Last year, two satellites the size of cereal boxes sped toward Mars as though they were on an invisible track in space. Officially called MarCO A and MarCO B, they were nicknamed WALL-E and EVE, after the animated robots from the Pixar movie, by engineers at NASA.

They were just as endearing and vulnerable as their namesakes from "WALL-E." The satellites, known as cubesats, were sent to watch over NASA’s larger InSight spacecraft as it attempted a perilous landing on the surface of Mars at the end of November.

Constellations of small satellites like the MarCOs now orbit Earth, used by scientists, private companies, high school students and even governments seeking low-budget eyes in the skies. But never before had a cubesat traveled 90 million miles into space.

On Nov. 26, as the InSight lander touched down, its status was swiftly relayed back to Earth by the two trailing cubesats. The operation was a success, and the performance of the MarCO satellites may change the way missions operate, enabling cubesats to become deep space travelers in their own right.

NASA engineers weren’t sure what to expect when the MarCO mission launched in May. "I think it’s opened up so many doors and kind of shattered expectations," said Anne Marinan, a systems engineer at the Jet Propulsion Laboratory in Pasadena, California. "The fact that we actually got as far as we did with both satellites working was huge."

About a month after dropping InSight onto Mars, NASA lost contact with the MarCOs. The agency may attempt to wake them up someday, but for now WALL-E and EVE are silently roaming the solar system, proof of a new space exploration technology that almost never got to the launchpad.

Uncanceling the cubesats

The MarCO mission was canceled repeatedly. After all, the primary goal of NASA’s InSight mission was to land a stationary spacecraft on Mars and listen for marsquakes, giving scientists an improved picture of the red planet’s internal makeup.

And multiple spacecraft orbiting Mars already relay information from its surface back to Earth. The cubesats wouldn’t play a direct role in InSight’s success or failure, so it was a challenge to persuade NASA to support a nonessential program using unproven technology.

The MarCO team fought hard, prevailing at last with the argument that at a cost of only $18 million, the idea was worth taking a chance on. If these two tiny satellites worked well, it would mean that not only could similar spacecraft support big planetary missions in the future, but also that cubesats might carry instruments of their own.

Proving the technology’s reach could stretch NASA’s funding, the engineers said, while creating opportunities for wider exploration of the solar system.

As InSight safely touched down on Mars, the MarCOs were zipping past the planet, collecting readings from the landing and relaying them home more swiftly than the satellites currently orbiting Mars could.

"We had some astonishing statistics," said John Baker, manager of the SmallSat program at JPL. "We ended up getting 97 percent of all the InSight data back. And that’s because we had two small spacecraft at exactly the right position over the planet to receive the signals."

On their six-month journey to Mars, both cubesats occasionally lost contact with Earth. A couple of months after launch, MarCO B sprang a fuel leak and started spinning out of control. The team thought they’d lost it.

"Management is slowly encroaching upon the room," said Andrew Klesh, MarCO’s chief engineer, describing the scene. "We started to look at all the data. We broke apart the problem, and within about 24 hours we had MarCO B back under control."

Just a day before landing, MarCO B stopped communicating with Earth again. The cubesat came back online just in time. The InSight probe moved into the Martian landing phase that NASA officials know as "seven minutes of terror," and both spacecraft spoke to Earth the entire time.

Bigger's not better

While inexpensive cubesats like the MarCOs may serve as real-time communication relays for future deep-space missions, NASA has more adventurous goals in mind, some of which were hinted at in last week’s budget proposals by the Trump administration.

"When we have big spacecraft, you don’t want to necessarily take it into a very risky situation," Baker said. "But you can take an inexpensive probe and send it down to search or to get up close to something and examine it."

Baker and others at JPL are working on planetary cubesat missions. One proposal, nicknamed Cupid’s Arrow, envisions using the spacecraft to study the opaque atmosphere of Venus.

In other proposals, the next iteration of interplanetary cubesats would be scouts deployed by larger spacecraft studying worlds that could be hospitable to life. They could be sent into the plumes of Enceladus, Saturn’s icy moon, which ejects water vapor into space. Or cubesats could descend toward the surface of Europa, the ocean moon of Jupiter.

"These spacecraft will allow us to act as the ‘Star Trek’ probes to go down to the surface of challenging worlds where we might not be able to take the risk of a much larger mission," Klesh said.

Private companies are working on shrinking scientific instruments to be placed aboard the next generation of Earth-orbiting satellites. As instruments become smaller, the options for singular scientific missions in deep space become greater, as does the potential for whole fleets of MarCO-like satellites.

Toughening tiny travelers

But much work remains before more cubesats can travel beyond the moon. The challenges that come with operating full-size planetary missions apply to small satellites, too. If you want to go to the Jovian system, you need heavy radiation shielding. If you want to go to Saturn, you need more efficient solar panels and ways to keep the tiny spacecraft warm.

"We think we can actually send a small spacecraft all the way to Jupiter," Baker said. "The problem is, I have to come up with a way of automating the onboard spacecraft so that it can fly itself to Jupiter or you only have to talk to it once a month. Or we create a way for it to only radio home when it needs help."

These are the kinds of engineering challenges the MarCO team worked to overcome with the journey to Mars.

"It’s really opened a door of possibilities now that we have shown that this has actually worked," Marinan said. "It’s not an impossible concept anymore."