Animation showing how Curiosity gets to the surface of Mars (Courtesy of Nasa)
Nasa might not be able to follow the progress of its big Mars rover all the way to the surface when it attempts to land on the planet on 6 August (GMT).
The Curiosity vehicle is aiming for a deep depression known as Gale Crater.
The US space agency will be tracking the descent with satellites, but its prime craft for the task may not now be in the correct place in the sky.
Engineers have been tackling a fault on the Odyssey satellite and it has yet to take up the best observational orbit.
Unless that is achieved in the next three weeks, Nasa will lose signal to the rover just as it is about to touch down.
This will not affect the outcome of the landing because Curiosity's descent manoeuvres are all performed autonomously, but it will give rise to some high anxiety as everyone awaits confirmation that the $2.5bn mission is safely on the surface.
"To be clear it won't have any impact on landing; it's all a communications issue," said Doug McCuistion, the director of Nasa's Mars exploration programme.
The 900kg robot's entry, descent and landing (EDL) will be the most dangerous aspect of the entire mission.
The rover, in its protective capsule, will hit the top of the Martian atmosphere at 20,000km/h (13,000mph) and attempt to slow to just one metre per second to execute a soft touch-down.
Continue reading the main storyCuriosity - Mars Science Laboratory
- Project costed at $2.5bn; will see initial surface operations lasting two Earth years
- Onboard plutonium generators will deliver heat and electricity for at least 14 years
- 75kg science payload more than 10 times as massive as those of earlier US Mars rovers
- Equipped with tools to brush and drill into rocks, to scoop up, sort and sieve samples
- Variety of analytical techniques to discern chemistry in rocks, soil and atmosphere
- Will try to make first definitive identification of organic (carbon rich) compounds
- Even carries a laser to zap rocks; beam will identify atomic elements in rocks
This rapid deceleration must be achieved in about seven minutes or Curiosity will smash into the ground.
Engineers have built a complex EDL system that includes a supersonic parachute and a rocket-powered crane. Everything must work on cue and in sequence.
It was expected that the Odyssey orbiter would track the whole descent, relaying UHF signals from the rover right up to the landing and for a few minutes beyond.
But the spacecraft recently experienced a reaction wheel failure.
This device is used to keep the satellite properly orientated, and because engineers have been investigating the issue they have not as yet moved Odyssey into the correct orbit to see the full landing sequence - and they may not do so.
This would leave Nasa blind for the final, nail-biting two minutes of the landing operation.
Antennas on Earth will be following the decent but they will lose contact as Curiosity hurtles into Gale, one of the deepest holes on Mars. The steep crater walls will block all direct radio transmission to the home planet not long after the supersonic parachute is opened.
The agency will have two other orbiters watching the decent - the Mars Reconnaissance Orbiter (MRO), and the Europeans' Mars Express spacecraft - but these only have a "store and forward" capability, which means they will not be able to confirm a landing has taken place until their data has been passed to Earth.
MRO is the key satellite here, and its data will not be available to engineers on Earth for three to four hours after landing.
As things stand, and unless Odyssey is moved, Nasa will have to wait until this satellite in its present orbit passes the landing site. This could be five to 10 minutes after the planned touch down.
The projected landing time is 22:31 PDT 5 August; 01:31 EDT, 05:31 GMT, 06:31 BST 6 August. This is "Earth receive" time - the time a signal sent from Mars is received on Earth.
The 250 million km between Mars and Earth on 5/6 August mean a radio transmission takes just under 14 minutes to pass between the two planets.
"If Odyssey is not able to be moved and it still remains late, that means it will fly over [Curiosity] after the spacecraft has landed, and we presumably will [then] be able to see transmissions from it. It would be some between 22:35 and 22:40 PDT," explained Pete Theisinger, the rover project manager at Nasa's Jet Propulsion Laboratory in Pasadena, California.
Curiosity - also known as the Mars Science laboratory (MSL) - is the most sophisticated space vehicle ever built to touch the surface of another world.
Assuming the robot lands safely, it will spend 98 (Earth) weeks scouring Martian soils and rocks for any signs that current or past environments on the planet could have supported microbial life.
Gale Crater was chosen as the landing site because satellite indicates there are sediments in the depression that were laid down in the presence of abundant water.
MSL-Curiosity is equipped with 10 advanced instruments. It also has a plutonium battery and so should have ample power to keep rolling for more than a decade.
It is likely the mechanisms on the rover will wear out long before its energy supply.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter
Source: http://www.bbc.co.uk/news/science-environment-18861463#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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