India's mission to Mars has hit a snag, after a planned engine burn failed to raise the spacecraft's orbit around Earth by the intended amount.
The problem occurred during a manoeuvre designed to boost the craft's maximum distance from 71,623km to 100,000km.
A problem with the liquid fuel thruster caused the 1,350kg vehicle to fall short of the mark.
But the head of the Indian Space Research Organisation (Isro) said the spacecraft remained "healthy".
As a solution, the Mars Orbiter Mission (MOM) - known informally as Mangalyaan, or Mars-craft - will be commanded to execute an additional thruster firing at 05:00 IST on Tuesday (23:30 GMT on Monday) to make up for the shortfall.
However, independent experts contacted by the BBC said they were puzzled by Isro's working hypothesis for the failure.
Instead of flying directly to Mars, the probe is scheduled to orbit Earth until the end of the month, building up the necessary velocity to break free from our planet's gravitational pull.
Mars mission payloads
- Lyman Alpha Photomoter (LAP): Measures amount of hydrogen and deuterium which will help scientists understand the loss of water from Mars
- Mars Exospheric Neutral Composition Analyser (MENCA): Studies the neutral composition of the upper atmosphere
- Mars colour camera: Takes images of the surface of Mars and its satellites Phobos and Deimos
- Thermal Infrared Imaging Spectrometer: Measures thermal emissions and helps map surface composition and minerals
This was the fourth in a series of five engine burns known as "midnight manoeuvres" because several constraints require that they are carried out in the early hours of the morning.
Speaking to Pallava Bagla, science editor at Indian broadcasting network NDTV, Isro's chairman K Radhakrishnan said: "The spacecraft is healthy and it encountered a problem when a specific redundancy test was being conducted and it failed to reach the desired velocity it was to achieve."
In that redundancy test, two coils in the liquid engine were supposed to be energised simultaneously.
"When you are going so far away, if one thing fails, you want to have a standby option. Everything is almost doubled up on the satellite, which is why they were not able to carry so much scientific equipment," Mr Bagla explained.
The failure of the test and the spacecraft's consequently reduced velocity raised the spacecraft's apogee (the point in its orbit farthest away from Earth) from 71,623km to just 78,276km - about 25% of the way to the target of 100,000km.
Mr Bagla told BBC News that the attempt on Monday morning used up about 2kg of the craft's 852kg fuel load.
But he added that the spacecraft's insertion into Earth orbit after launch on 5 November had been so precise, 6kg of liquid fuel had been saved. Even with Monday's glitch, the mission still had a fuel surplus of 4kg.
Nevertheless, Mr Radhakrishnan said that a failure analysis committee would examine why the problem occurred.
Cause to doubt?
But independent experts approached by the BBC said they were baffled by how the coil test could have contributed to a loss of thrust.
"Using the primary and redundant coils of the [engine's] solenoid valve of itself should not necessarily lead to a lack of thrust," a source told me on condition of anonymity.
"It should be a configuration they will have tested on the ground, which may mean this lack of thrust is nothing to do with the coils and that it's another issue."
The source explained: "It's very strange," adding: "The redundant coil should not act against the primary one in any way."
Alternative causes could include one or both propellant valves failing to open within the engine, or a lack of propellant in the first case. The latter scenario could indicate a catastrophic fuel leak that would almost certainly spell the end for the mission. But there are reasons to doubt this failure mode because less powerful liquid fuel thrusters on the spacecraft continued to work during the burn, the expert said.
Another possible reason for the loss of thrust could have been melting in the combustion chamber during a previous firing that broke the fuel nozzle.
However, all eyes will now be on the next engine burn, which should clarify whether or not there are bigger concerns over the health of the spacecraft.
The BBC's Sanjoy Majumder reports from inside the Satish Dhawan Space Centre
If the additional firing on Tuesday can successfully bridge the gap, a final midnight manoeuvre on 16 November will boost the apogee to 192,000km.
On 1 December, the engine will be fired again for its "trans-Martian injection", despatching the craft on a 300-day journey to Mars.
On 24 September next year, the engine will be fired again to slow down the spacecraft, enabling it to be captured by Mars' gravity and placed into orbit.
India's PSLV rocket - the second choice for the mission after a beefier launcher failed - was not powerful enough to send the MOM on a direct flight to Mars.
So engineers opted for a method of travel called a Hohmann Transfer Orbit to propel the spacecraft from Earth to Mars with the least amount of fuel possible.
At a cost of about $72m (£45m), the MOM is extremely cheap by the standards of planetary missions.
