Date: July 30, 2012
Title: 7 Minutes of Terror: Entry Descent and Landing of the Mars Curiosity Rover
Podcasters: Tony Rice
Description: Curiosity landing
Bio: Tony Rice is a Solar System Ambassador in Raleigh, NC and contributor to WRAL.com on spaceflight and astronomy topics.
Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
Getting to Mars isn’t easy. In the nearly 50 attempts, less than half have been successful. Many failed on launch, some failed en route, some ended up in smoking heaps on the surface. NASA’s Jet Propulsion Laboratory has a pretty good track record of putting rovers on Mars.
Early this week JPL made final course corrections to put the latest rover, the car sized Curiosity on course for landing at Mars Gale Crater in a little over a week’s time. There are 2 additional opportunities for corrections before the landing the night of August 5 into the morning of August 6, Earth time. And that landing, is nothing short of spectacular.
You may have seen “7 Minutes of Terror”, the video put out by JPL outlining the steps to take the rover from its current over 13,000 mph or 5,900 meters/sec down to zero on the planets surface.
17 minutes before landing, explosive bolts will fire detaching the cruise stage, a 880 pound, 400 kilogram ring shaped which provided power, trajectory corrections, and communication back to Earth during the 8 1/2 month trip. The entry, descent and landing instruments will also begin taking measurements.
16 minutes before landing, small thrusters on the aeroshell encasing the rover on its journey will stop the 2 rpm spin that kept it stable during that cruise phase. Those thrusters will then orient the spacecraft into the correct angle to enter the Martian atmosphere. A pair of solid tungsten 165 pound 75 kilogram weights will be jettisoned.
This will shift the center of mass from axis it had been spinning during the trip from Earth to a point where lift can be generated by the shape of the spacecraft as it enters the atmosphere, slowing it considerably. 9/10ths of that 13,000 mph will be removed as Curiosity, snug in her aeroshell, behind the largest heatsheild built for a planetary mission.
The nearly 15-foot (4.5 meter) heatshield built by Lockeheed Martin is more than 2 feet larger that ones used on Apollo capsules.
Though the rover is still nearly 80 miles (125 km) above the ground, the science has already begun. The Mars Science Laboratory Entry, Descent and Landing Instrument (MEDLI) instrument will gather information about the descent which will be used to not only better understand the Martian atmosphere but measure the effectiveness of the heatshield, data that will be very useful in any future manned missions to Mars.
It’s important to point out, that all of this is going on without assistance from controllers back on Earth. It’s preprogramed, according to schedule and little decision-making has been required by the spacecraft. The next phase is different thought.
As the spacecraft enters the atmosphere, a point called Entry Interface, it will begin performing “S” turns to both bleed off more speed and make any trajectory corrections needed to get back on target. This cant be preprogrammed as atmospheric effects such as wind cant be completely accounted for in advance.
About 75 seconds after entry interface, the heatshield will reach nearly 3,800 degrees F ( 2,100 C). 10 seconds later the spacecraft will reach 10-15g’s in deceleration during this violent descent.
Another pair of tungsten weights are then jettisoned to put that center of gravity back on the axis of symmetry. Another 6 weights, each about 55 pounds (25 kilograms) is jettisoned, balancing things out for the next phase: parachute deployment.
Engineers note that Mars’ largely carbon dioxide atmosphere is 100 times thinner than that of Earth. Thick enough to cause destructive heating, thin enough that parachutes are not enough to slow a landing spacecraft.
With under 3 minutes before landing, a parachute, the largest ever created for a planetary mission measuring 51 feet wide (nearly 16 meters) and capable of handling 65,000 pounds of force is deployed.
The spacecraft has slowed to 900 miles per hour (~405 meters / sec)
24 seconds after the parachute is deployed at an altitude of about 5 miles or (8 kilometers), the heatshield is jettisoned. The spacecraft has slowed to a more manageable 280 miles per hour (125 m/s)
So far, this isn’t very different from the previous 3 rover landings for the microwave oven sized Pathfinder mission in 1997, or the golf cart sized Spirit and Opportunity in 2003. Use the atmosphere to slow down. However those 3 lacked the guided entry Curiosity requires and were able to use airbags to cushion the landing. That works for the 374 pounds (170 kilograms) Sprit and Opportunity but not for the nearly 2000 pound (900 kg) Curiosity.
At this point, the Mars Descent Imager fires up and begins recording high definition video and radar, looking for the landing site. 85 seconds later and about 1 mile (1.6 kilometers) above the ground at a speed of about 180 miles per hour (80 meters per second), 8 retro rockets on the descent stage attached to Curiosity’s back will fire removing nearly all of the remaining descent speed.
Once the rover has slowed to 1.7 miles per hour (.75 meters per second) half the retrorockets will power down and the vehicle will hover about 66 feet (20 meters) off the surface of the red planet. Curiosity will then be lowered on a set of nylon cords as she unfolds here six wheels for the first time, like a traveler stretching after a long journey.
As the descent stage sense the sudden lack of weight on those cords as Curiosity is set down on those six wheels, the cords are cut and the descent stage flies off 150 meters or more, out of the rover’s way.
At least that’s how it’s supposed to work, and nothing can go wrong or it all will go wrong.
154,225,000 miles away a very excited group of scientists and engineers will be waiting in mission control at the Jet Propulsion Laboratory in Pasadena, CA. Waiting to hear the fate of the robot they’ve poured years of their lives into. Did she make it? When word comes around 10:31 Pacific Time, it will be in the form of initial telemetry from the rover relayed by one of a number of satellites in Mars orbit. Whatever the verdict, it will have happened 13 minutes 48 seconds earlier due to the time required for signals to pass between Mars and Earth.
Those JPL scientists and engineers wont be the only ones on the edge of their seats awaiting word from Curiosity. The event will be carried live on NASA TV. Here in North Carolina, events are planned at the Morehead Planetarium and Science Center in Chapel Hill the afternoon before the landing. The North Carolina Museum of Natural Sciences is planning a night at the museum to watch it unfold live. There are other events around the world planned to celebrate the landing including Australia, New Zealand, Spain, the UK and the US. For more information on these events as well as a wealth of information in this and other NASA Mars missions, visit http://mars.jpl.nasa.gov
End of podcast:
365 Days of Astronomy
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