The space missions looking to understand more about water in our Solar System | Planetary Science PhD Students

It is an incredibly exciting time to be studying asteroids – two incredible space missions are reaching the most exciting phases of their journeys! On the 27th June this year, the Japanese Space Agency mission ‘Hayabusa2’ arrived at the near Earth asteroid Ryugu, after travelling for three and a half years and travelling 3.2 billion kilometres. On the 3rd December the NASA mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) will arrive at the near Earth asteroid Bennu.

These two missions are ‘sample return’ missions, which means they will try and collect some material from the asteroids and then bring them back to Earth, so we can do detailed analyses, which would be impossible to do remotely.

Why these asteroids?

Asteroids like Ryugu and Bennu are incredibly old and primitive. They essentially formed right at the beginning of the Solar System (4.6 billion years ago!), and have existed more or less in the same form since then. They are little snapshots of what the Solar Nebula looked like before there were planets.

The reason Ryugu and Bennu in particular are so exciting is because they might be rich in hydrous minerals, which suggests they experienced some sort of aqueous or fluid alteration. That in turn implies there was a fluid, like water, present on these bodies at some point!

Bennu and Ryugu might have formed during the early stages of Solar System evolution, before planets. Image from NASA/JPL.

The age of these asteroids and the fact that they show evidence of fluid alteration is incredibly exciting, and the crux of why they were chosen as targets for Hayabusa2 and OSIRIS-REx.

When Earth formed in the early Solar System, it was too close to the Sun for water to exist as anything other than a gas. So Earth formed dry, without water. We think that the water present on Earth now might have been delivered by bodies such as Bennu and Ryugu, which may have formed in a different part of the early Solar System, and somehow intersected and collided with our planet. Studying material from them should therefore address some of these bigger questions: did they play a role in the delivery of Earth’s water and how did water influence their evolution?

Hayabusa2 and OSIRIS-REx are both at critical stages

OSIRIS-REx is on its approach phase to Bennu. It took the first images of its target on the 17th August, which although simply a collection of pixels moving across a static background, are very exciting. OSIRIS-REx is currently doing some ‘asteroid approach manoeuvres’, which mean the spacecraft is adjusting its speed and trajectory to match that of Bennu.

These are images from the OSIRIS-REx polycam instrument as the spacecraft continues its approach to Bennu. The images were taken on the 13th, 14th and 15th October. From

On the 31st of October, as a Halloween treat, OSIRIS-REx sent its first high resolution image of Bennu, from a distance of 330 km from the asteroid. The image is quite a stark one – a pitch black background with the bright asteroid looking like it is simply superimposed! OSIRIS-REx is due to arrive properly at Bennu on the 3rd December, at which point it will be in a ‘home position’ 20km above the surface of the asteroid.

The first high resolution image of Bennu. Image from

Hayabusa2 on the other hand, has already arrived at Ryugu, and the images are truly stunning. They show a roughly diamond shaped body, and a surface covered in boulders and rocks.

The spacecraft spent some time mapping the body, determining where to put its small landers down, before deploying two small robots which successfully landed on the surface on the 21st September. These robots (collectively called MINERVA-II1) are able to move around the surface using small motors and well to conduct experiments in multiple locations!

On the 3rd October Hayabusa2 deployed the larger MASCOT rover, which had more instruments aboard but was only designed to be operated for approximately 16 hours.

Image from JAXA.
The Hayabusa2 spacecraft caught these images of the MASCOT rover as it descended to the surface of Ryugu! Images from JAXA.

The MASCOT rover has a suite of instruments capable of measuring multiple characteristics of the asteroid including cameras to photograph physical and geological features and a magnetometer to determine the strength of Ryugu’s magnetic field. It also has a radiometer to measure surface temperature and an infrared spectrometer (my favourite instrument!) which can determine the mineralogy and composition of the asteroid surface. If the spectrometer can identify some minerals, which have been hydrated then it might be able to address some of those bigger questions about the role of water in the asteroids evolution.

In January of 2019 the main Hayabusa2 spacecraft will descend to the surface of Ryugu and try and collect some samples to return to Earth. This is slightly delayed from the original sampling timeline as scientists decided the surface of Ryugu was rougher than expected and so the sampling needed more planning. All this means is that when the sampling goes ahead there will be an extremely high chance of it being successful!

OSIRIS-REx and Hayabusa2 will hopefully go some way to answering some of the questions we have about the role of water in the evolution of the early Solar System, though I am sure they will also introduce some new questions for researchers to tackle over the next few decades. It is a real privilege to be studying planetary science whilst these two historic missions are going on, and I cannot wait to see what they find out.

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