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.

5 This is one of the last pictures taken by the MarCO mission before it disappeared forever Credit: Nasa

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.

"This mission was always about pushing the limits of miniaturised technology and seeing just how far it could take us," said Andy Klesh, the mission's chief engineer.

5 The two craft, known collectively as MarCO, passed Mars over a month ago (artist's impression) Credit: Nasa

"We've put a stake in the ground. Future CubeSats might go even farther."

WALL-E and EVE launched toward Mars in May 2018 alongside Nasa's Insight lander, which touched down on the red planet in November.

The two CubeSats, which are the first of this kind of spacecraft to fly to deep space, were near Mars during Insight's landing.

They helped Nasa to quickly transmit information about the landing as Insight descended to the surface, "blazing a trail" for future CubeSat missions, according to Nasa.

5 Each probe is about the size of a briefcase Credit: Nasa

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.

5 WALL-E and EVE transmitted data from Nasa's Insight Mars lander in November (artist's impression) Credit: Nasa

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.

5 Nasa scientists say the craft are AWOL until they circle back toward the sun this summer (artist's impression) Credit: Nasa

They won't start moving toward the sun again until this summer.

The team will reattempt to contact the CubeSats at that time, though it's anyone's guess whether their batteries and other parts will last that long.

Nasa plans to launch several new CubeSats in coming years.

"There's big potential in these small packages," said MarCO programme manager John Baker.

"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."

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WALL-E and EVE are not the first probes Nasa has lost.

Last August, it admitted that its Opportunity rover could be lost forever on Mars after a planet-wide dust storm smothered its solar panels.

Nasa discovered a lost Indian spacecraft in orbit around the moon in 2016.

You can read our summary of the best photos taken from Mars by clicking here.

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