After a successful mission that pushed the limits of small satellite technology, ground controllers have lost contact with two briefcase-sized CubeSats beyond Mars, NASA said Tuesday.

The pioneering Mars Cube One, or MarCO, mission set records for the farthest distance CubeSats have ever operated, accompanying NASA’s InSight lander to Mars after a May 5 launch atop an Atlas 5 rocket from Vandenberg Air Force Base, California.

The twin MarCO spacecraft relayed telemetry from InSight as it entered the Martian atmosphere Nov. 26 and successfully landed on the Red Planet, giving engineers at NASA’s Jet Propulsion Laboratory in California updates on the lander’s progress. albeit with an eight-minute delay due to the time it took radio signals to travel the 91 million miles (146 million kilometers) from Mars to Earth.

InSight could have succeeded without MarCO, but engineers would have had to wait hours to receive confirmation of the landing.

But MarCO was conceived primarily as an experimental mission to prove that CubeSats, with some modifications, could withstand the perils of deep space travel. CubeSats are much less expensive than larger satellites, and can cost less than $1 million to design and build for missions in Earth orbit.

The Mars Cube One mission cost $18.5 million, once engineers at JPL outfitted the satellites with a new type of radio, innovative antennas, a cold gas propulsion system, and other custom features needed for interplanetary spaceflight.

That’s still a fraction of the InSight mission’s $993 million cost.