An artist's concept of Mars Express. Image: ESA

In two year's time, the planet Mars will receive a Christmas present from planet Earth - a flotilla of spacecraft. Gifts from different nations, the spacecraft will share communications channels to solve a problem which will intensify as Mars exploration gathers pace: how to relay the data gathered by so many missions back to Earth.

First to arrive, just before Christmas 2003, will be the European Space Agency's Mars Express and its lander, Beagle 2, with the Japanese ISAS's Nozomi and NASA's two Rovers arriving in the New Year. They will join NASA's Mars Odyssey, which arrived at the Red Planet last October and will still be operating.

By early 2004, Mars will therefore have six visitors (three landers and three orbiters). Just as on Earth, these festive season guests could strain the available resources. Each of the spacecraft will need to send a steady stream of data back to Earth via limited communications channels.

Data can be relayed only when a spacecraft's antenna has an uninterrupted view of the receiving antenna on Earth. Because of the rotations of Mars and Earth and the movement of the orbiters around Mars, this condition is met intermittently for each spacecraft.

By sharing compatible communications technologies and algorithms, however, the spacecraft will be able to provide an open communications channel for most of the time. The availability of more than one communications channel also provides valuable insurance against the technical failure of one.

Thus, Mars Express will be able to forward data and messages between Earth and NASA's Rovers, if needed, and Beagle 2 can do the same via Mars Odyssey as well as Mars Express. Only Nozomi, which was launched too early to be included in the communications scheme, will look after its own data relay requirements alone.

Future missions are expected to follow a communications standard compatible with that now adopted by ESA and NASA, thus giving Mars Express and Mars Odyssey the potential of maintaining a useful role as communications relays long after their scientific missions have ended.

The spacecraft due to arrive at Mars around Christmas 2003 will thus do so in a spirit of cooperation entirely fitting for the season.