The mission team cheered wildly after Curiosity, the largest and most sophisticated interplanetary lander ever built, descended through the thin Martian atmosphere and made a successful, pinpoint touchdown inside Gale crater.
In the hours leading up to the arrival of Curiosity rover on Mars, the mood at NASA's Jet Propulsion Laboratory in Pasadena, California, was upbeat but understandably tense. Engineers had tested and retested every phase of the complex atmospheric entry, descent, and landing (EDL) — a perilous "seven minutes of terror" during which the spacecraft would slow from 13,200 miles per hour (nearly 6 km per second) to a dead stop.
Although optimistic — they believed the chance of success to be better than 95% — the team had nonetheless steeled itself for the possibility that something might go wrong. Would Curiosity "fly" itself to a small landing zone on the floor of Gale crater? Would the parachute deploy on time? Would the untested, rocket-assisted landing scheme work as designed?
They needn't have worried. At 10:32 p.m. Pacific Daylight Time (5:32 Universal Time on August 6th), NASA's Australian tracking station received radio word that Curiosity had arrived intact. "Touchdown confirmed," announced Allen Chen, the EDL operations lead. "We're safe on Mars." The 8½-month, 350-million-mile journey had come to its desired end, with the spacecraft basking in late-afternoon sunlight on the crater's floor.
The landing involved four distinct phases. Curiosity remained inside its streamlined, protective aeroshell when it first slammed into the top of Mars's thin atmosphere, about 78 miles (125 km) up. This part of the ride was abrupt and rough: the capsule's exterior reached an estimated 3800°F (2100°C) and 10 or 11 g of deceleration. Onboard sensors controlled the descent's trajectory with rapid-fire rocket bursts, essentially steering it with a precision never before attempted. This crucial innovation reduced the size of the ellipse-shaped target zone to just 12 by 4 miles (20 by 7 km), small enough to fit on the flat floor inside Gale.
Once the speed dropped to about 900 mph, at an altitude of about 7 miles (11 km), the largest parachute ever sent to another planet unfurled at supersonic speeds and slowed the descent further. Then the bottom of the aeroshell dropped away, exposing the Mars Descent Imager (MARDI) that snapped four high-definition frames per second from an altitude of roughly 1.2 miles (2 km) until touchdown.
The most anticipated — and anxious — portion of the descent occurred about a mile up, when the parachute and the remaining half of the heat shield were discarded. That's when the eight engines of the rover's "rocket backpack" fired up, slowing the descent further. On-board radar gauged the remaining distance to the surface, throttling the engines accordingly. But the mission's designers realized that the engines' exhaust would raise a large cloud of dust if they burned all the way to the surface. So about 70 feet (20 m) from touchdown, the rover was lowered on a trio of nylon cables. Once the wheels touched the ground, the cables were severed. This let the rocket stage veer off to the side so it could crash a safe distance away.
Four low-resolution snapshots were quickly radioed to Earth in the first minutes after touchdown, but don't expect to be wowed by dramatic vistas right away. The first 360° color panorama of the landing site won't reach Earth until Tuesday at the earliest, once mission controllers command Curiosity to raise its camera-topped mast. Also in the transmission queue are the first bird's-eye-view images taken by a camera on the craft's underside as it descended through the final 1¼ miles (2 km) of atmosphere.
In fact, it will be at least a week before Curiosity sets its six 20-inch-diameter wheels in motion. The time between now and then will be full of system checks and confirmation that its 10 experiments are ready to undertake the most exhaustive analysis of the Martian surface to date. Measuring 9.8 feet (3 m) long and nearly as wide, the spacecraft is roughly the size of a Mini Cooper and has a mass of nearly 1 ton. In fact, Curiosity's science payload alone — 165 pounds (75 kg) — is nearly half the mass of the entire rover Opportunity.
The $2½ billion Mars Science Laboratory mission has a lot of work ahead of it, expected to last at least a full Martian year (98 weeks). "This will be not a sprint but a marathon," notes Doug McCuistion, who directs NASA's Mars-exploration program. The main objective is to scale the slopes of the massive 3-mile-high mountain inside Gale. Named Aeolis Mons, it's actually a stack of sediments that, scientists hope, represent hundreds of millions of years of Martian history.
It's also informally known as "Mount Sharp," honoring the late Robert P. Sharp, who formed Caltech's planetary-geology group during the late 1960s and early 1970s.
Just as the Grand Canyon in Arizona exposes ancient strata at the base of its walls and younger ones higher up, it's hoped that the lowest layers within Aeolis Mons were laid down more than 3 billion years ago, when geologists suspect that the climate was warm enough to permit liquid water to flow abundantly across the surface.
Were these climatic conditions conducive to life? That's what Curiosity — part geologist and part chemist — hopes to learn. Its instruments will study the geology along its uphill route with cameras, spectrometers, and other instruments that will study the character and composition of its rocks — even revealing crystal structure for the first time. Moreover, it will determine isotopic ratios of carbon and oxygen of ground-up rock samples, looking for any hint of organic matter that might exist in the planet's harsh environment. One especially novel experiment, called ChemCam, will vaporize tiny spots on rocks up to 20 feet (7 m) away and record the spectrum (at ultraviolet, visible, and infrared wavelengths) of the brief incandescent burst of light.
The only spacecraft to have searched specifically for Martian life were the twin Viking landers, which touched down in 1976. While Curiosity will not look specifically for life, it certainly represents a new tack in NASA's exploration strategy: to follow not only the water, but also the carbon, that might exist on the Red Planet.
"The Curiosity story is just beginning," says John Grunsfeld, who manages NASA's space-science efforts.