Landing Site and Mission
The landing site for Curiosity and the overall mission objectives go somewhat hand-in-glove. Overall, MSL is charged with exploring and quantitatively assessing a local region on Mars’ surface as a potential habitat for life, past or present. This means that MSL is not equipped to detect extant processes that would betray present-day microbial life, nor does it have the ability to image microorganisms or their fossil equivalents.
Rather, MSL has the capability to detect complex organic molecules in rocks and soils and the analytical capability to probe other less unique biosignatures such as the isotopic composition of inorganic and organic carbon in rocks and soils. It will also be able to evaluate the concentration and isotopic composition of potentially biogenic atmospheric gases such as methane (which has recently been detected in the Martian atmosphere). In order to do this effectively, the rover requires access to as much of Mars’ geological history as possible, and in this respect, Gale Crater, residing in the southern hemisphere of Mars, is the ideal landing and exploration site.
Some 154 kilometres (96 miles) in diameter, it is thought that the crater may have once lain within a huge Martian lake. Of particular interest to scientists is an enormous mound of eroded sedimentary layers sitting on the central peak of the crater, rising for approximately 5 kilometres (3 miles) above the crater floor. Officially called Aeolis Mons, it is thought that the mound is all that remains of sediments laid down over a period of 2 billion years and which may have once completely filled the crater. While there is some debate around the nature and formation of the layers, they offer a unique insight into the three major periods of Mars’ geological history (Noachain, Hesperian and Amazonian), allowing the rover to study all three and the transitional periods between them, which should reveal a wealth of information about the planet’s past and their climatic conditions.
If all goes according to plan, Curiosity will touch-down in the north-western quadrant of the crater, in a region called Aeolis Palus, and then make its way south to Aeolis Mons.
The Seven Minutes of Terror
Before Curiosity can do anything on Mars however, it first has to land safely on the planet, and it will be doing so unlike any other mission ever undertaken anywhere in the solar system. Officially know as the Entry, Descent and Landing (EDL) phase of a mission, MSL’s mission planners have their own term for it: the seven minutes of terror. It requires six vehicle configurations, 76 pyrotechnic devices and the largest supersonic parachute ever built, all employed is a precise order during five distinct phases of EDL.
- Phase 1 – cruise stage separation: This will occur 10 minutes prior to atmospheric entry, when the section of vehicle charged with getting it to Mars separates from the main landing systems, which then “de-spin” (the entire vehicle having maintained stable forward momentum by spinning about its central axis while en route from Earth), before orienting itself for entry into the Martian atmosphere
- Phase 2 – guided entry: MSL will enter the Martian atmosphere contained within an aeroshell and protected by the largest heat shield so far flown in space (4.5 metres / 15ft in diameter). As the vehicle enters the atmosphere it will be travelling at some 21,000 kph (13,200 mph), and will decelerate to just under 1,700 kph (just over 1,000mph) in just 4 minutes, with a maximum deceleration of some 15G and a peak re-entry temperature of 2,090 °C (3,800 °F)
- Phase 3 – parachute descent: With the vehicle now some 10km (6 miles) above the surface of Mars and travelling at 470 metres per second (Mach 2.2 in the Martian atmosphere), the 16 metre (52 ft) diameter supersonic parachute is deployed and the heat shield jettisoned. The parachute will slow the craft to around 100 metres per second as it descends to some 1.8km above the surface of Mars. This phase will see MARDI commence its recording of the descent (from a height of some 3.7km)
- Phase 4 – powered decent: The rover, slung beneath a descent stage will fall clear of the aeroshell / parachute. Eight hydrazine-fuelled rocket motors will then ignite, bringing the vehicle’s horizontal movement over Mars to a stop and reducing its decent rate to around 1 metre per second at a height of some 20 metres above the Martian surface
- Phase 5 – skycrane: Curiosity is lowered on a 7.5-metre “bridle” so that it is suspended beneath the descent stage, which reduces thrust to just for of its motors while slowing the descent to just 0.75 metres per second. At the same time, Curiosity’s wheels unfold from their stowed configuration as the decent stage descends until the rover’s wheels make contact with the Martian surface. As soon as the on-board computers confirm this is the case, the rover detaches itself from the “bride” system, and the descent stage throttles-up to fly clear of the landing zone before powering-off to crash-land elsewhere.
This is a remarkably complex way of getting the rover safely down on Mars – certainly moreso than the use of airbags (employed on both the MER missions and with Mars Pathfinder and which would have been used by the UK’s Beagle 2 mission, had that reached the surface safely), which have seemed crazy enough in the past. However, Curiosity’s size and mass meant that airbags were impractical for the landing, while the use of a more “conventional” landing platform with landing legs and retro-rockets was deemed too complex and heavy a solution for the landing.
Even so, this landing represents the most high-risk so far used in any landing on Mars. Given the overall track record for Martian missions, with a failure rate of 53%, some might state that such a complicated landing system is tempting fate. Nevertheless, the United States has never yet suffered the loss of a rover vehicle, and hopefully, MSL will further add to that success rate.
Curiosity should touch-down at around 15:00 local time. The weather forecast for the Gale Crater region suggests an afternoon temperature of around -12 °C (10 °F), with a chance of some water ice haze in the upper atmosphere. Curiosity will have time to get systems up and running smoothly prior to the onset of its first night on Mars, when temperatures are liable to plummet to around -73 °C (-100 °F).
Communications with the rover vehicle will be handled by three orbiting vehicles: NASA’s 11-year-old Mars Odyssey and the 7-year old Mars Reconnaissance Orbiter (MRO), together with Europe’s 9-year-old Mars Express vehicle. All three will monitor MSL’s EDL, but only Odyssey will be able to relay X-band transmissions from the vehicle directly to Earth – at least for a few minutes during the landing phase; UHF transmissions from MSL will be stored by both MRO and Mars Express for later transmission to Earth (and might provide additional data in the event of the vehicle being lost).
Concerns were raised over Odyssey’s ability to function as a relay for the MSL’s landing in early July, when the orbiting spacecraft unexpectedly entered a “safe” mode – one of a number of difficulties it has experienced over the course of the last year. This meant that the vehicle was at risk of not being able to perform an orbital manoeuvre that would place it in a position to relay MSL’s X-band transmissions directly to Earth. However, mission managers were able to clear the problem, and on July 24th, Odyssey successfully completed the manoeuvre, ensuring it would be in position for Curiosity’s arrival.
A Social Mission
Another element that is marking MSL out as substantially different to other Mars missions is that it is perhaps the first interplanetary mission to engage with social media on a “personal” level. While missions such as the Mars Phoenix lander and the MERs (now just Opportunity, after Spirit was officially declared “lost” on May 24, 2011) have made use of Twitter, the Mars Curiosity team have gone a stage further, presenting Tweets about the mission in the first person – as though they are coming from the rover itself. These have given Curiosity something of a personality that has helped capture the attention of people around the world.
This has also lead to some exchanges “between” Curiosity and various personalities, such as astrophysicist Neil Tyson, further bringing the mission to life.
There is a lot riding on this mission: if Curiosity does find irrefutable evidence of organics in Gale Crater, it would be a eureka moment that goes a long way towards confirming Mars once had habitable environments in its ancient past. More than that, the success or failure of the mission may determine NASA’s long-term future in terms of the agency’s ability to undertake missions of planetary exploration. As it is, NASA is facing cuts of up to 21% to its planetary science budget, representing a 38% cut in the agency’s Mars Exploration Programme. If Curiosity survives its white-knuckle ride down to the surface of Mars, its success may just be enough to forestall what might otherwise be crippling cuts to NASA’s ability to operate a programme of exploration and discovery across the solar system.