In an unparalleled technological triumph, a 1-tonne nuclear-powered rover the size of a small car was lowered to the surface of Mars on the end of a 7.62-metre bridle suspended from the belly of a rocket-powered flying crane late on Sunday in the US to kick off an ambitious US$2.5 billion mission.
During the final hundred feet of entry, descent and landing, the Curiosity rover was lowered from its rocket-powered "sky crane" directly to the surface of Mars, as seen in this computer animation.
With flight controllers at NASA's Jet Propulsion Laboratory anxiously watching telemetry flowing in from Mars, 250 million kilometres away and 13.8 minutes after the fact, the Mars Science Laboratory rover — Curiosity — radioed confirmation of touchdown at 3.32pm AEST (GMT +10).
"Touchdown confirmed. We're safe on Mars!" said mission control commentator Allen Chen, as the flight control team erupted in cheers and applause.
"It's just absolutely incredible, it doesn't get any better than this," said NASA administrator Charles Bolden. "I was a basket case in there, I was really on pins and needles.
"It's a huge day for the nation, it's a huge day for all of our partners and it's a huge day for the American people," he said. "Everybody in the morning should be sticking their chests out, saying 'that's my rover on Mars'. Because it belongs to all of us."
The target landing zone was Gale Crater, near the base of a 4.82-kilometre-high mound of layered rock that represents hundreds of thousands to tens of millions of years of Martian history, a frozen record of the planet's changing environment and evolution.
While the rover's exact position was not immediately known, there were no obvious problems during the dramatic entry, descent and landing, and Curiosity presumably made it down inside a predicted footprint measuring 6.44km wide and 19.31km long — a pinpoint landing compared to previous missions.
The seven-minute descent to the surface provided high drama, as flight controllers monitored telemetry from the spacecraft, relayed through NASA's ageing Mars Odyssey spacecraft. As each major milestone ticked off, engineers clapped and cheered.
"We have acquired the ground with the radar," Chen reported. "Heat shield has separated, we have found the ground. We're standing by to prime the engines in preparation for powered flight. Six point nine kilometres and descending..."
A few minutes later, the rover and its descent stage fell away from the vehicle's braking parachute and eight rocket engines ignited to stabilise the craft and slow the fall to touchdown velocity.
"We are in powered flight," Chen reported. "We're at an altitude of 1 kilometre, and descending about 70 metres per second ... 500 metres in altitude ... standing by for sky crane. We found a nice, flat place, we're coming in ready for sky crane. Down to 10 metres per second, 40 metres altitude."
A few moments later, just above the surface, the rocket-powered sky crane lowered Curiosity to the surface on the end of a 25-foot-long bridle, a landing technique never before attempted.
While engineers did not expect pictures right away, blurry, low-resolution thumbnails from the rover's rear hazard-avoidance cameras were transmitted within minutes of touchdown, showing a wheel on the surface of Mars.
"Odyssey data is still strong," Chen reported. "Odyssey is nice and high in the sky. At this time, we're standing by for images..."
"We've got thumbnails," someone said.
"We are wheels-down on Mars!" Chen reported.
"Oh my God," someone said in the background.
Exploring the crater floor and climbing Mount Sharp over the next two years, Curiosity will look for signs of past or present habitability, and search for carbon compounds, the building blocks of life as it is known on Earth.
But before the rover's geological fieldwork can begin, engineers will devote several weeks to carefully checking out Curiosity's complex systems, and testing its state-of-the-art instruments and cameras.
At touchdown, cables connecting the rover to the sky crane descent stage were severed, setting the stage for initial tests and checkout.
Curiosity's landing represented the most challenging robotic descent to the surface of another world ever attempted, a tightly choreographed sequence of autonomously executed events with little margin for error.
NASA managers went out of their way to brace reporters for the possibility of failure.
"No matter what happens, I just want the team to know I am really proud and privileged to have worked with these guys and gals," Doug McCuistion, director of Mars exploration at NASA. "They're amazing. They've done everything humanly possible to make this happen."
Adam Steltzner, the leader of the EDL team at the Jet Propulsion Laboratory in Pasadena, California, said earlier that he was "rationally confident; emotionally terrified".
"This team ... has worked really hard for the better part of a decade, and the fruits of that labour get put to the test tonight," he said. "It's a little anxiety provoking. But I will say I slept better last night than I have slept in a couple of years, because she's kind of on her own now. When I look back on the hard work that we've done, I believe the team has done everything that we can to deserve success tonight."
And that's exactly what they got.
The Mars Science Laboratory spacecraft consisted of an interplanetary cruise stage, providing power and communications during the long flight out from Earth, and the Curiosity rover, which was cocooned inside a heat shield and aeroshell to protect it from the extreme temperatures of atmospheric entry.
After covering 566 million kilometres since launch from Cape Canaveral, Florida, last November, the cruise stage separated from the lander at around 3pm AEST. But because of the distance between Earth and Mars — 250 million kilometres — it took radio signals confirming critical events 13.8 minutes to reach the flight control team at JPL. That translated into 3.14pm "Earth-received time".
One minute later, thrusters fired to stop the entry vehicle's 2rpm rotation, and the spacecraft re-oriented its heat shield forward and slammed into the discernible atmosphere at 3.24pm at an altitude of about 125km and a velocity of 21,243km/h. At that point, it was about 628km — seven minutes — from touchdown in Gale Crater.
The Mars Science Laboratory was the first spacecraft to attempt a so-called guided entry on another planet.
To control its lift, which allowed Curiosity's flight computer to make a pinpoint landing, two 75kg tungsten weights were ejected just before entry, to change the spacecraft's centre of mass. During hypersonic flight, thruster firings controlled the orientation of the vehicle's "lift vector" to compensate for actual atmospheric conditions as it precisely controlled its path towards Gale Crater.
About one minute and 15 seconds after entry, the spacecraft's heat shield was expected to experience peak temperatures of up to 2100 degrees Celsius as atmospheric friction provided 90 per cent of the spacecraft's deceleration. Ten seconds after peak heating, that deceleration was expected to peak at 10 to 15 times the force of Earth's gravity at sea level.
Plummeting towards Mars, the rover's flight computer continued steering the spacecraft, firing thrusters to make subtle changes in the flight path as required by atmospheric density and other variables.
The guided-entry phase of flight was programmed to an end about four minutes after entry began. Six 25kg weights then had to be ejected to move the centre of mass back to the central axis of the spacecraft to help ensure stability when its braking parachute deployed.
Seconds later, at an altitude of about 7 miles and a velocity of some 1450 kilometres per hour, the huge chute unfurled and inflated to a diameter of 21 metres, delivering a 29,483.5kg jolt to the still-supersonic spacecraft.
The heat shield was expected to be jettisoned about 24 seconds later, at an altitude of about 8 kilometres and a descent rate of 450km/h, exposing the rover's undercarriage to view.
A sophisticated radar altimeter then began measuring altitude and velocity, feeding the data to the rover's flight computer, while a high-definition camera began recording video of the remaining few minutes of descent.
Six minutes after entry, now 1.61km up and falling toward the surface at roughly 290km/h, the rover and its rocket pack were cut away from the parachute and backshell, falling like a rock through the thin Martian atmosphere.
An instant later, eight hydrazine-burning rocket engines, two at each corner of the descent stage, ignited to stabilise and quickly slow the craft's vertical velocity to less than 3.2 km/h.
About 16 seconds before touchdown, at an altitude of just under 21 metres, Curiosity was lowered on the end of a 7.62-metre-long bridle made up of three cables. As the support and data cables unreeled, the rover's six motorised wheels presumably snapped into position for touchdown.
Finally, seven minutes after the entry began and descending at a gentle 2.73 km/h, Curiosity's wheels touched the surface of Mars. Radio confirmation of landing came in at 3.32pm; about 3pm local time on Mars.
Curiosity's flight computer, sensing "weight on wheels", then sent commands to fire small explosive devices that severed the cables connecting the rover to the still-firing propulsion system. Its work complete, the descent stage flew away to a crash landing a safe distance away.
"We have three different signals we would use to confirm touchdown, and we need all three of those things to look right before we say so," Steltzner said earlier. "One of those is a message from the spacecraft that says 'I touched down, and this is the velocity I touched down at and where I think I am'.
"The rover has an inertial measurement unit, a gyro and an accelerometer set, and we look at that stream to say the rover's not moving at all, that signal says 'I think I'm on the ground and I'm not moving'. And the third is, we wait a safe period of time and confirm we're getting continuous UHF [radio] transmission. And frankly, that's there to make sure the descent stage hasn't fallen back down on top of the rover. When all three of those signals are positive, we declare touchdown confirmation."
And that's exactly what Chen reported at 3.32pm.