The mass available to Beagle 2, unfortunately, is insufficient
to allow the lander to communicate directly with Earth. Therefore
signals can only be sent to and from the spacecraft via the
Mars Express orbiter when that is in view of the lander at certain
intervals during its orbit.
Communication sessions will often only be a few minutes
at a time. A particularly difficult time will be right at
the very beginning of the operations phase when Mars Express
is manoeuvring into position. Because this is a time of very
great interest, Beagle 2 has negotiated an alternative route
to send its information and receive commands. This will be
via the NASA orbiter Odyssey launched in 2001, on an extended
lifetime surveying Mars from orbit.
The Beagle 2 communication protocol has been designed with
the option of using Odyssey when Mars Express is unavailable.
However Odyssey will have its hands full meeting the requirements
of the Mars Exploration Rovers, Spirit and Opportunity.
See the diagram in the next section for details of the available
communication paths from Mars to Earth.