It is a tiny spacecraft by interplanetary standards, but size does not matter in this endeavour. The Indian Space Research Organisation (ISRO) is not trying to do exotic scientific experiments on Mars at the moment.This mission is simply to test the organisation’s ability to take something all the way up to Mars, keep it in good health during the journey, and make it go around the planet. Doing experiments while orbiting Mars is useful, but it is not the core part of the mission.On Monday next week, ISRO will face one of the biggest tests of this complicated project. It will switch on the engine that has been lying dormant for 10 months, and fire it for four seconds to slow down the spacecraft.If it fires and performs well, ISRO will fire it for a longer duration two days later and ease the spacecraft into an orbit around Mars. If it fails to ignite on September 22, the space organisation will nudge the spacecraft’s path towards a Martian orbit by firing eight smaller thrusters on September 24. In either case, barring completely unexpected situations, the Mars orbiter is expected to reach its destination within a week.Apart from a small glitch a few days after launch, the mission has gone very smoothly. The spacecraft has completed 98 per cent of its journey, and its trajectory is so close to the intended path that ISRO did not have to do a correction exercise planned for last month.“We have crossed several situations that we have not faced before,” says ISRO Chairman K Radhakrishnan. “We are now preparing for all contingencies on September 24.”The Mars mission, as planned by ISRO, was a sophisticated exercise. Compared to other Mars missions, ISRO had a smaller rocket and payload. This reduced the cost significantly but increased the mission’s complexity. Other Mars missions are not planned this way.The Maven spacecraft of NASA, which will reach Mars a few days before ISRO’s orbiter, was on its way to the red planet directly after launch. All it required was a fiveminute push from the powerful upper stage of the rocket, just 27 minutes after lift-off.

ISRO’s Mars Orbiter Mission, on the other hand, required repeated firings of the satellite thrusters and intricate manoeuvres before it could begin its journey to Mars. The launch was complex too. The PSLV rocket had a long coast of 25 minutes between the third and fourth stage, during which all engines were shut off. Once in orbit around the earth, the satellite had six orbit-raising manoeuvres that together lasted for 25 days before it left the earth’s grasp.

All of this required precise calculations that made the mission complicated. The spacecraft is now about 217 million km from the earth. It is going extremely fast, at 22 km per second with respect to the sun.When it reaches Mars, this translates to a velocity of just over 5 km per second with respect to the planet. At this speed it will shoot past Mars at an angle and move towards the outer reaches of the solar system.So, it has to be slowed down to a velocity of around 4.3 km per second with respect to Mars. This is done by reorienting the spacecraft and firing an engine the opposite way, just like the reverse thrust to slow down a plane after it lands.This is the job of the Liquid Apogee Motor (LAM). ISRO tested its LAM in the spacecraft during the orbital manoeuvres, and will be tested again on Monday. The LAM had two valves controlled by two separate coils of wire.It found out that fuel flow to the engine stopped when both coils were fired together, but the engine worked when they were energised alternately. This is not a problem that would hold up the mission.But as often happens in space missions, there could be other problems that ISRO has not foreseen. ISRO engineers are quite confident. There could be several reasons for failure of engines after a ten-month space journey. This could be corrosion or a leak or anything that cannot be foreseen, but ISRO has taken all precautions for foreseeable problems.The LAM is ISRO’s workhorse in its geostationary satellites, and it has been tested time and again for two decades. Satellites need such engines to bring them from transitory orbits to permanent orbital slots allotted to them.So the LAM has encountered the harsh environment of outer space before. Conditions are not too different for satellites around the earth and the Mars orbiter, as both operate for long periods in intense cold. The LAM worked well during the moon mission too. So everyone at ISRO is quiet confident.“Things can go wrong sometimes,” says Mars programme director Mylswami Annadurai. “But from a technical point of view, we do not expect problems.”ISRO had made a late decision to fire the engine on September 22. This was partly to correct the orbit and partly to test the engine. If the engine works well on Monday, ISRO engineers will be confident that it would work on Wednesday as well.On Monday it would fire for only four seconds, while on Wednesday it has to fire for 24 minutes, but the duration should not be a problem. “We have fired this engine on satellites for longer durations,” says ISRO scientific secretary Koteswar Rao.ISRO engineers have now uploaded the commands that have to be executed on September 22 and 24. Since the spacecraft is far away, and would be 224 million km from the earth on September 24, it will take 12.5 minutes for signals to arrive on earth from there.So a satellite near Mars cannot be operated in real time. It is no different in a rocket launch, even if it is on earth. The commands are fed to its computer just before launch, and then all decisions taken on board. The liquid engine will ignite at 7:17 am IST on September 24. The spacecraft would have been reoriented just before that, and it will go into eclipse – away from the earth’s view – five minutes before ignition.So the LAM would fire when the orbiter is not visible from earth. Signals from the ignition would reach the earth 12.5 minutes later. The eclipse ends at 7:37 am. At 7:52 am the manoeuvre would end, and the spacecraft will be in orbit around Mars. The first pictures will arrive by Wednesday afternoon, but they will be not the best due to poor illumination.A perfect execution of these commands would place ISRO at a high perch, as no country has gone to Mars without failures. Of course, some of these failures were a while ago, when several aspects of interplanetary travel were not well understood. ISRO has crossed two major hurdles already. The rocket lifted off and injected the satellite around the earth flawlessly.Three weeks later it was put on its journey to Mars, also with high precision. Both were causes of failures for other countries. Some other failures were near Mars, but scientists claim to understand the planet’s gravity much better these days. ISRO has managed the navigation to Mars well, and the satellite is in good health at the moment.If the LAM does not fire, ISRO has a back-up plan with eight thrusters originally meant for minor orbital corrections. This will result in a much higher orbit than planned, and the scientific objectives of the mission will then have to be tweaked accordingly.In either case, an Indian satellite would be orbiting Mars a week from now.