachusetts Institute of Technology.
Ground stations are receiving a clean carrier signal from the satellite.
But a modulator that varies this signal to carry data has failed. It is
as if a person’s lungs and windpipe are working as normal but their vocal
cords no longer vibrate.

According to Pettengill, engineers have little hope of reviving the
failed equipment. Instead, they will try to use the satellite’s second transmitter.
Last year, that transmitter started producing a spurious signal, or whistle,
that changes pitch as the equipment heats up. The whistle, which may come
from a cracked chip in the spacecraft’s electronic circuitry, sometimes
drowns out Magellan’s data.

Pettengill says that Magellan’s handlers will now leave this second
transmitter switched on, in the hope that the whistle will stabilise at
a particular frequency. If it does, engineers should be able to filter it
out, or transmit the spacecraft’s data on a different frequency.

Despite these difficulties, Magellan has been a great success. It has
already mapped 95 per cent of Venus’s surface to a resolution of 100 metres.
If the problem with its data transmission can be solved, the spacecraft
will return to areas that were missed earlier.

Pettengill says that scientists would also like to focus Magellan’s
radar on many areas from a different angle to create a ‘stereo’ image that
will reveal details of cliffs and mountains.

Plans to map Venus’s gravitational field are unaffected by Magellan’s
difficulties. To do this, scientists need only a clear, unmodulated radio
signal, which they will use to monitor slight variations in the spacecraft’s
orbit, says Pettengill.