Nasa's Curiosity rover edges closer to Mars
Nasa's Curiosity rover is on course to land on Mars on Monday, where it will search for clues in the Red Planet's rocks and soil about whether it could once have supported life.
The robot's flight trajectory is so good engineers cancelled the latest course correction they had planned.
To land in the right place, it must hit a box at the top of the atmosphere that measures just 3km by 12km.
Curiosity has spent eight months travelling from Earth to Mars.
The robot - -also known as the Mars Science Laboratory - has covered more than 560 million km.
"Our inbound trajectory is right down the pipe," said Arthur Amador, Curiosity's mission manager.
"The team is confident and thrilled to finally be arriving at Mars, and we're reminding ourselves to breathe every so often. We're ready to go."
The rover's power and communications systems are in excellent shape.
The one major task left for the mission team is to prime the back-up computer that will take command if the main unit fails during the entry, descent and landing (EDL) manoeuvres.
Curiosity - Mars Science Laboratory
- Mission goal is to determine whether Mars has ever had the conditions to support life
- 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
The robot was approaching Mars at about 13,000km/h on Saturday. By the time the spacecraft hits the top of Mars' atmosphere, about seven minutes before touch-down, gravity will have accelerated it to about 21,000km/h.
The vehicle is being aimed at Gale Crater, a deep depression just south of the planet's equator.
It is equipped with the most sophisticated science payload ever sent to another world.
Its mission, when it gets on the ground, is to characterise the geology in Gale and examine its rocks for signs that ancient environments on Mars could have supported microbial life.
Touch-down is expected at 05:31 GMT (06:31 BST) Monday 6 August; 22:31 PDT, Sunday 5 August.
It is a fully automated procedure. Nasa will be following the descent here at mission control at the Jet Propulsion Laboratory in Pasadena, California.
The rover will broadcast X-band and UHF signals on its way down to the surface.
These will be picked up by a mix of satellites at Mars and radio antennas on Earth.
The key communication route will be through the Odyssey orbiter. It alone will see the rover all the way to the ground and have the ability to relay UHF telemetry straight to Earth.
And mission team members remain hopeful that this data will also include some images that Curiosity plans to take of itself just minutes after touching the ground.
These would be low-resolution, wide-angle, black and white images of the rear wheels.
They may not be great to look at, but the pictures will give engineers important information about the exact nature of the terrain under the rover.
A lot has been made of the difficulty of getting to Mars, and historically there have been far more failures than successes (24 versus 15), but the Americans' recent record at the Red Planet is actually very good - six successful landings versus two failures.
Even so, Nasa continues to downplay expectations.
"If we're not successful, we're going to learn," said Doug McCuistion, the head of the US space agency's Mars programme.
"We've learned in the past, we've recovered from it. We'll pick ourselves up, we'll dust ourselves off, we'll do something again; this will not be the end.
"The human spirit gets driven by these kinds of challenges, and these are challenges that drive us to explore our surroundings and understand what's out there."
The mission team warned reporters on Saturday not to jump to conclusions if there was no immediate confirmation of landing through Odyssey.
There were "credible reasons", engineers said, why the UHF signal to Odyssey could be lost during the descent, such as a failure on the satellite or a failure of the transmitter on the rover.
Continued efforts would be made to contact Curiosity in subsequent hours as satellites passed overhead and when Gale Crater came into view of radio antennas on Earth.
"There are situations that might come up where we will not get communications all the way through [to the surface], and it doesn't necessarily mean that something bad has happened; it just means we'll have to wait and hear from the vehicle later," explained Richard Cook, the deputy project manager.
This was emphasised by Allen Chen, the EDL operations lead. His is the voice from mission control that will be broadcast to the world during the descent. He will call out specific milestones on the way down. He told BBC News there would be no rush to judgement if the Odyssey link was interrupted or contained information that was "off nominal".
"I think we proceed under any situation as though the spacecraft is there, and there for us to recover - to find out what happened," he said.
"That's the most sensible thing to do. There are only a few instances I think where you could know pretty quickly that we'd be in trouble."