When NASA selected a $450 million spacecraft mission last year to fly past seven different asteroids in 12 years, the agency probably thought it was getting a bargain. And by NASA standards, the 2021 Lucy mission isn’t terribly expensive.
Don’t tell that to Pekka Janhunen, though. He and his colleagues at the Finnish Meteorological Institute propose a fleet of nanosatellites that could study 300 asteroids in only three years, at a fraction of Lucy’s cost. If their project ever flies, it would return more photographs of these small bodies than have been taken in 60 years of space exploration.
For 60 million Euros ($70.5 million), a fleet of roughly 50 small spacecraft would each study six or seven asteroids. But there are trade-offs. The Asteroid Touring Nanosat Fleet would zoom past each asteroid at a range of about 1,000 kilometers (620 miles), a considerable distance given that some asteroids are only a few dozen feet in diameter. The tiny spacecraft would carry telescopes only an inch and a half in diameter, substantially smaller than the ones on conventional asteroid missions.
As a way to reduce overall cost, the mission designers accept that a certain number of the spacecraft would fail in flight, according to Janhunen, the project’s principal investigator. Including backup hardware to lower the risk of that happening would add weight, “which means that a smaller number of them can be flown with the given launcher,” he said in an e-mail. “The optimum is, in my opinion at least, to keep the spacecraft lean and mean.”
To reach his targets, Janhunen would equip the spacecraft with an electric sail or e-sail. Rather than rocket fuel, this approach uses the solar wind—charged particles streaming from the sun—for propulsion. Each spacecraft creates thrust by rotating a tether, with one side of the tether attached to the spacecraft. The tether would rotate about every 50 minutes, creating a small thrust.
Although the propulsion effect is not well tested in space, Janhunen calculates that an 11-pound spacecraft with a 12.4-mile-long tether would have an acceleration of 1 millimeter (0.04 inches) per second, when the spacecraft is operating at the Earth’s distance from the sun.
Janhunen’s group has produced a one-kilometer tether on the ground, and plans to test a 100-meter version on the Aalto-1 Cubesat that launched in June. The technology also flew in 2013 on the ESTCube-1 CubeSat, but a technical problem stopped the tether from deploying.
“My understanding is that there has been quite a lot of work doing computer simulations of the e-sail and investigation of the physics that allows it to work,” said Jonathan Eastwood, principal investigator for an instrument on a NASA solar sail project called Sunjammer, which ended in 2014.
“The main benefit of e-sailing is that you don’t need traditional propellant,” he added in an e-mail. “That saves weight, and therefore money. It also means that missions could last almost indefinitely, as long as the mechanisms used to electrically charge the tether continue to work. It also offers the possibility of allowing new space missions, which would normally be too difficult with regular spacecraft because they would need too much fuel. A drawback is that it relies on the solar wind pressure, and so you have to be in the solar wind for it to work.”
E-sailing may have application to more than just propulsion. If the polarity of the tether is reversed from positive to negative, the e-sails could be used to deorbit space debris around Earth, Janhunen said. The European Space Agency provided early-stage funding for his investigation under its CleanSat project; Janhunen calculated he could bring down a 880-pound satellite from an orbit as high as 530 miles, or a 220-pound satellite from 745 miles.
The astronaut fleet concept is at least a decade away from flying, and depends, as usual, on funding. ESA is studying it under its science program’s “call for new ideas” in 2016. Details of the Asteroid Touring Nanosat Fleet were presented at the European Planetary Science Congress in Riga, Latvia last month.
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