The current flight schedule is a bit lean, however. NASA's JUNO orbiter is en route and expected to arrive in July 2016, but it will orbit Jupiter to study the planet's atmosphere; it will not visit Europa. The European Space Agency is planning to launch a mission to Jupiter's icy moons in 2022, but it won't arrive until 2030. Budgets for big missions are even leaner.
Researchers have another idea: Send CubeSats – small satellites built up from cube-shaped modules about 4 inches on a side.
Typically, these tiny tots of the satellite set have been limited to serving as teaching tools for aerospace engineering students or test-beds for new space hardware, or for modest, short-duration missions in low-Earth orbit. But researchers have started to explore the possibility of turning them into the mighty mites of solar-system exploration.
To that end, Benjamin Longmier and colleagues at the University of Michigan are developing a small ion-drive propulsion system that could help CubeSats break the bonds of low-Earth orbit.
“People are interested in these sorts of missions – going to Europa, finding life in the solar system. Right now the only way we can do missions like that is through NASA or through the European Space Agency and maybe the Russians,” says Dr. Longmier, an assistant professor of aerospace engineering at the University of Michigan at Ann Arbor.
Such missions tend to be so expensive that that for any individual space agency they come around only about once every 10 years.
Go small, Longmier says, and the costs drop, increasing the opportunities.
For long-time observers of the US space program, that sounds a lot like the “faster, better, cheaper” approach former NASA administrator Dan Goldin championed in the 1990s, with mixed results.
Still, the prospect of giving CubeSats an ability to travel beyond low-Earth orbit is getting a serious look.
Last December, a team of researchers led by Robert Staehle, with NASA's Jet Propulsion Laboratory, produced a report for the space agency's innovative-advanced-concepts program exploring the technologies needed to turn CubeSats into low-mass tools that could be used for missions lasting up to five years and carrying price tags of less than $30 million.
The propulsion approach Dr. Staehle's group examined involved solar sails, which use the radiation pressure from sunlight to propel a spacecraft. NASA and Japan's space agency JAXA have tested solar sails on in orbit. Both were launched in 2010. NASA's version, NanoSail D-2, kept a three-module CubeSat on orbit for 240 days.
By contrast, Longmier's team is going high tech. It's working on a small version of ion propulsion – one in which the motor and the fuel, which the team envisions as water, fit into one of three CubeSat modules that would make up their spacecraft.
Nor is his team alone. A research group at the Massachusetts Institute of Technology also is working on ion propulsion for CubeSats – an approach that the group, led by Paulo Lozano, director of the institute's Space Propulsion Lab, initially plans to test as thrusters for steering CubeSats.
Ion technology has long been used to help full-scale satellites maintain their orbits. Over the past 15 years, ion engines have powered four deep-space missions launched by the US, Japan, and Europe.
Ion thrusters use electricity to ionize gas and accelerate it to provide thrust. They don't generate much instantaneous push, but they use their fuel more efficiently and can run continuously for years. Chemical rockets generate more thrust, but they gulp fuel in such large amounts that running them for long periods is impractical. Because ion thrusters can operate far longer than their chemical counterparts, the craft they propel can reach higher velocities.
Longmier's team calculates that under the right conditions, an ion-propelled CubeSat could reach Jupiter in about two to three years. ESA's JUICE orbiter would take eight years. JUNO is taking five years.
The challenge, Longmier says, it to simplify and miniaturize the thrusters.
One way is to use water as fuel instead of an inert gas, such as xenon. Big spacecraft store the gas in a pressurized cylinder. CubeSats have no room for that. Because liquids are more dense than gas, water represents a way to pack a lot of atoms – fodder for ionization – into a tight space and with simpler storage.
Another is to design the thruster so that it needs no additional source of charged particles to operate properly. Typical ion thrusters require this extra source, which generates a stream of electrons. These bind with the positively charged ions as they hurl from the thruster, returning the atoms to an electrically neutral state as they travel. Without this step, the positive ions from the thruster would merely be drawn to the spacecraft and form a halo around it rather than provide the push the craft needs.
Longmier's team has designed a system that uses the electrons the thruster strips from atoms initially as the electrons that will return the ions to their neutral state as they zip from the thruster.
The team is working toward a launch late next year of a test CubeSat that uses a prototype motor.
“Once we get into space, we're going to learn a lot about how to operate this propulsion system – what fails, what doesn't fail, what works well, what we need to change for the next iteration,” he says.