PASADENA, Calif.—Rapidly approaching Mars are the two smallest and cheapest spacecraft to ever cross between the planets, in the vanguard of what U.S. and European satellite designers hope one day will be swarms of tiny probes prowling the solar system.

The two 30-pound spacecraft are cruising alongside NASA’s newest Mars lander—an $828 million robotic probe called InSight that NASA engineers hope to land safely on the Red Planet on Nov. 26. While InSight makes its fiery descent, the tiny twin craft will hang back in orbit to act as a communications relay.

Hundreds of such tiny bargain-basement spacecraft, called CubeSats after their student-inspired standardized form, are transforming the business of space operations in Earth orbit. They image crops, beam internet service, gather weather data, track aircraft, monitor factories, and count the cars in shopping malls to gauge retail sales. Aerospace companies have filed plans to orbit thousands of these diminutive satellites in the coming decade.

But until now, no one knew whether these miniature satellites could withstand the rigors of deep space.

NASA’s two MarCO CubeSats, as the pair nearing Mars are called, are the first CubeSats to attempt an interplanetary journey. Launched this past May, each one is no bigger than a briefcase, is built from off-the-shelf commercial parts, and cost $18.2 million—a fraction of the price of the InSight craft they are escorting.