In the fall of 2005 at Cornell University, graduate student Justin Atchison and Mason Peck set out to create such miniature spacecraft. The aim of our project, called Sprite, is to fit everything a satellite might need on a 1-square-centimeter integrated circuit. The project finally took its first step into space on 16 May of this year, when the space shuttle Endeavour, on its final mission, carried three of our prototypes to the International Space Station. We’ll find out in a couple of years how these first chips withstood the rigors of space. If all goes well, we then plan to launch smaller Sprites into orbit on their own, where they can be used to test new forms of propulsion that could ultimately take them to other planets.
The Sprite prototypes weigh about 10 grams, but their successors will ultimately weigh between 5 and 50 milligrams and will likely be able to carry just one simple sensor each. One goal is a microchip 20 micrometers thick and weighing 7.5 milligrams. It is a tiny solar sails — Peck calls these ‘Sprites’ — that would have the right ratio of surface area to volume to accelerate at about 0.06 mm/s2, making them about ten times faster than the Japanese IKAROS sail.
Send tens of thousands of Sprites into orbits between Earth and the sun. These simple chips would have one task: to send a signal to Earth when the local magnetic field or the number of charged particles that hit the spacecraft exceeded some threshold. Taken alone, each chip would provide just one data point. But a network of these scattered chips could produce 3-D snapshots of space weather, something no traditional spacecraft, no matter how sophisticated, could ever do on its own. A payload of a million of the relatively heavy 50-mg version of the Sprite would amount to just 50 kilograms, about the mass of a single science instrument on one of NASA’s larger interplanetary spacecraft. So the launch costs of a Sprite network would be significantly lower than that of a traditional satellite.
Launching a million Sprites would be pointless unless a substantial number of them could survive the many hazards of space, including charged particles, micrometeorites, and extreme temperature swings. Their hardiness is one of the things we hope to gauge in our current experiment aboard the space station.
The low mass of Sprites should also allow them to harness the magnetic fields that surround planets and pervade the solar system. In this case, they’d be taking advantage of the Lorentz force, which bends the paths of charged particles that move in the presence of a magnetic field. Like radiation pressure, the Lorentz force dominates the dynamics of very small bodies.
A Sprite would need an electric charge to take advantage of this property of electromagnetism. In Earth orbit, charging a Sprite could be as simple as establishing a potential, via a power supply, between two wires that extend from the chip; the plasma in Earth’s ionosphere would do the rest. Lightweight free electrons would quickly neutralize the Sprite’s positive wire, but the heavier and slower positively charged ions wouldn’t be able to discharge the negative wire as quickly, leaving the spacecraft with a net negative charge. This charge would be maintained as long as the Sprite continued to power the wires.
At Cornell, we have begun testing this charging process by exposing Sprite-size spheres to a stream of xenon plasma. The setup mimics conditions in Earth’s ionosphere, and our early results suggest that the charging technique will work. If it can be accomplished in Earth’s orbit, Lorentz propulsion could allow Sprites to rendezvous with other satellites without releasing exhaust plumes that could damage delicate equipment. Charged Sprites would also be able to change their orbital inclination, enabling the chips to enter an equatorial or polar orbit regardless of their original launch location. Sprites could also raise their orbits, to counteract the tug of Earth’s atmosphere, and they might even escape Earth’s gravity entirely if the charge is high enough.
A more spectacular application of Lorentz propulsion could turn Jupiter into a particle accelerator. Jupiter’s magnetic field is 20 000 times as powerful as Earth’s, and so a charged Sprite could use this magnetic field to accelerate itself in orbit around the planet. Once it reached speeds of a few hundred or thousand kilometers per second, the chip would turn off its supply of power to the wires. Jupiter’s magnetic fields would then no longer confine the spacecraft, and it would be flung out of its orbit and indeed out of the solar system.
Sprites may be able to accelerate fast enough to reach the nearest star system, Alpha Centauri, in a few hundred years. That might not seem impressive; the speedup process itself could take decades, and the Sprites would arrive beyond any of our lifetimes. But consider the alternative: Solar sails, which have long been considered for interstellar trips, would take at least a thousand years—and probably a lot longer—to make such a journey.