It is midnight on 1 January 2004 and you want to send a
greeting on your mobile phone to a friend. Sorry, the line
is too busy, try again later. If you think you are alone
with this problem, you are wrong. 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.

USEFUL LINKS FOR THIS STORY

* More about Mars Express
http://sci.esa.int/marsexpress/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=31426&ooid=25718]
A visualisation of Mars, created from spacecraft imagery.
ESA’s first mission to the Red Planet is Mars Express,
planned for launch in June 2003. It comprises an orbiter
carrying seven scientific instruments to probe the
planet’s atmosphere, structure and geology, including a
search for evidence of hidden water. The main spacecraft
will also release the UK’s small Beagle 2 lander to
gather and test rock and soil samples on the surface.
As well as its science objectives, Mars Express will
also provide relay communication services between the
Earth and landers deployed on the surface by other
nations, thus forming a centrepiece of the international
effort in Mars exploration. Mars Express is ESA’s first
‘Flexible’ (F-class) mission. It is a pilot project for
new methods of funding and managing ESA space missions,
built more quickly and launched at a much lower cost
than any previous mission of a similar nature.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=31426&ooid=30192]
New ESA deep-space ground station. This is ESA’s first
35-metre deep-space ground station at New Norcia, 140
kilometres north of Perth in Australia. It will become
the main communications link between all future
deep-space missions and ESA’s operation centre, ESOC
(European Space Operation Centre), in Darmstadt
(Germany).

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=31426&ooid=28366]
Map indicating the locations of ESA and NASA ground
stations.