A Japanese spacecraft is set to “bite an asteroid” as it descends to collect a sample of rock from the surface.
The Hayabusa-2 probe will try to grab the sample from a pre-chosen site on the asteroid Ryugu at 23:00 GMT on 21 February.
The spacecraft reached asteroid Ryugu in June 2018 after a three-and-a-half-year journey from Earth.
It is is expected to return to Earth with the rocky material it has cached in 2020.
During sample collection, the spacecraft will approach the 1km-wide asteroid with an instrument called the sampler horn. On touchdown, a 5g projectile made of the metal tantalum will be fired into the rocky surface at 300m/s.
The particles kicked up by the impact should be caught by the sampler horn.
The spacecraft began descending from its “home position” of 20km above the asteroid’s surface in the early hours of 21 February (GMT) – several hours later than planned. However, controllers said they would slightly increase the speed of descent down to 5km, so that the original touchdown time was not affected.
Ryugu belongs to a particularly primitive type of space rock known as a C-type. The near-Earth asteroid (NEA) is a relic left over from the early days of our Solar System.
Prof Alan Fitzsimmons, from Queen’s University Belfast, told BBC News: “We think we understand how carbon-rich asteroids migrate from the asteroid belt to become near-Earth asteroids, but the samples from Ryugu will allow its history to be explored.
“After the Rosetta mission, it’s now clear that most of Earth’s water did not come from comets in the early days of the Solar System. We believe carbon-rich (C-type) asteroids may have significant amounts of water locked up in their rocks. It’s possible such asteroids may have brought to Earth both the water and the organic material necessary for life to start.
“These samples will be crucial in investigating this possibility.”
Hayabusa-2 has already dropped a small, reflective, beanbag-like “target marker” on to Ryugu. This will be used as a guide as the spacecraft descends to the rough surface of the asteroid.
Controllers will aim for the centre of a circle, some 6m in diameter, located about 4-5m away from the target marker.
The Japanese space agency (Jaxa) had originally planned to carry out the touchdown operation in October last year. But the asteroid’s surface was found to be much more rugged than expected, with numerous, hefty boulders making it hard to find a location that was large and flat enough to sample.
Controllers had hoped they would have an area of about 100m in diameter to target. But because of the surface properties, this had to be reduced to a 6m circle for what team members are calling a “pinpoint touchdown”.
The sampler horn that extends out from the bottom of the spacecraft has a length of 1m. It’s therefore vital that there are no boulders more than 50cm in height at the landing site, to reduce the chances that the body of the spacecraft could hit a rock.
Unexpected surface properties also have the potential to affect the amount of material collected. Before arriving at Ryugu, researchers had expected the surface to be covered in a powdery layer of fine-grained material – the regolith.
In fact, the upper layer turned out to be akin to gravel, consisting of rocky chunks that are centimetre-sized or larger.
Prof Fitzsimmons told BBC News: “This was a surprise, as other near-Earth asteroids we have visited previously have shown areas dominated by small particles.
“It might be due to the carbon-rich composition, as the previous NEAs are composed of silicate rock, which are more Earth-like. But the shape of Ryugu also implies it was spinning much faster in the past, so it’s possible this could have affected the particles’ sizes in some fashion.”
Scientists carried out additional tests in Japan to determine whether the sample material could still be gathered by the spacecraft.
They used a container of artificial gravel with a similar size distribution to that on Ryugu. In a vacuum chamber, they fired a tantalum bullet identical to that used by Hayabusa-2 into the gravel.
According to Jaxa, the results of the test exceeded expectations, with the tantalum projectile yielding fragments of rock in size ranges that should easily pass through the sampler horn.
This suggests that Hayabusa-2 should still be able to collect a sample.
In September, Hayabusa-2 deployed two robotic “hoppers” that propelled themselves across the surface of Ryugu, sending back images and other data.
Then, in October, the “mothership” despatched a French-German instrument package called Mascot to the surface.
Later this year, perhaps in March or April, Jaxa plans to detonate an explosive charge that will punch a crater into the surface of Ryugu.
Hayabusa-2 would then descend into the crater to collect fresh samples of material that have not been altered by aeons of exposure to space.
“We know that the surfaces of asteroids are changed over time by bombardment with energetic particles from the Sun and interstellar space,” said Alan Fitzsimmons.
“Yet studies with telescopes show that this ‘space weathering’ affects the surfaces of carbon-rich asteroids differently to those mostly made from more rock-like silicate minerals. We don’t know why this is, and the fresh sub-surface samples from Ruygu will play a very important role in understanding how this happens.”
Follow Paul on Twitter.