Space experts will meet in France later this week to thrash out plans for an ambitious robotic spacecraft mission to return rocks from Mars to Earth – arguably the most ambitious interplanetary adventure ever attempted.
"This has been the holy grail for Mars scientists for a very long time," says David Parker of the British National Space Centre in London, UK. "We think it is doable, but it is going to take international collaboration and a lot of hard work to make it happen."
Mars is a hub of robotic activity, with NASA's Phoenix lander currently digging there and future landers such as the Mars Science Laboratory and ExoMars in the pipeline.
But no mission has ever returned Martian rocks to Earth, something that scientists consider essential to understanding the history of Mars and whether it ever hosted life.
One report previously identified 55 pressing studies to do on Mars. More than half of them could be only answered with a high degree of accuracy by a sample-return mission, while 13 of them could only be done at all by bringing rocks back to Earth.
Earth-based lab tests are essential for accurately assessing the abundances of different isotopes and trace elements, as well as identifying ancient life in the form of nanofossils.
With samples on Earth, scientists can double-check conclusions with a battery of different tests in labs around the world. Landers that look for life on Mars have the drawback that you have to guess the right chemical tests in advance; with rocks in the lab, scientists can change their tack as information unfolds.
Scientists also need to study Martian rocks in Earth-based labs to accurately determine their ages.
"You need large-scale lab equipment that you can't credibly take to the Martian surface," says Parker, chairman of the International Mars Exploration Working Group (IMEWG), which was set up in 1993 as a forum to discuss Mars exploration strategies.
Last year, IMEWG asked a committee of scientists and engineers called IMARS (International Mars Architecture for Return of Samples) to draw up draft plans for a sample-return mission. They will discuss their progress on Wednesday at a meeting in Paris.
The team envisages launching two sets of spacecraft to Mars. One would comprise a lander, rover and ascent rocket that travels directly to Mars, gathers samples then launches them back into Martian orbit.
The second would be an orbiting mothership that receives the samples, propels them back to Earth then drops the samples into Earth's atmosphere. Both sets of spacecraft will require heavy-lift launchers, such as Europe's Ariane 5.
The rover should roam at least a few kilometres away from the lander and drill several centimetres beneath the surface to gather rocks of different ages and types, including sedimentary rocks that record the history of water. "It's not just a dumb rover, it needs to select the right kinds of rock," says Parker.
After gathering at least 500 grams of Martian material, including samples of surface dust and the atmosphere, the rover will return to the lander, where the samples will be sealed into separate containers within a capsule. An ascent rocket will carry the capsule into orbit then transfer the samples to the mothership, a feat never performed before.
"You have to play a game of interplanetary pass the parcel," says Parker. This will have to be done autonomously, because Mars lies from 3 to 22 light-minutes away from Earth, so mission controllers cannot control the docking by communicating with the spacecraft in real time.
"Most likely you would have an intelligent system aboard the orbiter spacecraft, which searches for the capsule and locks onto it," says Parker. "The capsule would probably have a radio transmitter on board to say 'here I am'."
Next, rockets will have to fire to return the capsule to Earth, then drop it into the Earth's atmosphere for some kind of soft landing with parachutes and airbags, or possibly helicopter collection.
There are various possible timelines for the mission, depending on the position of Mars. The earliest possible launch would be May 2018, with the sample returning in mid-2022.
The Soviet Union successfully returned rocks to Earth from the Moon during robotic missions in the 1970s. But since then, such complex sample-return missions have been regarded as prohibitively complicated and expensive.
But Parker says the cost of returning Martian rocks, roughly estimated to be "at least $3 billion", is now viable if many nations team up to foot the bill.