By Paul Rincon, Science editor, BBC News website
A Japanese spacecraft is about to deliver to Earth the first large sample of rock and soil from an asteroid.
The Hayabusa-2 probe will release its precious sample cache, which is expected to parachute down to a safe landing in the Australian outback.
It grabbed the cosmic treasure trove last year from Ryugu, an asteroid regarded as a particularly primitive relic from the early Solar System. Studying the rocks could provide insights into how life itself began.
The operation should begin at about 05:30 GMT on Saturday, when the capsule containing asteroid samples will separate from the main spacecraft.
While the capsule heads home to Earth, the “mothership” will change its orbit to bypass our planet.
Later, on Saturday evening, the capsule should be visible to skywatchers in Australia’s Coober Pedy region as a fireball streaking overhead.
It is scheduled to touch down within a 100sq km (39sq mile) area at the Royal Australian Air Force’s Woomera range between 17:47 GMT and 17:57.
A beacon on the capsule will transmit data about its position, which will be analysed at Woomera to find where it has landed.
Once it has been located, it will be taken to a “quick-look facility” for inspection before being airlifted to Japan.
The capsule will be transported to a curation chamber in Sagamihara for analysis and storage.
The mission planned to collect a sample of more than 100mg.
“Fortunately, the spacecraft has been flying very smoothly… we are in very good condition,” said Yuichi Tsuda, project manager for the mission.
“I hope it works well, and I hope the weather in Australia is very good. I’m very excited and looking forward to it.”
Ryugu belongs to a particularly primitive class of space rock known as a C-type asteroid. Objects like it may have seeded the early Earth with water and the organic material that was necessary for life to get started.
“Having samples from an asteroid like Ryugu will be really exciting for our field. We think Ryugu is made up of super-ancient rocks that will tell us how the Solar System formed,” Prof Sara Russell, leader of the planetary materials group at London’s Natural History Museum, told BBC News.
“We think that this asteroid may have organic material and water which can give us information about how these things were delivered to the early Earth.”
Hayabusa-2 was launched from the Tanegashima Launch Center in far southern Japan on 3 December 2014. It reached the 1km-wide, spinning top-shaped object known as 162173 Ryugu in June 2018.
The samples were collected by detonating an explosive charge just above the asteroid. This drove a copper projectile into the surface, punching a 20m-wide crater in Ryugu.
Hayabusa-2 subsequently descended into this depression with the intention of collecting fresh rocks that had not been altered by aeons of exposure to the environment of space.
With the samples safely cached, the spacecraft began making its way home to Earth in November 2019.
The first Hayabusa spacecraft was launched in 2003 and reached the asteroid Itokawa in 2005.
Despite being hit by several mishaps, it returned to Earth in 2010 with a small amount of material from the asteroid.
