A spacecraft hurtling toward Mars is preparing to fire its thrusters and put itself on course for an August landing.
NASA engineers closely tracked the one-ton rover nicknamed Curiosity, which has been cruising along since rocketing from Earth last November.
The engine firing is the most important task Curiosity will perform during its 352-million-mile flight to the red planet, but it is not unprecedented. Previous robotic explorers have had to adjust their paths several times en route to ensure a bull's-eye landing.
"Just because this is a well-travelled road to Mars given the number of trips we've made, I'm very careful to not let that experience cause us to be complacent," said Arthur Amador of the NASA Jet Propulsion Laboratory, which manages the $2.5 billion mission.
At the time of the course correction, Curiosity will have racked up 80 million miles and will be travelling at 10,200 mph relative to the Earth.
The day before, the team uploaded commands calling for the spacecraft to fire its eight thrusters for nearly three hours, redirecting it closer to its target. Though it will execute the move without human interference, engineers will be on standby in the off chance that they have to abort.
"We should be very, very close to our desired aim point at the top of the Martian atmosphere" after the manoeuvre, Amador said.
If Curiosity did not tweak its position, it will miss Mars altogether. Engineers did this by design to prevent the rocket's upper stage that hoisted the spacecraft from hitting the planet. Once Curiosity separated from the upper stage and was on its way, the team has several chances to fine-tune its trajectory before touchdown.
Curiosity, whose formal name is the Mars Science Laboratory, is aiming for a 96-mile-wide crater near the Martian equator that boasts a towering mountain in the centre. The six-wheel, nuclear-powered rover planned to drive to the lower flanks and examine the layered deposits to determine whether the area once had conditions capable of supporting microbial life.
Armed with a suite of instruments including a laser to zap into bedrock and a jackhammer, Curiosity is more sophisticated than previous Mars surface spacecraft. Despite its capabilities, it won't be able to detect life. Instead, it will hunt for the chemical building blocks of life during its two-year mission.