9 mins read 22 May 2020

From the Belt to Oz: Asteroid Sample Incoming!

The Hayabusa-2 spacecraft has departed Ryugu after studying the asteroid and taking samples, and currently on its return journey to Earth. The Australian Space Agency and Japanese Space Agency (JAXA) are working collaboratively for the landing, targeted for the outback deserts of South Australia.

We’ve all been waiting for a lot of deliveries lately, huh?  I know that in my attempts to keep my favourite vineyards in business, every knock at the door has me wondering ‘what did I order again?’  But there’s a special package on its way, that a big team of people have been waiting for since 2014, waiting to see what got ordered. Instead of a knock on the door though, this package will be hurled from space into the red sands of the outback Australian desert. Oh, and it’s coming from an asteroid some 165 million kilometres away.

Asteroids are one of the deep unknowns in our solar system, mostly living between the orbits of Mars and Jupiter, remnants from the formation of our solar system.  They range in size from pebbles to potential planet-killers, and although we’ve counted nearly a million of them it's thought their whole mass wouldn’t add up to that of our Moon.  There’s quite a few that are known to wander away from the main asteroid belt, and the ones we watch closely are those in near-Earth orbits.

As well as tracking near-Earth asteroids so we don’t go the way of the dinosaurs, they do also present a marvellous opportunity.  Each asteroid is a pristine record of the early solar system, and studying even a small piece of one gives us the opportunity to find out more about our planet, and the origins of many others.

Science Check: Asteroids and the Asteroid Belt

Relative size of Asteroids, including the two visited by Hayabusa 1 and Hayabusa 2: Itokawa and Ryugu. Credit: Osiris-X team, NASA .

The planets of the Solar System follow a known hypothesis that states that each planet will reside at a distance at roughly twice that of its predecessor. Mathematically, you could express this function by commencing a sequence with 0, then applying 3, 6, 12, 24, 48, etc. doubling each number from the prior. If you add 4 to these values and then divide it by 10, this represents a fairly close outline of the distances the major planets in our system tend to have. 

Known as the Titius-Bode law, this hypothesis proved rather peculiar when the planet Uranus was found - as it matched where the Titius-Bode law had predicted. Profound in that, a new planet was discovered to fit the model - but even more so, that the number that represented the value that should reside between Mars and Jupiter - had yet to be discovered. 

So astronomers decided to look in this region, and then they found it. A new world. They called it Ceres. But then, just over a year later, they found another one. Then another. Then another. Slowly, this region of the Solar System started presenting itself as many worlds!

Some of these were round, like the major planets. But most of them were irregular shaped, like a potato with dents in it. So they called them “Asteroids” - which means “star-like”. 

As technology advanced, observation techniques improved and more people around the world started building up the knowledge banks of astronomical bodies in our system, the number of asteroids increased to thousands, and it was soon fairly obvious that these objects occupied a torus-shaped belt, residing between the orbits of Mars and Jupiter. 

Asteroids are composed of rocks, ices, carbonous materials, and metals - all of which are from the early formation of the Solar System. Some scientists believe that the Asteroid Belt is all that remains from a planet that never formed in this region, due solely to the strong gravitational influence of Jupiter. In fact, Jupiter’s gravity is so strong on the belt - that the Gas Giant planet has kidnapped a collection of asteroids that now reside in its orbit - known as the Trojans and the Greeks.

About half of the mass of the Asteroid Belt is contained within the four largest objects (Ceres, Vesta, Pallas, Hygiea) with the remaining mass scattered across the remainder of objects. There’s like millions of asteroids spread across the belt, ranging from the size of a phone through to the size of mountains - though, these are spread across such a large space that there is plenty of room between them. In fact, numerous spacecraft have passed through the asteroid belt without any problems. 

All objects in the Asteroid Belt orbit the Sun, though these are not all in a linear, circular orbit like the planets. Some of the asteroids cross paths with the planets - and those that cross paths with Earth, are termed Near-Earth Objects (NEOs). When these objects are over a certain size and come within a certain distance - these are termed “Potentially Hazardous Asteroids”, but at this stage - none are deemed dangerous for our planet. That’s not always the case though, as impact scars from around Earth (and other objects like the Moon) indicate that every now and then, we do get hit by a few that are large enough to do damage (though, these events are rare). 

From The Belt to Oz

The Hayabusa 1 returning to Earth, over South Australia. Credit: National Geographic.

So why don’t we just pick up a piece and bring it back to Earth for a closer look?

Sadly, to date, there have been very few sample-return missions, reflecting that it is enough of a challenge getting to space and to the target. Compounding that is the task of getting the craft back and landing the sample safe on Earth, an enormous engineering effort.  To date only three samples have been returned to Earth: from the solar wind (NASA’s Genesis mission), from the tail of a comet (NASA’s Stardust mission), and a very small piece of the near-Earth asteroid Itokawa (JAXA’s first Hayabusa mission). 

It was the Hayabusa mission that was Australia’s first ‘interplanetary’ delivery.  It's not very feasible to land these missions in the sea - they are small and there isn’t the naval support that the Apollo missions had.  Instead JAXA needed access to a largely uninhabited land area, and that’s where Australia came in.  As the Hayabusa spacecraft performed its swan song diving into the atmosphere of Earth, it released the samples it had collected from Itokawa which landed at Woomera, South Australia.   However, not all had gone to plan on Hayabusa’s trip to Itokawa, the craft had gone into safe mode just before collecting the precious samples.  So, although this was the first return of a piece of an asteroid to Earth, there wasn’t as much as the JAXA scientists were hoping for. So they are trying again. 

Hayabusa, Round 2

The Hayabusa 2, with annotated features including the Minerva-II rovers. Credit: JAXA.

Though the Hayabusa 2 mission shares its name with its predecessor - the mission has been much more complicated and so far, very successful.   The target this time is a near-Earth Asteroid named Ryugu, of which the Hayabusa 2 reached in June 2018, after travelling for more than four years after its 2014 launch. From then on, the search was on to find a good sampling location and to map the whole of this little rocky world.  

The main sample collection carried out on the 21st February last year, went particularly well and has enabled the team behind the mission to write a Science paper before the main sample has even got back to Earth.  Dipping close to the surface, Hayabusa 2 released a small impactor that churned up the rubble and allowed the spacecraft to collect what had been knocked off.  As well as showing us great detail of the sample collection site, the team constructed a history of the asteroid Ruygu and have come to a surprising realisation. Though its orbit seems stable now, the variation of the surface suggests that Ruygu has wandered around the Solar System previously - showcasing stripes of redder terrain from being heated a bit. The asteroid has a tan!  

This can’t happen on its current orbit, so the thinking is that about 8.5 million years ago Ruygu took an ‘excursion’ to a warmer orbit close to the sun (who doesn’t need a holiday every so often?).  But this seems to be a one-off event, as Ruygu then moved back to being one of the Near-Earth asteroids tracked today, and from how the asteroid evolved after its holiday, we can now admire its tan lines.

The Journey Home

Artist illustration of the Hayabusa-2. Credit: NASA.

There’s lots more science to come.  As well as the main sample collection - Hayabusa 2 deployed a series of surface rovers, two MINERVA-II robots, and the Mobile Asteroid Surface Scout (MASCOT), that hopped about the asteroid’s surface.  The mission also threw a bigger impactor into the asteroid (ouch) creating a new crater on the surface, then maneuvered Hayabusa 2 to see what had been dugout.  And this is all before we get that precious stowed sample back to Earth.  

After two years of chipping away at the target, Hayabusa 2 packed up and is now heading back to Earth.  In December as it approaches Earth it will release the stowed sample, which is due to land in Woomera, deep in the South Australian Desert where Japanese and Australian scientists will be waiting for it.  Once safely recovered, these precious fragments of an orbiting artifact will become the most studied pieces of rock on Earth.  It’s hoped that they will help us tell a bit more about the story of the solar system.  What about Hayabusa 2 though?  Well, unlike its predecessor (who ended in a fireball above Australia) with all going well, the plan is that Hayabusa 2 will head back out to space to carry on its asteroid investigations. 

The Hayabusa-2 journey around the Solar System. Credit: JAXA.

Dr. Helen Maynard-Casley

Dr. Helen Maynard-Casely is a planetary scientist, her own research focuses on the icy materials that make up the dwarf planets of our solar system. She loves to share stories about science to all.

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The latest findings from Ryugu appears in the Journal Science