Slow a rocket booster tearing through space at nearly 8,000 km/h; guide the 12-storey behemoth safely back toward Earth's surface; and land it, upright, on an autonomous platform floating in Pacific Ocean swells.
This is the endeavour that SpaceX, the space exploration firm owned by futurist billionaire Elon Musk, will attempt with the launch Sunday of its newest Falcon 9 rocket.
Mastering ocean landings is a significant step to deploying reusable rockets like the Falcon 9 to get equipment, and some day people, into deep space with high velocity launches.
Musk has repeatedly said that a manned mission to Mars is the ultimate goal of the Falcon 9 program.
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Fuel is, after all, the most valuable currency in space flight — currency best spent on delivering bigger, better or more useful tools into orbit, rather than flying rocket boosters back to immovable landing pads on terra firma.
SpaceX has already proven it can successfully land the first stage of the Falcon 9 rocket.
On Dec. 21, 2015, the rocket delivered 11 satellites into low-Earth orbit before touching down about 10 minutes later at a designated site near Cape Canaveral, Fla.
The feat made waves in the space exploration industry.
"It's always cool to see something that's never been done before," says Jon Goff, CEO of Altius Space Machines in Broomfield, Colo.
"They've shown they can navigate back to a platform. They've shown they can land it. But now the question is, can they land it on a platform that is pitching and rolling in the ocean?"
The answer until this point has been a firm no. Two attempts conducted last year off the coast of Florida ended in disaster.
During the first attempt in January, the booster slammed into the platform at a nearly 45-degree angle and exploded.
The second try a few months later was only slightly more encouraging. The booster touched down on the landing pad, but the lateral force caused by the angle of approach snapped two of the landing legs. The booster tipped over and ignited into a fireball.
No 'trivial task'
The mission planned for Sunday will launch from California's Vandenberg Air Force Base inside a 30-second window that starts at 1:42 p.m. ET. It proved to be a perfect opportunity to test another ocean landing because the Federal Aviation Administration (FAA) did not grant it an environmental approval for a landing in Florida.
The Falcon 9 will deliver the Jason-3 satellite into low-Earth orbit. From there it will measure ocean surface levels as part of a joint monitoring effort by NASA, the National Oceanic and Atmospheric Administration and multiple European weather agencies.
Aiming to launch this weekend and (hopefully) land on our droneship. Ship landings needed for high velocity missions https://t.co/n6j0mExAqM— @elonmusk
After separation from the payload about 100 kilometres above Earth, Falcon 9's first stage will be guided back toward the 46-by-76 metre autonomous spaceport drone ship — drolly named Just Read the Instructions after a spaceship in Iain M. Banks's classic sci-fi series Culture — waiting several hundred kilometres from the coast line.
Each part of the trip is perilous. After the initial engine cut-off, cold gas thrusters on the outside of the booster position it for its return trip. Once in position, three of the nine Merlin rocket engines are restarted and the burn toward the drone ship begins.
"Taking the booster from hypersonic to subsonic speed and back into Earth's thick atmosphere is not a trivial task for something shaped like a rocket," says Canadian Space Agency astronaut Lt.-Col. Jeremy Hansen. "It wants to tumble."
As it approaches the platform with only a single engine left burning, four extendable legs are deployed and the booster slows to as close to zero velocity as possible.
'Less room for error'
"This is a lot like landing a plane on an aircraft carrier vs. a normal runway," wrote Musk in a blog posted before the successful Dec. 21 landing.
"A lot less room for error."
Ultimately, if SpaceX succeeds, ocean landings will open the door for more launches and could help to significantly lower the cost of sending equipment and people into space.
"Once that barrier to entry is eliminated, we're going to see more and more experimentation, more and more proverbial spaghetti being thrown against the wall," says Goff.
"The more we throw, the more is going to stick and that means more problems being solved and boundaries pushed."