Cheap, small satellites have swarmed into Earth orbit over the past decade, cutting the cost of studying our home planet from space. Now, these spacecraft, some no bigger than a briefcase, are becoming capable enough to venture into deep space—or at least the inner solar system. Two are halfway to Mars, more than a dozen planetary probes are in development, and scientists are coming up with ever more daring ideas for doing cheap, high-risk interplanetary science.
"Planetary is definitely getting excited," Lori Glaze, head of NASA's planetary science division, said last week at a symposium on small deep-space probes at Goddard Space Flight Center in Greenbelt, Maryland. Earlier this year, NASA began to accept proposals for a line of small planetary missions, with costs capped at $55 million. Glaze says 12 teams have submitted proposals, and the agency plans to select several finalists in February 2019. Europe, too, has plans for small planetary probes, also known as CubeSats for the cube-shaped modules from which they are built. "We see now the potential for interplanetary CubeSats," says Roger Walker, the European Space Agency's technology CubeSat manager in Noordwijk, the Netherlands.
Small satellites can be assembled from low-cost components and released by the dozen from a single rocket. But systems key to interplanetary flight, including propulsion, communication, and navigation, have traditionally been too bulky to fit into a small package.
A mission called Mars Cube One (MarCO), twin craft launched in May along with the Mars InSight lander, is breaking that size barrier. Built from six standard, 10-centimeter cubes, they are meant to provide a communication relay for InSight as it descends to the surface. But Glaze says the craft, which passed the halfway point in their journey last week, are already pioneers. "These CubeSats have flown farther than any ever before," she says. "They've already demonstrated the ability to do a comm relay." An unfurled radio antenna panel, three times the size of the CubeSats themselves, transmits a trickle of data directly to Earth using the CubeSats' limited solar power.
MarCO also showcases a miniature guidance, navigation, and control system developed by Blue Canyon Technologies in Boulder, Colorado. The technology has helped make CubeSats attractive for space science, says Dan Hegel, Blue Canyon's director for advanced development. "CubeSats were tumbling around, not doing much," he says. "There was no motivation before to try and shrink your instrument." The company shrank reaction wheels, gyroscopes, and star trackers into a system that sells for less than $150,000 and fits in half a cube.
Propulsion is a lingering concern. The small craft may need to change course, or slow down to orbit a planet, moon, or asteroid. Although MarCO's propulsion system occupies half of the craft, it holds only enough fuel to make small trajectory adjustments en route to Mars, and it squirts pressurized gas like a fire extinguisher, an inefficient approach. As a result, the CubeSats will helplessly coast past the Red Planet after completing their mission.
CubeSats in Earth orbit have tested solar sails, thin mirrored foils that deliver a gentle push from the pressure of sunlight. Other developers are betting on solar electric propulsion systems. A device built by ExoTerra Resource in Littleton, Colorado, uses electricity from solar panels to bombard a xenon gas "fuel" with a beam of electrons, creating a charged plasma. An electric field shoots the plasma out the back, generating a feeble thrust. No bigger than a hockey puck, the device, called a Hall thruster, uses fuel much more efficiently than conventional rockets do, ExoTerra President Michael VanWoerkom says. "If you're willing to wait longer to get there, you can package a lot of propellant into a very small space," he says.
A big test of propulsion technologies will come at the end of 2019, when NASA's heavy lift rocket, the Space Launch System, is due for its maiden voyage. It will carry 13 CubeSats, many of them focused on moon science. "Almost all are using different propulsion technologies," says Goddard's Barbara Cohen, principal investigator for one of the missions, Lunar Flashlight, an effort to confirm the presence of ice in permanently shadowed regions of polar craters by shining lasers into them.
Better propulsion could help solve another problem facing planetary small satellites: a lack of rocket rides. CubeSats often piggyback on larger mission launches, but rideshares beyond low-Earth orbit are rare. Solar electric propulsion systems could help craft released into low-Earth orbits make an escape. A small satellite equipped with a Hall thruster could spiral out from Earth to the moon in a few months, VanWoerkom says. Reaching Mars would take a few years.
Scientists are starting to have big dreams for their small packages. Tilak Hewagama, a planetary scientist at the University of Maryland in College Park, wants to send a small satellite to intercept a comet on its first arrival in the solar system. Most comets have swung around the sun many times, and their once-pristine surfaces have grown weathered. But nearly every year, astronomers discover a few that are swooping in for the first time. By then, it is too late to develop a spacecraft to study them, Hewagama says. But a small satellite already parked in a stable orbit could maneuver in time to witness the comet's passage up close—a risky plan that Hewagama says NASA wouldn't be willing to pursue for a larger, more expensive craft.
Timothy Stubbs, a planetary scientist at Goddard, wants to use two 30-kilogram satellites to explore the origin of curious bright swirls on the surface of the moon. One idea is that weak magnetic fields in moon rocks—implanted by comet impacts or a long-extinct magnetic dynamo—might be repelling the solar wind particles that weather and darken the surrounding soil. But understanding the interactions between the particles and the fields requires skimming the moon in a close, unstable orbit that would require large amounts of fuel to maintain. Stubbs's solution: Orbit two small satellites in tandem, linked by a thin Kevlar tether 25 kilometers long, so that a satellite in a higher orbit can stabilize its mate a mere 2 kilometers above the surface.
Both teams plan to submit proposals to the new NASA funding program—if they can whittle costs down to fit the $55 million cap. Small satellites may be cheap, but developing a deep-space mission traditionally requires a big team and lots of testing to pare down risk. Symposium organizer Geronimo Villanueva, a Goddard planetary scientist, says NASA officials are working on changing the rules for small satellites headed for deep space so that higher risk levels are acceptable. "We need to change the way we do business," he says.