By Dr Rok Nežič

(Credit: DLR – German Aerospace Centre)
Caption: Artist's impression of Hayabusa2 firing its ion thrusters

If you’ve been following space news in the last couple of years, you will have heard a lot about two space missions to nearby asteroids: Hayabusa2 – operated by JAXA (Japan Aerospace Exploration Agency) – and OSIRIS-REx – operated by NASA (the US National Aeronautics and Space Administration).

Both missions are notable for their long-term stays at specific asteroids, doing extremely valuable science for understanding not only asteroids specifically, but the early Solar System environment in which they formed as well. Both missions also feature a ‘sample-return’ part, meaning that a sample (or multiple samples) are taken from the surface of the asteroids and then flown to Earth, where they can be examined in detail by scientists in dedicated laboratories. This is a lot more complicated than analysing material ‘in situ’ (on the asteroids themselves), but laboratories on Earth have much more sophisticated equipment than anything we can put on small spacecraft like those two.

(Credit: NASA.)
Caption: An animated series of photographs taken by OSIRIS-REx upon its sample collection (‘touch-and-go’) manoeuvre on 20th October 2020. The collection arm TAGSAM (Touch-And-Go Sample Acquisition Mechanism) is seen in the centre.

In October 2020 OSIRIS-REx was in the news because it performed a successful touch-and-go manoeuvre and sample collection on asteroid 101955 Bennu, its target of interest. In the following days it transpired that it perhaps acquired too much material, since the seal meant to close the material in was not fully closed and pieces were slowly leaking out of the sampler head. The mission control had to move quickly to enclose the sampler into the Sample-Return Capsule (SRC) and prevent further loss of material. This meant they couldn’t perform some the planned spinning manoeuvres which would have allowed them to ‘weigh’ the sample, but the fact that some material was escaping is a sign that plenty was collected in the sampler head, which has a maximum capacity of 2 kg (4.41 lb).

The SRC from OSIRIS-REx will land on Earth in September 2023, whereas Hayabusa2’s SRC was just in the news because it landed in Woomera Prohibited Area in Australia on 5th December 2020. It is expected to have brought in over 100 mg of material from two separate sampling events performed on asteroid 162173 Ryugu – the main mission target – in 2019.

(Credit: JAXA)
Caption: Hayabusa2’s Sample-Return Capsule recovered in Woomera, Australia on 5th December 2020.

As its name suggests, Hayabusa2 is not the first mission of this kind. The return of samples from the asteroid is hugely important for Solar System science and it marks only the second time material was brought to Earth from an asteroid with a space mission. The accolade for the first sample return from an asteroid goes to Hayabusa mission, its predecessor, which landed on 13th June 2010, also Woomera Prohibited Area, Australia.

Hayabusa achieved many other firsts, too, and OSIRIS-REx wasn’t even the first to experience some difficulties during sample collection: Hayabusa’s sample-collecting mechanisms failed at both attempts they made, but luckily a small amount of material found its way into its capsules anyway and was safely delivered to Earth. This is essentially the opposite of the problems OSIRIS-REx team had with too much sample material, but all’s well that ends well. Hayabusa2, then, might not have collected quite as much material as OSIRIS-REx, but at least it did so without any difficulties of the other two missions!

(Credit: JAXA)
Caption: Photo taken by Rover-1A (HIBOU) on from surface of the asteroid in mid-hop. The white region to the right is due to sunlight.

Hayabusa2 mission was launched on 3rd December 2014. It arrived at asteroid 162173 Ryugu (or just Ryugu for short) in June 2018, where it stayed until 12th November 2019, doing a lot of interesting science along the way. Its design was based on the Hayabusa mission, and included a lot of cameras – optical, infra-red, a spectrograph, and even a LIDAR (Light Detection And Ranging, used to measure distance between the spacecraft and the asteroid; radar, but with visible light) – and four rovers, the largest (MASCOT) weighing almost 10 kg!

As you may imagine, asteroids are quite small and therefore have very weak gravity, so any object going too fast on Ryugu’s surface might have flown straight off. Besides, the rocky terrain can be very unpredictable. That’s why Hayabusa2’s rovers didn’t drive around with engines, but instead moved around by means of small hops. Three of the four rovers were a great success: Rover-1A (HIBOU) and Rover-1B (OWL) – both collectively called MINERVA-II-1 – and MASCOT (Mobile Asteroid Surface Scout). Rover-2 or MINERVA-II-2 had some issues before being deployed. That one was, in the end, released into orbit around the asteroid to perform gravitational measurements before eventually hitting the surface. The rovers had a variety of instruments on board, mostly cameras, but also a spectrometer, thermometers, and a device for measuring magnetic fields.

(Credit: JAXA)
Caption: Hayabusa2’s two MINERVA-II-1 rovers: Rover-1A (HIBOU) on the left, Rover-1B (OWL) on the right, with their storage container in the back.

The missions also used a number of other, more immobile objects, including an interestingly-named ‘Small Carry-On Impactor’, which was, essentially, a small explosive charge Hayabusa2 dropped and detonated on the asteroid in order to make a crater and so reach material below the surface for one of its sample-collection tasks.

All three missions mentioned here – Hayabusa, Hayabusa2, OSIRIS-REx – have visited asteroids (25143 Itokawa, 162173 Ryugu, and 101955 Bennu, respectively) which are near-Earth objects (NEOs) and, in fact, ‘potentially hazardous asteroids’, meaning they approach the orbit of Earth to less than 0.05 AU (19.5 lunar distances) and are large enough to cause significant regional damage if they impact the Earth. None of them are expected to actually hit the Earth in the next couple of hundred years, but being so near, small gravitational perturbances in the future make further predictions less reliable.

Bennu, 490 metres across, is the most ‘potentially hazardous’ of the three (indeed, it is the second most hazardous object on the NASA JPL Sentry System list), and its highest likelihood of impacting the Earth is 1 in 11,000 in the year 2196, although due to the nature of orbits and probability calculations, the total probability of an impact in the late 22nd century is somewhat higher. By far the most likely end for Bennu will actually be falling into the Sun with no harm to the Earth, based on a computer simulation presented by Lauretta et al. (2015).

Ryugu – Hayabusa2’s main target and about 1 km across – is a lot better behaved than Bennu, however a much smaller member of its asteroid group (estimated to have been 20 m in size) ended its life as the Chelyabinsk meteor in February 2013. Ryugu is the undersea palace of the Dragon God in Japanese mythology. In one legend, a fisherman was rewarded with a visit to Ryugu and brought back a box with a secret. Similarly, Hayabusa2 has visited Ryugu (the asteroid) and now brought back its own mysterious box: the capsule with samples from the asteroid.

(Credit: ISAS/JAXA)
Caption: An animation of asteroid 162173 Ryugu, showing the entire surface through rotation around the equator.

Ryugu is round-ish with an equatorial ridge and looks a lot like a spinning top, much like the smaller Bennu. Another thing they both have in common – and which likely contributed to this interesting shape – is the fact that they are both what we call ‘rubble pile’ asteroids. This means that rocks that make them up are not sticking together at all, but simply held loosely in place by their combined gravity; moving the material around would be much like dealing with a pile of rubble or gravel. Most smaller asteroids are expected to be ‘rubble piles’, and about 50% of their volume is empty space. Asteroids are also defined by their spectral type, which can depend on the colour, albedo (brightness), and spectral shape and features of the object. Ryugu is quite unusual, as it falls between the very common C-type (carbonaceous) and uncommon B-type (‘blue’) asteroids. The images from MASCOT showed that there were two different types of rocks on the surface, both very dark: one is smooth with sharp edges, while the other has crumbly surfaces. The ‘Small Carry-On Impactor’ has shown that the rocks below the surface are even darker.

Lots will still be learned from Hayabusa2 mission about the asteroid Ryugu in the coming years, not least from the samples which have just arrived on Earth. The story of Hayabusa2 is not over, however! The mission went so smoothly that about half of its ion thruster propellant remains in store. The spacecraft therefore simply deposited the Sample-Return Capsule and will continue its journey through the Solar System. The first port of call will be asteroid 2001 CC21, another unusual (L-type) asteroid, but that will only be a very fast fly-by in 2026. Between 2021 and 2026, Hayabusa2 will also be observing exoplanets. It will swing by Earth again in December 2027 and in June 2028. Both manoeuvres will help direct it to its next target, a fast-spinning asteroid only 30 m or so in size: 1998 KY26. The future of Hayabusa2 mission therefore looks bright and interesting!

(Credit: JAXA)
Caption: Hayabusa2’s Sample-Return Capsule falling through the skies of Australia creating a bright streak in this long-exposure photo (middle). The dimmer, shorter streak to its left might be the main Hayabusa2 spacecraft continuing its journey through space.

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