US "beachball" due to bounce down on Mars on Independence Day

A US spacecraft packed into a 5-foot diameter "beachball" will later today bounce down on to the surface of Mars, the conclusion to a seven month, 309 million-mile journey from Earth.

If successful, the Mars Pathfinder mission will signal a spectacular return to inter-planetary exploration that should give us an unprecedented view of the Red Planet. A six-wheeled, self-propelled rover will be released by Pathfinder, and sent out to explore the surface and conduct experiments. It may help determine whether life ever existed on the planet.

It also marks an entirely new approach by the US National Aeronautics and Space Administration, which involves sending a series of low cost spacecraft to Mars, culminating in an attempt in 2005 to collect Martian rocks and soil and return them to Earth.

The Pathfinder mission will be the first to land on Mars in more than 20 years and the first to send out a rover. NASA describes it as a "discovery class" mission, which involves using low-cost robotic spacecraft that have limited but highly focused scientific ambitions.

It represents the beginning of a new era in Mars exploration, which will see pairs of spacecraft dispatched to the Red Planet every 26 months. Pathfinders "twin" is the Mars Global Surveyor, which is due to go into orbit around Mars in September and begin a two year surface mapping programme.

NASA very consciously chose American Independence Day, July 4th, for the touchdown of Pathfinder. It has also employed a novel new method to bring Pathfinder safely to rest airbags.

"This is a new way of landing a spacecraft on a planet and the first time a US mission will use airbags to absorb the shock of landing and protect the lander from the rough, rocky terrain," said Mr Brian Muirhead, Mars Pathfinder flight system manager at NASA's Jet Propulsion Laboratory in California.

Pathfinder will enter the atmosphere above Mars just before 6 p.m. this evening and begin a wholly automated 40-minute landing sequence. It will approach Mars at 16,600 mph, initially using a heat shield to protect and to slow the spacecraft.

It next releases a large parachute, and jettisons the heat shield. About eight seconds before landing Pathfinder will inflate a series of airbags and then fire rocket engines to nearly halt the craft for an instant in mid air just before impact.

It will then freefall for about 100 feet, protected by its l5ft-diameter cocoon of airbags. Pathfinder may bounce as high as a 10-storey building depending on what it hits and may take several minutes to come to rest.

It arrives during the Martian night and will first have to deflate its airbags before opening up and signalling its safe arrival to its controllers in California. All going well this signal will be received by 6.07 p.m. Irish time, but it will be later tonight before NASA knows how well Pathfinder survived the impact.

It will be able to do very little more until after the Martian dawn when Pathfinder switches from batteries to solar power. Then a period of electronic "tick-tacking" between the spacecraft and its handlers begins to discover whether everything has remained intact.

"By around 3 p.m. (11 p.m. Irish time) we'll have the critical data that we need to determine whether we have a basically healthy spacecraft in reasonably good condition, or whether we'll need to start thinking about contingency operations," Mr Muirhead said.

"Frankly, we will be very surprised if everything goes just right, since there are so many conditions that are unknown until we actually arrive at the landing site."

All going well, Pathfinder will begin opening up its camera equipment and will send back high quality pictures of the local terrain. At this point controllers will also decide whether it is safe to release the rover, called Sojourner, which provides much of the drama of the mission.

Sojourner is expected to be deployed during the first three days after arrival. This box, measuring 48 inches long by 18 inches wide by 12 inches high, sports six wheels to help it cope with the rough terrain. It has its own camera, but more importantly it carries equipment that will allow it to travel to pick up promising rock samples and then analyse them for chemical content.

While mission controllers do not expect to establish conclusively whether life ever existed on Mars during this foray to the Red Planet the Sojourner camera may reveal something.

The potential for variety, but also a possible insight into a time when the Martian surface had liquid water, provided the reasoning for Pathfinder's landing site, the Ares Vallis. This is a massive flood basin, the scene of a catastrophic release of water sometime in the planet's past.

There will be a great variety of rocks washed down into the basin bye this event, so there will be promising pickings for the rover as it scours the surface.

Assuming everything works, the rover will complete its planned mission in seven days and pathfinder will have fulfilled its role in a month. It will take years to sift through the data provided by the spacecraft, however, adding immeasurably to our understanding of our planetary neighbour.