A pair of Nasa probes have mysteriously disappeared after a close flyby with Mars. It's been over a month since Nasa last heard from the £14million spacecraft, which were named WALL-E and EVE after the Disney animated robots.

Engineers say it's unlikely they'll ever hear from the probes again, but have hailed the mission, dubbed Mars Cube One (MarCO), as a great success.

That's because it proved that experimental "CubeSats" – a new type of low-cost satellite about the size of a briefcase – could make it into deep space and beyond.

Nasa hopes to use the compact craft in decades to come to explore distant corners of the universe at a fraction of the cost of most space missions.

Following Insight's successful touch down, WALL-E and EVE continued beyond Mars to test how far they could travel.

But it seems the spacecraft have finally reached their limit.

WALL-E was last heard from on December 29, while the last message from EVE was received on January 4.

Based on their trajectories at the time, WALL-E is currently more than 1million miles past Mars, while EVE is almost twice that distance from the planet.

                                   The suitcases pass by Mars in this illustration.

The mission team isn't sure why the probes have disappeared, but has several theories.

WALL_E has a leaky thruster, and issues with controlling the direction of the craft could be causing them to wobble and lose the ability to send and receive commands.

Problems with the brightness sensors that allow the CubeSats to stay pointed at the Sun and recharge their batteries could be another factor.

The MarCOs are in orbit around the sun and will only get farther away from the star as February wears on.