(CNN) When NASA's InSight lander was on its seven-month trip to Mars, it wasn't alone. Two suitcase-size spacecraft, called MarCO, followed InSight on its journey. They were the first cube satellites to fly into deep space. And now, they've fallen silent.

During InSight's entry, descent and landing on November 26 , the MarCO satellites received and transmittedcommunication from the lander to let NASA know that InSight was safely on the surface of the Red Planet. They were nicknamed EVE and WALL-E, for the robots from the 2008 Pixar film.

They were the great experiment accompanying the InSight mission, and their success was measured by survival, according to NASA. The fact that the tiny satellites made it to Mars, flying behind InSight through space, excited engineers.

But the experiment has come to an end.

After InSight was safely on Mars, MarCO kept on flying. At the time, the MarCO team collected data from each satellite to determine how much fuel they had left and took a deeper look at how they performed.

Engineer Joel Steinkraus uses sunlight to test the solar arrays on one of the MarCO spacecraft.

"WALL-E and EVE performed just as we expected them to," MarCO chief engineer Andy Klesh at NASA's Jet Propulsion Laboratory said at the time. "They were an excellent test of how CubeSats can serve as 'tag-alongs' on future missions, giving engineers up-to-the-minute feedback during a landing."

Engineers last heard from WALL-E on December 29 and EVE on January 4, the agency said. They estimate that EVE is now 2 million miles past Mars, and WALL-E is more than 1 million miles past Mars.

The loss of communication isn't entirely a surprise. The team believes that WALL-E's leaky thruster, wobble-causing control issues, the lack of being able to send and receive commands or even battery issues could be at play. The satellites have sensors to point them toward the sun and recharge.

The satellites are orbiting the sun and will only gain more distance from Earth. If there is any hope of future communication, the satellites will have to keep pointing their antennas to be able to transmit to mission scientists. The team believes there is another chance to try to contact the satellites this summer, when they're moving toward the sun, but they don't know whether the batteries or other components will last that long in deep space.

Little satellites have a big future

But MarCO was still a successful experiment, they said.

"This mission was always about pushing the limits of miniaturized technology and seeing just how far it could take us," Klesh said. "We've put a stake in the ground. Future CubeSats might go even farther."

MarCO is a demonstration of potential future capability, said Jim Green, director of NASA's Planetary Sciences Division.

In addition to assisting with communication about the landing, MarCO-B took an image of Mars from 4,700 miles away during its flyby.

"WALL-E sent some great postcards from Mars!" said Cody Colley of JPL, MarCO's mission manager. "It's been exciting to see the view from almost 1,000 miles (1,600 kilometers) above the surface."

MarCO-B took this image as it approached Mars just before NASA's InSight spacecraft landed.

And also during the flyby, MarCO-A transmitted signals through the edge of the Martian atmosphere. That atmosphere causes interference to change the signal when it's received on Earth, a way for scientists to detect how much atmosphere is present and even its composition.

"CubeSats have incredible potential to carry cameras and science instruments out to deep space," said John Baker, JPL's program manager for small spacecraft. "They'll never replace the more capable spacecraft NASA is best known for developing. But they're low-cost ride-alongs that can allow us to explore in new ways."

And for the team that worked on MarCO, the success of the mission is just the beginning.

"MarCO is mostly made up of early-career engineers and, for many, MarCO is their first experience out of college on a NASA mission," said Joel Krajewski of JPL, MarCO's project manager. "We are proud of their accomplishment. It's given them valuable experience on every facet of building, testing and operating a spacecraft in deep space."

This paves the way for new CubeSats that NASA will launch.

"There's big potential in these small packages," Baker said. "CubeSats -- part of a larger group of spacecraft called SmallSats -- are a new platform for space exploration that is affordable to more than just government agencies."

So how is InSight doing?

The stationary lander has been quietly toiling away and sending back selfies to assure everyone that it's doing just fine, even as MarCO disappears into the void.

InSight placed its seismometer, which can detect quakes on Mars and provide mission scientists with a sense of the planet's deep interior, on the surface in December. It's been making adjustments to that for the past few weeks.

NASA's InSight lander deployed its Wind and Thermal Shield to cover InSight's seismometer.

This week, InSight placed a protective domed shield over the seismometer to make sure the data it collects is accurate, since factors like wind can create disruptions. The shield will keep the instrument, which is incredibly sensitive, from being affected by wind. It was designed with aerodynamics in mind so the wind wouldn't flip it off the instrument and instead would help press it down.

Because InSight's location includes a few rocks, the shield has a chain mail skirt around the bottom to help it settle over the surface. The shield was also designed to help stabilize temperature. The temperature here can fluctuate by about 170 degrees Fahrenheit over the day, the agency said.

"Temperature is one of our biggest bugaboos," InSight Principal Investigator Bruce Banerdt said. "Think of the shield as putting a cozy over your food on a table. It keeps SEIS from warming up too much during the day or cooling off too much at night. In general, we want to keep the temperature as steady as possible."

Next week, InSight will deploy more instruments on the Martian surface.