|A NASAs artists conception of what ProSEDS will look like as the Icarus satellite, at upper right, begins to pull the tether away from the spent Delta II rocket booster, at left.|
Weighing in at only 50 pounds and about the size of a small microwave oven, the box-like satellite is named Icarus after the Greek legend of the man who flew too close to the sun, melted his wax-covered wings, and plunged into the Aegean sea. If everything goes right, the painstakingly hand-built satellite will meet a similar fate, burning up in the Earths atmosphere a week or two after its mission begins sometime in the fall 2000.
Though student teams at the U-M have flown experiment packages on the space shuttle and participated in building sub-systems of satellites before, the Icarus project will be the first entirely student-built U-M satellite to be flown by NASA.
It was made to function as an autonomous satellite, said student project manager Jane Ohlweiler, a masters degree student in space systems engineering. The packages primary function is to act as a weight to pull a nine-mile-long string called a tether out from a spool on a spent Delta II rocket booster. If everything works as designed, Icarus will end up sailing along at the end of this tether like an orbital plumb bob. First and foremost, were a dead weight at the end of the tether, Ohlweiler acknowledges.
But rather than making the box a mere lump of mass, the students have covered Icarus with solar cells and crammed it full of instruments that will gather data on the tethers motion and position and then beam that information down to listening stations around the Earth. The finished satellite will be a rectangular box about 18 inches long and a foot high, and all of itthe aluminum box, the instruments inside it, the complex network of wiring, and even the circuit boards that make it workwere designed, built and tested entirely by the student team.
Our mission is to prove that we can do this, said B.T. Cesul, a senior in chemical engineering and assistant manager of the project. And were helping NASA prove the smaller, faster, cheaper model.
Icarus is part of a larger mission (ProSEDS, the Propulsive Small Expendable Deployer System) in which Brian Gilchrist, associate professor of electrical engineering and computer science and associate professor of atmospheric, oceanic and space sciences, is participating. The primary mission of the Delta II rocket launch will be to lift a Global Positioning System satellite into orbit sometime this fall. Once that is done, ProSEDS will get to work. Icarus, the spool of tether, and an array of instruments to gauge the experiments success will be mounted on the side of the rockets second-stage booster.
Normally, a spent booster like this would take as much as a year and a half to tumble into reentry and burn up, but ProSEDS aims to bring it down in 21 days or less. Gilchrist and his colleagues who have been studying varying uses for space tethers think the fuel-free source of thrust created by a 15-kilometer kite string could be a boon to satellite operators. Space has become a fairly hazardous place to fly, with thousands of bits of dead spacecraft and spare parts zinging around. A cost-effective way to quickly deorbit spent payloads, pull big things like space stations into higher orbit, or even to do mundane tasks like taking out a space stations trash, would be a great improvement. For example, if tethers were used to help keep the new International Space Station aloft for 10 years, the savings over conventional fuels would be about $2 billion, NASA estimates.
Icarus also represents the latest in a new trend in engineering education being pursued at Michiganstudent team projects. Tackling authentic engineering problems and working in an interdisciplinary team helps prepare students for the way engineering is currently being practiced in the real world. Its the kind of thing you dont learn in the classroom, said Ohlweiler, sitting shoulder to shoulder with her teammates in a cramped office plastered with posters from U.S. and Russian space missions. Nearly 100 students from six different engineering disciplines have been involved. Weve had everybody from freshmen to Ph.D.s participating and doing things they never thought theyd get a chance to do, Ohlweiler said.
How it works
At 250 miles of altitude, the drag created by the tether isnt from air resistance, its from the Earths magnetic field. The first 5 kilometers of the tether are a conductive wire that captures passing electrons and sends them streaming toward the Delta II rocket booster. The interaction between that electrical current and the Earths magnetosphere results in a sort of drag that slows the rocket stage down and makes it start to fall. The tether also generates about 100 watts of electricity that can recharge the experiments batteries and keep its instruments running. Icarus is powered by some space-grade C batteries and solar panels.
Why use tethers?
Tether propulsion should work near any planet with a magnetic field, including Jupiter. And it wouldnt be just for taking things down. This fuel-free source of thrust could also be used to lift satellites and space stations into a higher orbit. One proposal envisions a fleet of tether-powered tug boats in space that would lift satellites up to higher orbits after theyve been carried aloft by rockets. Thats what NASA terms a low- recurring-cost space asset or a good deal.