For the finale to its Fall Lecture Series, last month the Owens Valley Radio Observatory (OVRO) and Cerro Coso Community College’s “Explore Your Universe” talk pulled back from black holes and the edge of the universe to focus on happenings closer to home. In this case, it was our nearest neighbor Mars that got the attention. Scientists Pam and Tom Hoffman from NASA’s Jet Propulsion Laboratory discussed the challenges of getting there in a talk entitled “Current and Future Missions to Mars.”
When it arrived on Mars, people from all over the country (including a crowd in Times Square, New York City) watched the final descent of the Mars Science Laboratory (MSL) Curiosity rover, which left Earth in 2011, on pins and needles, though it’s likely that most of us really had no idea how historic its touchdown really was.
The Red Planet, you see, isn’t named after the mythical god of war for nothing. Landing on the Mars surface doesn’t happen without a battle, one in which Earthlings have mixed results, with a historical success rate of only about 50%. “Getting to Mars is hard, we’ve sent 40 missions there over the last 40 years, and landed 16 times,” Pam noted. It is, however, a challenge the Hoffmans are looking forward to. Their project, the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) Mars lander, will follow Curiosity, which is currently exploring the Red Planet’s surface.
Tom Hoffman has served as the InSight Project Manager. Prior to InSight, Tom was Deputy Project Manager for the GRAIL mission, which conducted gravitational measurements of the Moon. During his 25 years at JPL, he has also worked on previous Mars Exploration rovers (Spirit and Opportunity), as well as the STARDUST, SeaWinds, Cassini, NSCAT and Voyager programs.
Pam is currently Instrument Mechanical Manager for the InSight’s Soil Moisture Active/Passive (SMAP) project, which will measure water exchange between the atmosphere and the ground to better model the Earth’s water cycle and climate. Pam managed the development of the Cruise, Entry, Descent and Landing (CEDL) mechanical hardware (including the aeroshell, the deployment mechanisms and the decent stage) for the Mars Science Laboratory (MSL) project from 2003-2009. She has also been part of the mission support team for the Mars Exploration rover program (Spirit and Opportunity), Cassini, NSCAT and Galileo missions.
In the last decade, Mars Odyssey is still orbiting, Mars Express, rover Opportunity is still at work, the Mars Reconnaissance Orbiter, the Phoenix mission in 2007 is completed, Curiosity is still at work, the Maven mission is set to launch by 2014. Following that, another Curiosity-sized rover will be launched in 2020, which will in part test sample gathering techniques that will lead to actually bringing back samples to Earth during the 2020s.
Landing is always the most critical part of any Mars mission. After hurtling toward the planet at thousands of miles per hour, and even after slowing down in the atmosphere, the craft is still falling at about 100 mph. There are three options: small rovers use cushions and retro rockets to bounce along at 30 mph. Larger craft use a combination of rockets and landing legs to touchdown at about 6 mph. Large rovers use a rocket pack to slow down to about 2 mph, and lower the lander via a sky crane.
Atmospheric conditions vary from day to day, and even hour-to-hour, with windstorms a common occurrence on Mars. A series of thrusters allowed for more maneuverability and more accurate placement in the chosen landing zone.
On a scale of 1 to 10, NASA puts landing a rover on Mars at a 20. The “Six minutes of terror” starts with atmosphere entry,” Pam said, slowing the craft from 12,000 mph down to 1,000 mph. Deployment of the parachute, the largest of which was 66 feet on the MSL mission, brought the craft down to 200 mph or less.
The aeroshell used by the MSL, a bit larger in diameter than a BMW Mini Cooper, was designed to use ballast weights that could be ejected during atmosphere interface to bring the vehicle in at an angle, as opposed to pointing the heat shield right into the atmosphere. It used a similar method to that used by the Apollo capsule when it re-entered the Earth’s atmosphere back in the 1960s and 1970s. “We keep getting bigger, I guess the next rover will be the size of an SUV,” Pam quipped.
The finished heat shield for NASA’s Mars Science Laboratory is the largest ever built for any spacecraft descending through the atmosphere of any planet. “We knew that because we were so much bigger, we’d be coming in hotter,” Pam explained. The first arc jet test of the heat shield was deemed a failure, she noted. The gaseous flow hit the shield dead on and survived just fine. However, because of the lift caused by the offset entry angle, the flow doesn’t come in nice and neat. The speed and weight of the vehicle caused turbulence. A similar program has plagued capsule research for manned missions, which use larger vehicles as well.
The heat shield is first a thermal protection system, SLA-561V, which had been used on various satellites and Mars spacecraft. During testing, though, SLA yielded a lot of bubbles and pitting, and in some sections completely burned down to the spacecraft’s hull. “Not good,” Pam summarized. The manned program looked at other materials, and developed the Phenolic Impregnated Carbon Ablator or PICA system, which had been developed for the STARDUST capsule, but wasn’t used because of its small size. For the MSL lander, PICA was adapted into a tile-like format, similar to that used on the Space Shuttle.
MSL Curiosity was the very first of any Mars mission to capture photos of the heat shield separation, just seconds after it was detached during final descent.
Parachutes were another issue. At full inflation, the first few tests revealed flaws in the seams and cord material, leading to breakage. And kinking occurred in the umbilical cords used to lower the lander to the planet’s surface.
Another hurdle involved engineering the retro rockets, which were similar to those used on the two Viking landers. Much of the early development work done on the Viking vehicles in the 1970s has been lost, and required almost entirely new research to make them compatible for the MSL mission.
Tom focused on the upcoming InSight mission. Assuming it arrives safely, some of InSight’s tasks will be to perform Seismology and Heat Flow measurements to better understand the formation and evolution of terrestrial planets, and how those could enable or limit the evolution of life.
“Curiosity is mainly looking for signs of life … but InSight has a very different goal,” Tom said. “We’re not really a Mars mission. We just happen to be going to Mars because it’s the best place to do the science we want to do. We want to find out more about terrestrial planets, those rocky planets … Earth, Venus and Mars.
“We’re going to put a seismometer to study how the crust mantle and core relate on Mars. We know a lot about the Earth, but we don’t know a lot about the other planets. We know some things about the moon from the Apollo missions. Mars is a good choice. It’s very similar to the Earth in terms of size. And there are Mars quakes, so we can pinpoint where they happen and get a lot of information out of that.”
A seismometer has been SENT to Mars before on one of the Viking missions, but it remained on the Viking lander’s deck, and was never coupled directly to the ground. To this day, Tom said no signals have ever been received from it.
“We’re also going to study how the planet rotates, and that will tell us a lot about the core, it’s size, whether it’s fluid or solid,” he added. “And then we have this device that’s going to hammer in 15 feet below the surface and measure how much heat is coming out of Mars, which will tell us how hot the core still is. We should come away with a good understanding of the geophysical properties of Mars.”
InSight’s equipment not only has to work, it has to work within Mars’ changeable weather conditions. According to Tom, the seismometer is sensitive enough to measure meteor strikes on the other side of the planet and the planet’s “tidal” activity. Thus, it has to be shielded from the wind and weather, and all the equipment has to be aerodynamically engineered to prevent being lifted or blown over by the “dust devils” that can occur there.
InSight is scheduled to launch in March 2016.
(Photo courtesy NASA and JPL)