SAO Guest Contribution
Mars Exploration Rovers - the View from Inside
Randii R. Wessen
Dr Randii Wessen has been an employee of the California Institute of Technology's Jet Propulsion Laboratory for twenty years. He is currently the Navigator Program Engineer. Previously, Randii was the Telecommunications and Mission Systems Manager for the Mars Program, the Supervisor for the Science System Engineering Group, Manager of the Cassini Science Planning & Operations Element, the Galileo Deputy Sequence Team Chief, and the Voyager Science Sequence Coordinator for the Uranus and Neptune encounters.
Randii received his Bachelors of Science in both Physics and Astronomy from Stony Brook University, a Masters of Science in Astronautics from the University of Southern California, and a Doctorate in Operations Research from the University of Glamorgan, Wales, England. He co-authored the book Neptune: the Planet, Rings and Satellites and was the recipient of NASA's Exceptional Service Medal for his contributions to the Voyager 2 Neptune Encounter. He also has six NASA Group Achievement Awards and is a fellow of both the Royal Astronomical Society and the British Interplanetary Society.
It was 2004 January 3rd and about an hour and a half away from "bounce-down" for the first Mars Exploration Rover (MER). This rover was named Spirit and it was rapidly approaching Mars. The culmination of almost four years of work was literally minutes away from being a spectacular success or an embarrassing failure. I began to think about my earlier role as the Telecommunication & Mission Services Manager for Mars. In that capacity I was responsible for coordinating the Deep Space Network (DSN) resources (e.g., antennas, ground support equipment, personnel, etc.) for any missions going to Mars and requiring the use of the United States' world-wide deep space communication network.
Back than, five spacecraft were heading to Mars and would arrive late in 2003 and early 2004. Each craft would require the services of the DSN antennas for communications and navigation. The Japanese spacecraft Nozomi would be the first to arrive at Mars on 2003 December 13. This would be followed by the European Space Agency's (ESA) Mars Express with its Beagle II Lander. The Beagle II Lander would separate from Mars Express about 6 days before the mother spacecraft attempted orbit insertion. Finally the two United States Mars Explorations Rovers would "land" on 2004 January 3rd and then 21 days latter on January 24th.
With all of this activity, the use of DSN assets would have to be carefully managed between all the spacecraft. This task was made more difficult by the fact that 2/3 of all the spacecraft ever launched to Mars fail. A strategy for reallocating antennas in the case one vehicle declared a spacecraft emergency would have to be developed. And of course the problem of taking an antenna away from one spacecraft to help another in distress was that every mission wanted more tracking time, not less.
Looking back at that activity revealed two things, 1) space exploration is not easy and 2) a little luck goes a long way. Results of all our contingency planning was successful but Mars would not give up her secrets with out forcing us humans to pay a price. The Japanese Nozomi spacecraft and the Beagle II Lander both failed. As of January 2rd, two of the three spacecraft that were heading to Mars had ended in mission loss. What would happen to the next two missions to Mars?
With an hour and a half to go the flight team was updating the last of the entry parameters for the first rover. At this time the cruise stage was changing the attitude of the entire spacecraft for entry. This new attitude made sure that the rover's heat shield would take the brunt of the shock as the spacecraft slammed into the atmosphere of Mars. The turn to this attitude was painfully slow. It took about 15 minutes to complete the turn. For me, working in the Pressroom, there was nothing to do but wait. And waiting is horrible. All you can do is think of the myriad of things that can go wrong. What happens if the heat shield is loose? What happens if the parachute doesn't open? What happens if we land on a sharp rock? The list goes on and on.
None of the reporters were asking questions. Everyone was glued to the TV monitors that displayed the activities from the Mission Support Area. At 21 minutes away from "bounce-down" Spirit executed the commands to jettison the cruise stage that had successfully guided the spacecraft to Mars. With Mars rapidly filling the field of view of the rover and the entry path already achieved, the cruise stage was no longer needed. The excess mass and irregular shape would just make the vehicle tumble as it entered Mars' atmosphere. As the cruise stage silently separated from the entry stage a flight controller called out, "Thanks for flying with us, you're now on your own."
Atmospheric entry was only 15 minutes away. From atmospheric entry to touchdown was only six minutes. At that point it hit me. With a one-way communication light time of over nine minutes, Spirit would enter the atmosphere, fire all of its explosive devices, and hit the ground before telemetry from Spirit indicated that it had reached the upper atmosphere of Mars! In essence, the spacecraft would already be on Mars (either a successfully landing or in pieces), before the telemetry let its Earth-based controllers know that the entry stage had reached the atmosphere of Mars.
Finally it began. At landing minus six minutes (approximately 120 km (75 miles) above the surface), Spirit slammed into the planet's atmosphere traveling at a speed of 5.4 km/sec (12,100 miles/hour). Even though Mars has a very thin atmosphere, temperatures were expected to reach peak levels of 1,447 C (2,637 F) at approximately two minutes from "bounce-down." The heat shield was expected to absorb the shock of this for four of the six minutes that it would take the craft to traverse the Martian atmosphere. These four minutes dragged on and on. With the event already over on Mars, all you can do is keep your fingers crossed and watch the telemetry.
At an Earth-received time of 113 seconds away from reaching the surface, the tone from the spacecraft reached Earth that indicated that the hypersonic parachute had been commanded to be explosively shot from the back of the lander. This occurred at an altitude of 8.6 km (5.3 miles) and at a speed of 472 km/hour (293 miles/hour). Twenty seconds later explosive bolts were fired that held the heat shield to the lander. The heat shield continued on to the ground as the lander with its parachute slowed. Unfortunately, with a very thin atmosphere, the parachute was not going to do enough. If the entry vehicle did nothing else, it would hit the ground with a speed of 193 km/hour (120 miles/hour).
Designers knew this and had leveraged information learned from 1997 Pathfinder mission to Mars. The solution, which was used six years early and again with the Mars Exploration Rovers, was airbags and small rocket motors. The airbags were placed on each side of the tetrahedron shaped lander and would cushion the rover from impact. However MER ground testing had indicated that the airbags could not tolerate much horizontal velocity. At almost twice the entry mass of Mars Pathfinder, the MER spacecraft were susceptible to airbag ruptures.
To counter the effect of crosswinds, a new system was added to MER. It was called DIMES (Descent Image Measuring Estimation System). This system would take three images of the terrain as the entry vehicle approached the ground. An inertial measuring system coupled with the descent images would allow the rover to determine if the crosswinds were too great for the airbags. If the system did determine that the winds were too great, the rover would command the Transverse Impulse Rockets (TIR) to fire to cancel out this horizontal motion.
At 35 seconds from the ground and at a height of 2.4 km (1.5 miles) the onboard RADAR acquired the ground and began acquiring the descent images. We know now that the winds were too great and commands were issued to fire the TIR. With the ground still rushing up very fast, and impact only 8 seconds away (284 m (932 feet) above the ground), Spirit inflated its airbags. Two seconds later the Rocket Assisted Descent (RAD) rockets fired along with TIR. The RAD system was designed to fire in such a way as to completely cancel out all of the vertical velocity for an instant. With the fall toward the planet completely stopped for the moment, the bridle connecting the parachute/backshell/RAD rockets from the airbag/lander/rover was cut. At this point the rover was three seconds away from impact and at a height of just 7 meters (23 feet). From there the entry vehicle just fell.
Randii Wessen (right) in the Pressroom during a Mars Exploration Rover landing.
|In the Pressroom we listened to each call from the Flight Controller. We received the tones from the rover that indicated that the RAD rockets had fired. When we had confirmation of the tones, we cheered! However, we did not expect to hear the tones after bounce down since the vehicle would be tumbling. We waited impatiently to see if everything had worked. Then, there it was! A tone was received that indicated the rover had survived past the first bounce. The room exploded again with cheers. People were hugging, crying, and shouting. Reporters were furiously snapping pictures. It was a melee of sound, lights and emotions. However, there was no second tone from Spirit. The cheers started to fade as everyone began to once again stare at the TV monitors. Did we really see a tone in the telemetry? Could the flight controllers be wrong? The minutes went by. With each passing moment, the chances that Spirit had survived the landing faded. More than eight minutes had passed and still nothing. As the chances of success slowly slipped away, the flight team exploded with joy as the next tone was received. Spirit had landed safely on Mars.|
Atmospheric entry was now over, and the flight team began the work on rover egress from the lander. This effort would take more than a week to accomplish. In the mean time, mission controllers had to assess the health of the spacecraft, determine where "they" were on Mars, and then begin the exploration of the Gusev Crater. The crazy part about all of this was ...we had to do this all over again in three weeks!
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