Curiosity Rover (2012): Sky Crane Landing on Mars

Curiosity is a NASA rover that landed in Gale Crater on Mars on 6 August 2012, using a daring rocket-powered “sky crane” to lower itself gently onto the surface. The largest and most capable rover sent to Mars up to that time, it went on to show that an ancient lake on Mars could once have supported microbial life. More than a decade later, it is still exploring.

Background: the Mars Science Laboratory

Earlier missions, from Mars Pathfinder in 1997 to the Spirit and Opportunity rovers of 2004, had found growing evidence that liquid water once flowed on Mars. NASA’s next step, the Mars Science Laboratory, was designed to ask a bigger question: whether Mars ever had environments where life could have existed.

The rover, named Curiosity after a 2009 student essay contest, was much bigger than its predecessors, with a mass of about 899 kilograms. It carried ten science instruments, including a laser that could vaporise rock from several metres away (ChemCam), a drill, and a miniature chemistry laboratory. Instead of solar panels it used a radioisotope power system fuelled by plutonium-238.

The project suffered technical problems and delays, missing its planned 2009 launch window. Its cost rose to around US$2.5 billion.

Several instruments came from international partners. France’s space agency helped build ChemCam with the Los Alamos National Laboratory, Spain supplied the weather station (REMS), Russia a neutron detector (DAN) to look for hydrogen below the surface, and Canada an X-ray spectrometer for the robotic arm. The core laboratory, Sample Analysis at Mars (SAM), could heat rock powder and identify the gases released.

Choosing Gale Crater

Scientists debated the landing site for about five years. More than 30 candidates were narrowed down to four finalists: Eberswalde crater, Holden crater, Mawrth Vallis and Gale Crater. In July 2011 NASA chose Gale.

Gale’s attraction was Mount Sharp, whose layers had been mapped from orbit and appeared to contain clay and sulphate minerals that form in water. A rover able to drive from the crater floor up the mountain could read a long sequence of Martian history. The new guided landing technique made it possible to aim at a target ellipse only about 20 by 7 kilometres, small enough to fit on the flat crater floor beside the mountain.

A new way to land

Curiosity was too heavy to land on airbags like earlier rovers. Engineers at the Jet Propulsion Laboratory (JPL), with Adam Steltzner leading the entry, descent and landing team, devised a sequence that many outsiders thought too complicated to work.

The spacecraft would steer itself through the upper atmosphere to aim for a smaller landing area, deploy the largest supersonic parachute yet sent to another planet, and then drop free on a descent stage with eight rocket engines. Near the ground, this “sky crane” would hover and lower the rover on cables about 7.5 metres long before flying away to crash.

JPL publicised the sequence in a video titled “Seven Minutes of Terror”, because the whole landing took about seven minutes and the craft had to carry it out automatically: radio signals needed around 14 minutes to reach Earth.

The landing

The mission was launched on an Atlas V rocket from Cape Canaveral on 26 November 2011. After a journey of about 560 million kilometres, it entered the Martian atmosphere on 6 August 2012.

Curiosity touched down at about 05:17 UTC; signals relayed by the Mars Odyssey orbiter confirmed the landing on Earth about 14 minutes later, at 10:31 p.m. on 5 August in California. Scenes of celebration in JPL’s control room were broadcast worldwide, and the first low-resolution pictures arrived within minutes. The Mars Reconnaissance Orbiter even photographed the spacecraft hanging from its parachute. Later orbital images showed the scattered pieces of the landing system: the heat shield, the parachute and back shell, and the crashed descent stage, each lying some distance from the rover.

The landing point, later named Bradbury Landing after the writer Ray Bradbury, lay on the floor of Gale Crater, a basin about 154 kilometres wide with a mountain of layered rock, Aeolis Mons or Mount Sharp, rising about 5.5 kilometres at its centre.

Evidence of a habitable Mars

In September 2012 Curiosity found rounded gravel cemented into rock outcrops, evidence of an ancient stream that had flowed at about ankle to hip depth.

In February 2013 the rover drilled its first rock sample at a site called Yellowknife Bay. The analysis, announced in March 2013, revealed fine-grained mudstone containing clay minerals, formed in a lake of fresh, not very salty water. The rock held sulphur, nitrogen, hydrogen, oxygen, phosphorus and carbon, key ingredients for life. The scientists concluded that this environment could have supported microbes, although no evidence of life itself was found.

Later results added detail. Curiosity detected organic molecules preserved in ancient rock and found that methane in the air at Gale Crater varies with the seasons, both announced in 2018. Its radiation detector measured the dose that astronauts would receive on a journey to Mars.

By the numbers

Climbing Mount Sharp

From 2014 Curiosity began climbing the lower slopes of Mount Sharp, reading its layers like pages in a history book. The lower layers record a wetter Mars with long-lived lakes, while higher layers show the planet becoming drier, with sulphate-rich rocks formed as water evaporated.

The rover’s aluminium wheels were damaged by sharp rocks, forcing controllers to plan routes more carefully. Despite this, Curiosity has driven more than 30 kilometres and continues to operate well beyond its two-year prime mission.

Myths and facts

Curiosity’s famous “selfies” are sometimes taken as proof that another camera must be on Mars. In fact the rover photographs itself with the Mars Hand Lens Imager on the end of its robotic arm, taking dozens of overlapping pictures from different positions. These are stitched into a mosaic, and the arm holding the camera is left out of the frames, which is why it does not appear.

Another common belief is that Curiosity was sent to find life. Its goal was to judge whether Mars had ever been habitable, by studying rocks, minerals and chemistry. Instruments able to look for possible signs of past life were carried by its successor, Perseverance, which also collected rock samples intended for later return to Earth.

Timeline

• 2004, December: NASA selects instruments for the Mars Science Laboratory.
• 2009, May: The rover is named Curiosity after a student essay contest.
• 2011, 26 November: Curiosity is launched from Cape Canaveral.
• 2012, 6 August: The sky crane lowers Curiosity into Gale Crater.
• 2012, September: The rover finds evidence of an ancient streambed.
• 2013, March: NASA announces that Yellowknife Bay was once habitable.
• 2014, September: Curiosity reaches the base of Mount Sharp.
• 2018, June: Scientists report ancient organic molecules and seasonal methane.

Why it matters

Curiosity proved that heavy payloads could be landed precisely on Mars, and the same sky crane carried the Perseverance rover and the Ingenuity helicopter to the surface in 2021. Its discovery that ancient Mars had habitable lakes shifted the search from water to signs of past life. It builds on a line of exploration reaching back to Viking in 1976.