August 2, 2012
If all goes according to plan, the Mars Science Laboratory, or MSL, will approach the Red Planet late on Sunday, Aug. 5, before slowing, deploying its parachutes, and lowering the rover "Curiosity" via cable to explore the surface.
Designing an autonomously controlled spacecraft to go from an initial speed of 13,000 miles per hour to almost zero in just seven minutes - on a planet where winds howl and temperatures are frigid - requires off-the-charts engineering acumen, as well as in-depth knowledge of Mars' atmospheric conditions.
Researchers at Oregon State University have been working for the past four years with the Jet Propulsion Laboratory in Pasadena, Calif., on a computer model of the Martian atmosphere that the project engineers have used to make adjustments in the spacecraft's control system for the landing.
"They call it 'the seven minutes of terror' because so much will happen in such a small window of time - and it is when the greatest risks to the mission take place," said Jeff Barnes, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. "MSL is one of the most robust space vehicles ever built but there will still be a lot of tension until those few minutes are over and we know that the landing was a good one."
The OSU Mars atmospheric model is one of two that NASA/JPL engineers have been using to make small adjustments to the on-board software that will guide the entry, descent and landing of the Mars Science Laboratory. Based on orbital observations of atmospheric conditions gleaned from the Mars Reconnaissance Orbiter, the model calculates and predicts what conditions are likely to be. It can be adapted in different "nests" to simulate the Mars atmosphere over a very wide range of spatial scales.
And the higher the model resolution, Barnes says, the better.
"The critical atmospheric factors are wind, temperature and density,' Barnes noted. "Density is the most important because you are trying to slow the spacecraft down and enable it to land within 10 to 15 kilometers of the prime target for science. Densities lower than expected could be real trouble, because the spacecraft will automatically 'dive' to lower altitudes to find higher densities in order to slow down sufficiently. If it gets too low before the parachutes are deployed, a safe landing would be jeopardized."
"Our Mars model has a spatial resolution that can get down to a horizontal scale of 4-5 kilometers, which provides the engineers with very good information about local atmospheric conditions," Barnes said.
Created by Barnes and OSU research associate Dan Tyler, the Oregon State atmospheric model of Mars is a continuation of their previous research on the Red Planet. Both OSU scientists worked on the Phoenix Mission, which landed in the north polar region of Mars in 2008, and Barnes' involvement in Mars research dates all of the way back to the historic Viking mission. More recently, Barnes was heavily involved in the 1997 Mars Pathfinder mission, which operated the first rover on the Mars surface.
But this is the most ambitious, and expensive (at about $2.5 billion), NASA Mars mission yet. The Mars Science Laboratory is designed to descend inside the very large Gale crater, hover at about 20 meters above the surface, and lower the Curiosity rover via cables to the surface. Past missions have "bounced" rovers down inside of giant airbag padding, but this rover, weighing one ton, is much bigger and heavier than those in the past."It's about the size of a Mini-Cooper," Barnes said, "so they've built a sophisticated 'sky-crane' system to lower it to the surface, then explosively sever the cables, and fire rockets to move the spacecraft away from the area so it doesn't fall onto and crush the rover. This is all totally new - it's never been done before."
The greatest risks, Barnes said, begin at about 15-20 kilometers above the surface.
"In this altitude region, the craft begins to fly almost horizontally over the Martian surface, which buys more time to slow itself down to a reasonable speed before the parachute deployment," Barnes said. "That's where the automatic control adjustments based on the expected temperatures - actually the speed of sound - and the winds and densities become the most critical."
The craft will land just south of the Martian equator - the first time a spacecraft has landed in the planet's southern hemisphere. It is mid-winter in the south, approaching the spring equinox and conditions are relatively mild. Predictions for the week of the landing include minimum temperatures of minus-110 degrees Fahrenheit, maximum temperatures of a balmy minus-10 degrees, and winds of about 10 miles an hour very near to the surface - though atmospheric winds will be stronger at higher altitudes. The biggest atmospheric threat to the landing is dust storm activity.
"If there are orbiter observations of a dust storm forming that could cause large changes in the dustiness of the atmosphere near Gale Crater, then there will be discussions about making last minute modifications to the onboard programming," Tyler said. "But I think that this is unlikely. There is a good deal of confidence now that the spacecraft system is very capable of dealing with the natural variability and will be able to land safely with great accuracy."
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