Mars rover Opportunity visits heat shield impact site
Posted: December 28, 2004
NASA's Mars Exploration Rover Opportunity has returned this view of its discard heat shield, which crashed to the planet's surface during the craft's descent and landing last January.
The main structure from the successfully used shield is to the far left. Additional fragments of the heat shield lie in the upper center of the image. The heat shield's impact mark is visible just above and to the right of the foreground shadow of Opportunity's camera mast. This view is a mosaic of three images taken with the rover's navigation camera.
Opportunity has driven well over one mile on the plains of Meridiani while searching for evidence of past water on the Red Planet.
The rover recently climbed from the stadium-sized crater called Endurance. Once out, the rover examined some of its own tracks that it had laid down prior to entering the crater. It compared them side-by-side with fresh tracks in order to observe any weathering effects in the intervening 200 martian days. Opportunity then made its way toward the heat shield, which was located about 200 meters (220 yards) from the edge of Endurance.
Here is the full description of the entry-to-landing procedure for both Mars Explorations Rovers, according to the NASA pre-landing press kit:
The planned sequence of events for entering the atmosphere, descending and landing is essentially the same for each of the two rover missions, though the operation will take several seconds more for Spirit because its landing target is at a slightly lower elevation than Opportunity's. On both spacecraft, 15 minutes before atmospheric entry, the protective aeroshell encasing the lander and rover will separate from the cruise stage, whose role will at that point be finished. Each cruise stage will ultimately impact Mars.
Each spacecraft will hit the top of the atmosphere, about 128 kilometers (80 miles) above Mars' surface, at a flight path angle of about 11.5 degrees and a velocity of about 5.4 kilometers per second (12,000 miles per hour). Although Mars has a much thinner atmosphere than Earth does, the friction of traveling through it will heat and slow the spacecraft dramatically. The surface of the heat shield is expected to reach a temperature of 1,447 C (2,637 F). By 4 minutes after atmospheric entry, speed will have decreased to about 430 meters per second (960 miles per hour). At that point, about 8.5 kilometers (5.3 miles) above the ground, the spacecraft will deploy its parachute.
Within 2 minutes, the spacecraft will be bouncing on the surface, but those minutes will be packed with challenging events crucial to the mission's success.
Twenty seconds after parachute deployment, the spacecraft will jettison the bottom half of its protective shell, the heat shield, exposing the lander inside. Ten seconds later, the backshell, still attached to the parachute, will begin lowering the lander on a tetherlike bridle about 20 meters (66 feet) long. Spooling out the bridle to full length will take 6 seconds. Almost immediately, a radar system on the lander will begin sending pulses toward the ground to measure its altitude. Radar will detect the ground when the craft is about 2.4 kilometers (1.5 miles) above the surface, approximately 35 seconds before landing.
The Mars Exploration Rover design has two new tools, absent on Mars Pathfinder, to avoid excessive horizontal speed during ground impact in case of strong winds near the surface. One is a downward-looking camera mounted on the lander. Once the radar has sensed the surface, this camera will take three pictures of the ground about 4 seconds apart and automatically analyze them to estimate the spacecraft's horizontal velocity. The other innovation is a set of three small transverse rockets mounted on the backshell that can be fired in any combination to reduce horizontal velocity or counteract effects of side-to-side swinging under the parachute and bridle.
Eight seconds before touchdown, gas generators will inflate the lander's airbags. Two seconds later, the three main deceleration rockets on the backshell -- and, if needed, one or two of the transverse rockets -- will ignite. After 3 more seconds, when the lander should be about 10 to 15 meters (33 to 49 feet) above ground and have zero vertical velocity, its bridle will be cut, releasing it from the backshell and parachute. The airbag-protected lander will then be in free fall for a few seconds as it drops toward the ground.
The first bounce may take the airbag-protected lander back up to 15 meters (49 feet) or more above the ground. Bouncing and rolling could last several minutes. By comparison, the airbag-cushioned Mars Pathfinder bounced about 15 times, as high as 15 meters (49 feet), before coming to a rest 2-1/2 minutes later about a kilometer (0.6 mile) from its point of initial impact.
Twelve minutes after landing, motors will begin retracting the airbags, a process likely to take about an hour. Then the lander petals will open. No matter which of the four petals is on the bottom when the folded-up lander stops rolling, the petal-opening action will set all four face up, with the rover's base petal in the center. Opening of the petals is expected to take about 20 minutes if the spacecraft has rolled to a stop with its base petal down, about 35 minutes if one of the three side petals is down, or more than an hour if the rolling ended with the lander nose-down.