Pasadena, California

Ready to roll: Curiosity's first glimpse from Gale Crater. Credit: NASA/JPL-Caltech

“It’s the wheel! It’s the wheel!”

The jubilant shout was heard over cheers and exclamations as team members with NASA’s Mars Science Laboratory — also known as Curiosity — watched as the first image from their spacecraft flashed up on a control-room screen here at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. Taken just minutes after the craft had landed, the picture capped off a dramatic descent sequence that deposited the rover on the surface of Mars at 10:32 p.m. Pacific daylight time on 5 August.

Although not a beautiful image — it was shot through a lens cover by the rover’s rear hazard camera — the picture was enough to show one of Curiosity’s wheels resting firmly on the Martian soil. In the distance, the curving horizon beckons.

After an 8-month journey, Curiosity survived a violent, 7-minute fall through the thin atmosphere of Mars before touching down at a speed of less than 1 metre per second, the softest landing in the history of Mars exploration. The feat proved that the mission's complicated landing system was just as robust as advertised.

The 900-kilogram rover touched down at the bottom of Gale Crater, a 154-kilometre-wide pit roughly the size of Kuwait, where it will begin its search for environments that might once have been habitable.

Indication of a safe landing came almost immediately through Mars Odyssey, an 11-year-old orbiter that relayed telemetry data from the spacecraft to large radio antennas in Canberra, Australia, part of NASA’s Deep Space Network.

But Adam Steltzner had to be sure. The man in charge of the entry, descent and landing phases, easily spotted in the control room by his pomaded hair, was looking for confirmation that the inertia sensors on board the rover were no longer registering movement. When it came, he counted to ten to make sure that the descent module hadn’t just fallen back down on the rover. Only then did he point at Al Chen, the voice of mission control.

“Once I had all three things, I gave Al Chen the A-OK to say ‘touchdown confirmed’.”

At the press conference, the mission leaders sat on a dais, with the rover’s second picture projected behind them: this one taken by a front hazard camera and showing the shadow of the rover in the late afternoon Sun. “There is a new picture of a new place on Mars,” Steltzner says. “And for me, at least, that’s pay-off.”

The event marks the seventh successful Mars landing for NASA, of eight attempts.

Lofty goals

Although the precise location of the rover is not yet known, the craft's target was a wide, smooth plain between the steep crater walls and Aeolis Mons, a 5.5-kilometre-tall mountain in the centre that mission scientists have dubbed ‘Mount Sharp’.

The landing proves the viability of a radically different landing system — one that gives NASA the ability to deliver a lander more precisely on the surface, and therefore closer to interesting targets for scientific study. One innovation was the use of guided entry, borrowed from the Apollo programme, which allowed the spacecraft to steer its way through the thin Mars atmosphere to a landing ellipse just 20 kilometres long. A second change involved swapping the air bags used on previous rovers for a ‘sky crane’, which unspooled the rover on three nylon cords, and lowered the six wheels gently to the surface.

The rover won't take its first drive for about a week and with a top speed of 4 centimetres per second, it will be many months, if not a year, before it arrives at Aeolis Mons. But there is plenty of science to be done even at the landing site; the rover is expected to have landed near the foot of an alluvial fan, a triangle-shaped feature that would reflect a time when water spilled over the crater rim and deposited sediments.

But the mystery of Aeolis Mons remains paramount (see ‘Crater mound a prize and puzzle for Mars rover’). The science team wants to understand how the mountain, which is taller than the rim of the crater, managed to form. But it also wants to understand the hundreds of millions of years of history that are embedded within the mountain. At the bottom of the mountain are layers about 3.8 billion years old that contain water-altered minerals such as clays and sulphates. Farther up the mountain, however, are layers that show no evidence of water at all. Somewhere in that stack lie clues to Mars’ “great desiccation event”, says project scientist John Grotzinger, a geologist at the California Institute of Technology in Pasadena. “How did Mars go from being a wet planet to a dry planet?” he asks. Among the rover’s 100-kilogram science payload are instruments that will help to address whether those watery environments in the deep past were conducive to life.

But those questions were far from the minds of relieved engineers at the JPL. As hundreds of science team members were gearing up for years of work, the engineers could relax for the first time in months. 

Ed Weiler, NASA’s recently retired science chief was one of the senior officials on hand. “I was there for the conception of this 12 years ago,” he told Nature. “It was a long, long pregnancy. But the baby sure is beautiful.”

Triumph and relief followed the apparent success of Curiosity's novel 'sky crane' landing system. Credit: NASA/JPL-Caltech