It was a beautiful day, with clear skies, perhaps a few wispy clouds. The temperature was perfect for the season, although it gets quite chilly at night. There is a dust storm several hunded miles away, but that was expected to subside within the next few days. This isn’t the forecast for Pasadena, California, home of NASA’s Jet Propulsion Laboratory however – it is for the planet Mars the new home of the Mars Science Laboratory (MSL) rover Curiosity. In one of the more amazing capabilities developed during the 15 years of continuous presence at Mars, scientists can monitor and predict the weather on a planet approximately 130 million miles away.
Although it is the middle of summer in North America, Gale crater on Mars will not reach the spring solstice until Sept. 30.
The Martian atmosphere, while far thinner than the Earth’s, contains the same ingredients to create active weather systems on the Red Planet as here on our home planet. Sunlight heats the surface, lifting columns of the atmosphere upwards which cools at it rises. Water vapor is scarce, but enough exists to create thin ice clouds high in the atmosphere. Strong temperature differences between the surface and the layers of the atmosphere cause the air to rotate forming miniature tornados. Jet streams, which play a key role in moving weather systems on Earth, perform the same role near the Martian polar caps.
Even at their strongest, the winds in the thin atmosphere are hard to feel. A future astronaut caught in a Martian dust devil is in no danger of being blown over. If Curiosity could see its parachute, over 600 meters away, it would be lying flat on the surface, far too heavy to be reinflated by the winds.
One of the primary science objectives for two of the spacecraft orbiting the Red Planet, the Mars Reconnaissance Orbiter and Mars Odyssey, is to photograph the planet and take temperature measurements. Drawing upon the experience of creating weather forecasting on Earth, scientists have learned to predict how weather patterns move and evolve on Mars.
In the days before Curiosity’s landing, scientists poured over now-daily weather reports generated by Malin Space Science Systems to gain information on how the winds and dust might affect the accuracy of its landing. Unlike violent weather that can make airliner travel dangerous, MSL can pass through even the strongest dust storm with little worry about its safety. However, one of the biggest concerns for engineers controlling the spacecraft in the period prior to the rover’s landing – was how the weather might have affected MSL’s attempt landing.
During MSL’s Entry, Descent and Landing, weather had its biggest effect soon after the spacecraft hit the top of the atmosphere. A day that is warm and dusty creates a higher and less dense atmosphere that keeps spacecraft from slowing down fast enough, but landing in cold and clear weather finds MSL hitting a denser atmosphere and possibly landing short.
Unforecast winds might have pushed the lander away from its intended target its parachute opened. Although traveling above Mach 1, even a gentle breeze could have pushed Curiosity in unexpected directions.
The Mars Science Laboratory was designed to handle a wide range of weather conditions, but was told beforehand what the “forecast” was to best prepare it for entry. The predicted weather turned out to match reality, and there were no problems with the rover arriving at its intended landing spot at Gale Crater. However, Martian forecasting can be as tough as it is on Earth, and surprises could have cropped up at the last moment.
In the days leading up to landing, meteorologists discovered a dust storm was forming in the southern hemisphere that could have reached Gale Crater within several days. From years of observing similar storms, scientists were confident that the storm would break down and dissipate within a few days. As Curiosity drew closer to entry, weather conditions were ideal. The atmosphere was clear and dense, and was expected to remain that way for several more days. The threatening dust storm began to dissipate, and was unlikely to be a factor. With a high level of confidence, controllers updated parameters onto the spacecraft compute, essentially telling Curiosity the forecast for landing day.
Slamming into the upper Martian atmosphere at 13,200 miles per hour, Curiosity found the conditions just as predicted. The cold dense air slowed the spacecraft considerably, and the parachute opened at Mach 1.7, comfortably below the worse-case predictions.
Once freed of the backshell and parachute, Skycrane with its rover stowed beneath was no longer at the mercy of the winds. Weighing several thousand ponds and able to control its position with its descent rockets, Curiosity ‘s computer sensed any drifting and change its flight path to compensate.
“We touched down in conditions that were on the more benign side of… expectations”, said Adam Stelzner, MSL Entry, Descent and Landing Lead.
Once on the surface, Curiosity will have its own weather station aboard. The Rover Environmental Monitoring Station (REMS) will record information on daily and seasonal weather. Mounted on finger-like booms on a mast several feet above the ground, data will be collected on temperature, pressure, humidity, and wind direction and speed.
