A Japanese space craft has made a close approach to the Earth in an attempt to gather speed for its journey to Mars.
By Helen Briggs
BBC News Online science reporter
The probe, Nozomi, which means hope in Japanese, was damaged by a solar flare soon after launch and may not reach its destination.
Nozomi cost an estimated $848m
Its heating system is not working and must be fixed somehow to stop the space craft missing Mars and getting lost in space.
Dr Yasunori Matogawa, director of Japan's Kagoshima Space Centre, told BBC News Online: "I think there is a 50% chance of succeeding in mending the Nozomi space craft based on the information I have at the moment."
Mission planners will not know for about another week whether the Earth approach was successful, the Associated Press news agency quoted the Japanese Institute of Space and Astronautical Science as saying on Friday.
The probe is already five years behind schedule and is low on fuel. It has been beset with problems since it was licked by an explosion of energy from the Sun.
The solar flare damaged the space craft's onboard communications and power systems.
The altitude control heating system is no longer working which will pose a particular problem when Nozomi moves further from the Sun.
Its fuel could freeze and it would be unable to fire its onboard thrusters to go into orbit around Mars.
"They're in big trouble," said Dr David Williams of the Nasa Goddard Space Flight Centre in Greenbelt, MD.
"If the heating system doesn't work, it will shoot past Mars. They've got to get it working whatever."
Nozomi made its closest approach to the Earth at 18,000 km altitude. The gravity of the planet gave it a push in the direction of Mars.
"Without this fly-by, Nozomi will not be able to reach Mars," said Dr Matogawa.
This is the craft's second swing-by of Earth; its first, in December 2002, failed to give it sufficient speed.
If it makes it to the Red Planet, Nozomi will study the Martian atmosphere and its interaction with the solar wind.
It should arrive about the same time as the European and US Mars missions launched this month.
The European Space Agency's Mars Express and the British built lander Beagle 2 were launched from the Russian Cosmodrome of Baikonur in Kazakhstan on 2 June.
The first of Nasa's two Mars Exploration rovers, Spirit, was despatched a week or so later. The second launch, of its twin, Opportunity, is set for 25 June.
Previous missions to Mars, have had a high failure rate, with two out of three ending unsuccessfully.