OK, our next caller is... from Mars!

Space agencies have had to work out ingenious solutions to prevent similar 'engaged, call later' tones from happening on Mars. For the first time, there will be seven spacecraft on the Red Planet at the same time. Will they all be able to 'phone home'?

Mars is a popular place to go nowadays. ESA's Mars Express mission will be entering orbit around Mars and releasing its lander Beagle 2 at the end of December 2003. The two NASA Mars Exploration Rovers will be landing about the same time. Japan's Nozomi, after five years in space, will be entering Mars's orbit not long after also. Finally, there are two other probes already orbiting the Red Planet: NASA's Mars Global Surveyor and Mars Odissey. What happens if they all try to communicate at the same time?

Scientists need to transmit messages continuously to their interplanetary spacecraft. Receiving data concerning the status of the instruments and their results is essential. Since the late 1950s, they use antennas in three NASA stations, which form the Deep Space Network (DSN). The stations are located in Goldstone (California), Madrid (Spain), and Canberra (Australia).

The core of each DSN station is an enormous 70-metre-diameter antenna. It can capture the signal of a spacecraft more than 16 thousand million kilometres away from Earth. Several smaller antennas surround that dish. The goal of the network is to constantly talk to spacecraft.

Soon, ESA's completely new ground station will help us listen to deep space also. Located in Australia, just outside the town of New Norcia, near Perth, the station has a new 35-metre antenna. It will become the main communications link between all future ESA deep-space missions and ESA's operation centre, ESOC (European Space Operation Centre), in Darmstadt (Germany).

Thanks to this new ground station, there will not be any 'busy-line' signals, explains Rudi Schmidt, Mars Express Project Manager. "We're not expecting communications difficulties between the ground stations and the orbiters. NASA, ESA, and the Japanese spacecraft use different frequencies to transmit their signals. In addition, each spacecraft has its own internationally recognised 'call sign', as an identifier."

Communications between landers and orbiters will be even more complicated. ESA and NASA have worked very closely to ensure crossover support between American and European landers and orbiters. ESA's Mars Express will therefore be able to talk to the NASA Rovers and ESA's lander Beagle 2 with NASA's Mars Odissey. "This guarantees that we receive the maximum amount of science data possible. In addition, this makes sure that we can help each other if there are problems," says Schmidt.

Don McCoy, Mars Express Project Engineer, adds: "Another good trick we will use is to store the lander's data on the orbiter, and then transmit them to Earth later. The orbiter will be visible longer from the Earth and it can use a bigger and more efficient antenna than the lander, which has a smaller solar panel and uses less transmission energy."

According to Agustin Chicarro, Mars Express Project Scientist, "We'll need DSN as a back-up during critical mission phases, such as the orbit intersection manoeuvres. The important thing is the coordination: don't forget that DSN will also have to track other important missions, such as Cassini and Deep Space 2."

Scientists want to be able to hear what exciting news their spacecraft have to tell once they reach their destinations. Now there is more certainty that they will.