“Volcanologists go to volcanoes for their field work, but meteoriticists never go to asteroids!”
Shogo Tachibana is an associate professor at Hokkaido University and one of the key scientists on the team behind Japan’s new Hayabusa2 mission. As he talked, he plucked from a shelf in his office a small grey rock embedded in a cottonwool filled container. It was a fragment of a meteorite that had fallen to Earth in Australia in 1969.
Appearing as a bright fireball near the town of Murchison, Victoria, the ‘Murchison meteorite’ became notorious not just for its size, but because embedded in its rocky matrix were the basic building blocks for life. The amino acids found in the Murchison meteorite were the first extra-terrestrial examples of these biological compounds, and their discovery threw open the door to the possibility that the seeds for life on Earth came from space.
“Meteoriticists can only pick up stones, observe the asteroids and imagine what happened,” Tachibana pointed out, tapping the meteorite. “But by visiting the asteroids we can get a much clearer idea of our solar system’s evolution.”
It is this goal that drives the Hayabusa2 mission. Translating as ‘peregrine falcon’ in English, Hayabusa2 will chase down an asteroid, before performing three surface touchdowns to gather material to return to Earth. It is a project that recalls the recent Rosetta mission by the European Space Agency, and with good reason, since the missions have a common goal.
All life on Earth requires water, yet in the most favoured planetary formation models, our world forms dry. To produce the environment for life, water must have been delivered to Earth from elsewhere. One possible source is the comets, which Rosetta visited. A second is the asteroids.
Situated primarily in a band between Mars and Jupiter, asteroids are leftover parts that never made it into a planet. Broadly speaking, they come in two main classes. The ‘S-type’ asteroids match the most common meteorites that fall to Earth. During their lifetimes, they have been heated and their original material has changed. The second type are the ‘C-type’ asteroids, and these are thought to have altered very little since the start of the solar system 4.56 billion years ago. It is this early remnant of our beginning that Hayabusa2 is going to visit.
The asteroid target is ‘1999 JU3’: an alphabetical-soup name derived from its date of discovery on 10 May 1999. Observations of 1999 JU3 have tantalisingly hinted at the presence of clays that could only have formed with water. Since 1999 JU3 has a meagre size of approximately 1 km across, it does not have the gravity to hold liquid water. Instead, it must have formed from parts of a larger asteroid. Such a history fits with 1999 JU3’s orbit. The asteroid is termed a ‘near-Earth asteroid’, on an unstable orbit between Mars and the Earth. Since it cannot maintain its current orbit for more than about 10 million years, it must have originated from a different location. One of Hayabusa2’s missions is to investigate this possibility by examining the presence of radioactive elements in the asteroid’s body. Atoms that are radioactive decay into other elements over time. However, rays from the Sun may trigger their formation. As the asteroid has changed its path, the abundance of radioactive elements will shift depending on if it moved closer or further from the Sun. By measuring the quantities of these elements, Hayabusa2 can glean a little of the asteroid’s past.
The delivery of water alone is not the only reason why asteroids may be important. Discoveries from meteorites such as the one that fell in Murchison have revealed a connection between the evidence of water and the presence of amino acids. Even more excitingly, these reactions with water appear to be correlated with the production of left-handed amino acids. Like your left and right hand, amino acids can exist as mirror images of one another. While laboratory experiments produce an equal number of left- and right-handed molecules, life on Earth strongly favours the left-handed version. How this preference came about is not clear, but it is possible the selection began on the asteroid that brought this organic material to Earth. When the samples from Hayabusa2 are analysed, scientists will be holding the final evolution of organic matter on the asteroid and possibly, the beginning of our own existence.
Hayabusa2 launched from the Japanese Aerospace Exploration Agency’s (JAXA) launch pad on the southern tip of Japan at the start of last month. It will reach 1999 JU3 by mid-2018, and spend the next 18 months examining the asteroid before return to Earth in December 2020. During its visit, three touchdowns to the asteroid’s surface are planned to gather material to return to Earth. The first touchdown will occur after a thorough examination to search for the presence of hydrated clays. Sweeping down to the surface, Hayabusa2 will launch a 5g bullet at the asteroid to kick-up the material to be gathered by the spacecraft. It will also dispatch a lander called ‘MASCOT’ that in turn, will deploy three small rovers. MASCOT (Mobile Asteroid Surface SCout) was developed by the German Aerospace Center and French space agency, the team also behind the successful Philae lander for Rosetta. MASCOT has a battery designed to last for 15 hours and the ability to make at least one move to a second location. The destination for the trio of rovers will be decided once the initial data from Hayabusa2 reaches Earth.
After its first landing, Hayabusa2 will drop to the surface another two times. Since the asteroid is thought to be formed from fragments of a larger body, different surface locations may have very different properties. Prior to the third decent, Hayabusa2 will launch an impactor carrying 4.5 kg of explosives at the asteroid’s surface. During the collision, Hayabusa2 will hide behind the asteroid to avoid damage, but send out a small camera to watch the impact. While the resultant crater size will depend on the structure of 1999 JU3, the maximum extent is expected to be around 10m in diameter. Hayabusa2 will then drop to gather its final sample from this area, which will consist of material below the surface of the asteroid.
The gathering of sub-surface material is particularly important for investigating 1999 JU3’s formation history. Exposure to cosmic and solar rays can cause ‘space weathering’ that changes the surface of the asteroid. This is something Japanese researchers are well aware, since this incredibly ambitious mission has actually been performed before.
As the name implies, Hayabusa2 is the second mission of its kind. In 2003, the first Hayabusa spacecraft began its seven year round-trip journey to the asteroid Itokawa. Unlike 1999 JU3, Itokawa is an S-type asteroid, and the evidence from Hayabusa suggests it had been previously heated up to temperatures of 800 celsius. While the mission to Itokawa was not designed to unravel the solar system’s early formation, it did reveal a large amount of information about the later evolution of space rocks, including the effects of weathering. In addition to the science, Hayabusa’s journey was also a lesson in what was needed to pull off such an ambitious project.
While Hayabusa was ultimately successful, not everything went according to plan. During its landing sequence to the asteroid’s surface, one of Hayabusa’s guidance lasers found an obstacle at the landing site. This should have terminated the operation, but rather than returning to space, Hayabusa continued with its decent. In a problem that the Rosetta lander, Philae, would later mimic, Hayabusa bounced. It returned to the asteroid’s surface but stopped, staying there for half an hour rather than the intended seconds needed to gather a sample.
“This was really dangerous,” Tachibana explains. “On the surface of the asteroid, the temperature is very high and it’s not good for the spacecraft to be there long.”
Despite this, Hayabusa survived and was able to make a second landing. Yet even here there were issues, since neither landing had deployed the bullets needed to stir up the surface grains. This left scientists unsure how much material had been successfully gathered. Hayabusa’s lander also met an unfortunate end when an automated altitude-keeping sequence mistakenly activated during the lander’s deployment. Missing the asteroid’s surface, the lander could not be held by the rock’s weak gravity and it tumbled into space.
While Hayabusa2’s design strongly resembles its predecessor, a number of key alterations have been implemented to avoid these problems happening again.
“We are sure Hayabusa2 will shoot the bullets,” Tachibana says confidently. Then he pauses, “… but just in case, there is a back-up mechanism.”
The back-up mechanism uses teeth on the end of the sample-horn that can dig into the asteroid. As the teeth raise the grains on the surface, Hayabusa2 will decelerate, allowing the now faster moving grains to rise-up inside the sample chamber.
The sample jar itself also has a different design from Hayabusa, since 1999 JU3 is likely to contain volatile molecules that will be mixed with terrestrial air on the return to Earth if the jar seal is not perfect. Tachibana is the principal investigator for the sample analysis once Hayabusa2 returns to Earth. In preparation for this, a new vacuum chamber must be set up in JAXA to ensure no terrestrial pollution when Hayabusa2’s precious payload is finally opened.
“It was not easy to get support for a second mission,” Tachibana explains. “JAXA have a much smaller budget than NASA and there are many people who want to do different missions. We had to convince the community and the researchers in various fields how important this project was.”
It is a view shared by the international scientific community and JAXA have no intention of completing this work alone. In addition to the MASCOT lander‘s international origins, NASA’s network of ground stations will help track the spacecraft to ensure continual contact with Earth. When Hayabusa2 returns to Earth, the analysis of the samples will then be performed by a fully international team. Hayabusa2 will also be able to compare results from the new OSIRIS-REx NASA mission, which will launch in 2016 to visit and sample the asteroid, ‘Bennu’. Such comparisons are vitally important when samples are this difficult to collect.
“International collaboration is very important for space missions,” Tachibana concludes. “For the future, we don’t want to be closed. We want to make the best team to analyse the samples.”