Nasa has lost a key monitoring satellite ahead of an attempt to land a car-sized robot on Mars, which it said will be its "hardest ever" unmanned mission.
The Curiosity rover is set to land on Mars at about 5.30am GMT on 6 August, ahead of a two-year mission to explore the planet's surface.
If the rover makes it down safely - and Nasa insists it remains on track - it will be used to look for signs of life and water which may have once existed on the surface.
But landing the vehicle in the targeted 96-mile-wide crater will not be easy. The rover weighs more than a ton, and the 'air bag' technique used to launch previous, smaller robots won't work - meaning a new, relatively risky landing method has to be attempted instead.
The craft will have to "fly like a wing", Nasa said, before a backpack with retro rockets will fire, controlling the speed of the craft before lowering the rover on three nylon cords.
Even worse, it has now emerged Nasa may not know for several hours whether Curiosity has landed at all.
A satellite on which Nasa was relying to provide real-time data about the spacecraft's landing at the Gale Crater, near the equator of the planet, had been offline for a month - and has now been recovered in the wrong position.
Two other satellites monitoring the landing are not able to provide live updates in the crucial last minute of flight.
"The Curiosity landing is the hardest NASA mission ever attempted in the history of robotic planetary exploration," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate, at NASA Headquarters in Washington. "While the challenge is great, the team's skill and determination give me high confidence in a successful landing."
But despite the issues, Nasa remains confident about the mission.
It is also on track to return to the red planet with astronauts by the end of the 2030s, it said.
Nasa is even preparing the menu for its inter-planetary travellers, according to a new report.
The mission would likely see a crew of six to eight astronauts spend 30 months flying to the planet, staying on its surface and returning home.
Nasa is already spending $1m a year on researching the menu - but with an overall budget of just $17bn it is unclear whether the agency will be able to make it to Mars at all.