Astronomers Planning Close-ups of Mars from a Baloon
The two Mars rovers, Spirit and Opportunity, have accomplished more than their creators thought possible. They have muscled across the planet’s rugged terrain for more than two years, collecting data about the composition of its rocks and soils in the process.
Even the most dogged workhorses, however, have limitations. In their entire time on the planet, the slow-moving rovers have traversed less than 10 square miles, and their sensors cannot collect data more than a few feet above the ground.
To survey areas of the planet that remain unknown, researchers affiliated with the privately financed German Mars Society in Munich are proposing a different kind of explorer: a vehicle inspired more by dirigibles (飞船) like the Hindenburg than by land-rover predecessors. Projected to reach Mars in 2009, the balloon craft, named Archimedes, would hover much closer to the planet’s surface than a satellite, snapping crisp, full-color images similar to those that an Earth photographer might take from a helicopter.
In an hour-long descent to the ground, the balloon craft would use an array of sensors to take temperature, wind and humidity readings. These data, collected at a variety of altitudes, would provide the raw material scientists need to understand Martian weather patterns.
The balloon craft was the brainchild of Hannes Griebel. In 2002, as an advanced engineering student at the Technical University of Munich, Mr. Griebel was looking for a Mars project that would not retread ground that rover missions planned to cover. “There’s so much about Mars we don’t know,” he said. “I started to think, ‘What are the data we haven’t been able to get?’”
A helium (氢)-filled balloon craft, he reasoned, could obtain a more comprehensive profile of the planet’s surface and atmosphere than any other vehicle. Best of all, the project would be cheap enough — under $2 million — that he could pursue it with money from private donors instead of depending on government financing.
Dr. Hausler, head of the Institute for Spaceflight Technology at the University of the Federal Armed Forces in Munich, was initially skeptical when Mr. Griebel approached him with the plan. “Hannes was proposing this crazy idea that we try to put a balloon into Mars’s atmosphere,” he said. Still, seeing the project’s potential, he decided to offer Dr. Griebel a faculty position instead of turning him away. “I told him, ‘Let’s attack this.’”
At first, Mr. Griebel envisioned a balloon that would inflate once its launch vehicle had entered the atmosphere. But in Mars’s thin carbon dioxide surroundings, “there isn’t much to slow the craft down as it falls, so you need to inflate something big in a very short time,” he said. Achieving this would require a complicated and expensive groundbased control system, so he scrapped(放弃) the idea, in stead in favor of inflating the balloon in deep space and then propelling the buoyant (有浮力的) object into the Martian atmosphere. That option presented its own problems. As a spacecraft reaches the threshold between the vacuum of space and a planet’s outer atmosphere, large amounts of friction turn the energy its orbital motion has generated into heat.
At a little over 170 pounds, the craft would generate less than a quarter of the friction that a space shuttle might, but its filmy surface would still heat up to about 620 degrees Fahrenheit during atmospheric entry. The project team plans to coat the 33-foot-wide balloon with a heat-tolerant chemical material.
Making sure the inflation mechanism would work in a zero-gravity environment also proved daunting(困难的). If pockets of gas became trapped in the folded balloon’s plastic crevices(缝隙), they would expand rapidly in the vacuum of space, possibly jamming the delicate mechanical components that position the balloon for inflation.
The team scheduled a test flight last spring on the Zero-G Airbus A300 of the French company Novespace. The plane flied in an up-and-down pattern that made objects in the cabin weightless for seconds at a time. Project scientists deployed(展开) the balloon more than 80 times during these weightless moments, and it inflated as expected about 90 percent of the time.
Last month, the researchers sent the craft into true zero gravity, strapping the balloon and a camera-containing module to a sounding rocket that went more than 60 miles above Earth’s surface. Although a few grainy images that cameras transmitted to Earth showed that the balloon inflated properly upon reaching deep space, the remainder of the test was a comedy of errors. The rocket collided with the recently detached camera module, preventing it from acquiring planned high-resolution footage of the craft. Another trial is planned for October.