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Posted by David Williams on October 23, 2006, 8:28 pm
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-> During that time, all spacecraft have become virtually incommunicado
-> for
-> about two weeks every two years. The reason is solar conjunction, which
-> occurs again from October 18-29, 2006. Solar conjunction is the period
-> when Earth and Mars, in their eternal march around the Sun, are
-> obscured
-> from each other by the fiery orb of the Sun itself. Like dancers on
-> either side of a huge bonfire, the two planets are temporarily
-> invisible
-> to each other.
Umm... Well, almost. In fact, Mars is not passing exactly behind the sun
as seen from Earth. The two planets' orbits are not exactly in the same
plane, so Mars usually passes a bit "above" or "below" the sun in the
sky. Radio signals from them are not physically obstructed by the sun.
In fact, they can sometimes be focussed by passing through the sun's
atmosphere, so they arrive at Earth with *greater* strength than they
would have if the sun were not there. However, the sun also emits radio
waves, and when Mars is very close to the sun in the sky, dishes that
are aimed at Mars are inevitably aimed pretty well at the sun too. This
means that the sun's radio waves are received by the dishes, and
interfere with the signals from the spacecraft at Mars. Likewise,
dishes on the spacecraft that are aimed at Earth inevitably pick up the
sun's radio emissions. This interference, rather than any loss of
signal strength, is what makes communications difficult at this time.
dow
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