|“Curiosity: The Next Mars Rover” - RAeS lecture notes
Mad Scientess Jane Expat
Since Curiosity is soon to make its necessarily complicated landing on the surface of Mars, I thought I’d share the lecture notes I made when I went with three of my work colleagues to the Royal Aeronautical Society (not to be confused with the Royal Astronomical Society) in London a couple of weeks ago. The lecture was given by Matt Wallace, Lead Systems Engineer for Curiosity at NASA-JPL. It was therefore tech-focused rather than science-focused. I imagine this is mostly information that can be found in various places on the interwebs, but it was fun hearing it put together from the engineering perspective. I enjoyed the lecture immensely.
He began briefly reminding us that rover-based exploration of Mars is pretty modern. 1997 saw the landing of Sojourner, which lasted for 90 days and carried about 5 lbs of science instrumentation. 2003 saw the landings of Spirit and Opportunity, which carried about 11 lbs. Curiosity is a big step forward, carrying 185 lbs of science instrumentation and weighing about a ton. Opportunity saw sedimentary rock near its landing crater, which indicated that surface water must have been on Mars. It’s hoped that Curiosity will be able to measure whether or not another key ingredient for the evolution of life as we know it, carbon, is present in sufficient quantities to have potentially existed on Mars.
Curiosity will land on the surface of Mars on 6 August 2012. It lies flat in its 4.5 metre diameter entry capsule, which looks a bit like it might contain an oversized Pokemon. Or a Mini Cooper. (A photo of the entry capsule sitting over a Mini Cooper may have been shown.) The landing process looks straightforward at first. The cruise stage releases the capsule, spinning at 2 rev/minute. The entry capsule, doing 17000 miles per hour during descent, deploys its parachute after firing thrusters to correct its trajectory. It splits in half and lets the bottom shell drift away as it slows to 200 miles per hour. Then the top half of the shell detaches to allow the descent stage and rover to slow to 2 miles per hour.
Now comes the weird bit. The descent stage - the SkyCrane - lowers the rover on a bridle (which is controlled by the rover) and ever so gently releases it onto the surface of Mars. It then expends its remaining fuel and flies off and crashes 150 to 200 metres away from the rover. [The animation linked below makes it look like it lands miles away, for some reason!] The SkyCrane is the really unique piece of kit involved in Curiosity. Such a large, heavy rover needed a very controlled delivery mechanism. The Doppler accuracy on the descent stage has to be accurate to within a few metres when it and the rover are still 11 km above the planet’s surface.
Mr Wallace showed several interesting touchdown testing clips on both rough and smooth terrain, as well as the deployment testing in a wind tunnel of the disk-gap band parachute. The parachute is the biggest of these NASA has built - capable of withstanding 6000 lbs of impulse. It was also field-tested with an F-18. Curiosity has 17 cameras. Some are for practical purposes, like hazard avoidance, and others for science.
Curiosity will not be all that much faster than Spirit and Opportunity. It will travel up to 6 cm/s (less than 0.1 mph), meaning it can go about 100 metres a day. However, it has one major advantage over them because it’s nuclear rather than solar powered. It uses radioisotope thermoelectric generators (RTGs). This is unusual for a non-outer-planetary mission, but it was deemed necessary to achieve the science objectives of the rover. It means it can be operational whenever it likes, instead of being restricted by its battery capacity and view of the Sun.
Thermal testing was pretty extreme. The 25-foot test chamber was raised to daytime temperatures (+20 degrees Celsius), then pumped out and flooded with liquid nitrogen to simulate night conditions (-130 degrees Celsius). Curiosity will be subjected to this thermal cycle on a daily basis, so the electronics needed to be very robust.
The landing site is at the base of Mount Sharp, near what looks like alluvial flow. The site combines a crater with a mountain in the middle of it, where the crater had filled in with sedimentary material that subsequently eroded away. All this makes it quite a rich science target. Curiosity will spend its two-year prime mission traversing up the lower slopes. The landing ellipse (19 km by 7 km) is much reduced from what Spirit and Opportunity had, since it’s quite a hazardous location.
Landing will be completely autonomous, as the landing phase takes 7 minutes, while communication time from Earth to Mars is about 13 minutes. That will be fun for the mission engineers after waiting through Curiosity’s eight-month journey.
Here’s hoping all goes well on 6 August!
JPL’s animation of descent and landing (first 4 minutes), and rover operations. Curiosity pootles along the edge of the crater, checks out several different rocks until it finds one that’s interesting enough to “taste” and then samples it. YouTube video, 11:20, no narration.