Five billion years ago, our solar system was little more than a gassy, dusty disc whirling about the infant, weak sun.
Eventually, most of that matter clumped together to form grains, pebbles, rocks, then planets – worlds that evolved over billions of years to become gas-trapping giants, barren deserts or, in the case of Earth, a water-filled oasis.
But the leftover pieces – which became asteroids and comets – were consigned to whiz around the solar system, occasionally bumping into other objects, but generally remaining pristine.
Studying these relics can tell planetary scientists plenty about the formation of the solar system. Did early asteroids carry molecules and water essential for life to Earth? And what’s the likelihood that life arose elsewhere in the solar system?
This is why two missions – the Japanese space agency JAXA’s Hayabusa 2 and NASA’s OSIRIS-REx – aim to deliver pieces of asteroid to Earth and with them, answers to those fundamental questions.
The Hayabusa 2 mission is the second asteroid sample return attempt by JAXA. The first, Hayabusa (Japanese for “peregrine falcon”), launched in 2003 and rendezvoused with the asteroid Itokawa in September 2005 and started mapping the asteroid’s shape, spin, density and composition from around 20 kilometres away.
Itokawa doesn’t look particularly special. A 535-metre long ellipsoid, its orbit crosses that of our neighbour, Mars.
But it’s what’s known as a chondrite asteroid – a stony “rubble pile”, formed as fragments of early solar system material stuck together. And as it’s made mostly of stony materials and nickel-iron, it’s called a silaceous, or “S-type”, asteroid.
After a couple of months of viewing from a distance, Hayabusa landed on Itokawa’s surface for around half an hour and collected a few tiny grains of material from the asteroid’s surface.
The mission, though, was plagued with problems. Rocket failures delayed its launch for a year. A blast of intense radiation from the sun damaged a solar panel while en route to Itokawa, slowing its progress.
And a mini-lander named MINERVA (Micro/Nano Experimental Robot Vehicle for Asteroid), which was supposed to pop out of Hayabusa, land on Itokawa and “hop” along the surface, deployed at the wrong time. Instead of being gently dragged onto the asteroid’s surface, it escaped its gravitational pull and tumbled away into space.
Nevertheless, some Itokawa samples did make it to Earth. The return capsule landed near Woomera in South Australia, in June 2010 holding around 1,500 grains of asteroid rock.
The mission’s second incarnation – Hayabusa 2 – is already on its way to asteroid Ryugu. With major improvements, it set off in December 2014, is due to arrive in July 2018 and will, if everything goes to plan, return to Earth with samples in December 2020.
Its scientific payload is much the same as Hayabusa, but with one notable addition: an explosive device to blast and collect fresh, sub-surface material off the asteroid.
Unlike Itokawa, Ryugu is a carbon-rich, “C-type” asteroid. It’s also more spherical in shape and larger – around 900 metres wide.
• Small Carry-on Impactor (SCI): A two-kilogram copper lump (called “Liner”) will be dropped to the surface of the asteroid at two kilometres per second to make an artificial crater. How the asteroid’s surface reacts to the thump will let planetary scientists infer what’s inside.
• Near InfraRed Spectrometer (NIRS3) and Thermal Infrared Imager (TIR): By measuring the amount of heat the asteroid emits in the night-time, these instruments will be able to determine if its core is one solid piece of rock or if it’s a conglomerate.
• Small rovers (MINERVA-II): These three rovers are successors of the MINERVA aboard the Hayabusa. Each will hop along and probe the surface.
• Small lander (MASCOT): Manufactured by the German aeronautics and space research centre and French space agency, the lander will make detailed observations of its immediate area.
NASA’s foray into asteroid sample return is slated for launch in September 2016. Its destination: Bennu, a near-Earth, carbon-rich asteroid around 500 metres wide.
OSIRIS-REx will sneak up behind Bennu and, in October 2018, start surveying. It will map the asteroid’s surface for more than a year, looking for potential sample-collection sites.
With sites decided, a sampling arm will extend from the spacecraft and touch the asteroid’s surface for around five seconds, blasting nitrogen gas onto the rock and collecting soil and shards that fly up.
If unsuccessful, the spacecraft has enough nitrogen gas for just two more attempts, and can collect up to two kilograms of material to store in its return capsule.
In March 2021, OSIRIS-REx will start its two-and-a-half-year journey back to Earth. But before re-entering the atmosphere, the return capsule will unhitch from the craft and make the final leg of the trip on its own, parachuting to the US military’s Utah Test and Training Range.
Only a quarter of the returned Bennu material will be immediately analysed. The rest will be set aside for future analyses.
And it may not be the last time humankind sees Bennu. Every six years its path brings it very close to Earth, and scientists have calculated a “high probability” that late next century, it will crash into our planet. By understanding its movements and composition, we may divert or destroy it before impact.
• OSIRIS-REx Camera Suite (OCAMS): Three cameras will record the spacecraft’s approach, along with mapping and sampling manoeuvres.
• OSIRIS-REx Laser Altimeter (OLA): A scanning LIDAR (Light Detection and Ranging), it uses reflected light to measure distance. It will also support other instruments as well as navigation and gravity analyses.
• OSIRIS-REx Thermal Emission Spectrometer (OTES), OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) and Regolith X-ray Imaging Spectrometer (REXIS): These instruments will measure radiation emitted from Bennu, which will provide elemental and mineral signatures and physical properties of the surface, such as particle size.
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Belinda Smith is a science and technology journalist in Melbourne, Australia.
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