Hayabusa-2: a sample return mission to an asteroid
Sen—Chasing down an asteroid, blasting a crater in it, releasing a rover and three landers, and collecting samples to bring home. The "to do" list of Hayabusa-2 is certainly impressive.
JAXA's Hayabusa-2 mission launched on Dec. 3 2014 on its way to rendezvous with the asteroid 1993 JU3 in June 2018. The design of Hayabusa-2 is based on the original Hayabusa craft, which gave the first close-up look of a Near Earth Object (NEO) in 2005. Hayabusa eyed up the asteroid Itokawa, which is classified as an S-type asteroid as it is rich in rocky material. After successfully landing on the asteroid to collect dust grains, Hayabusa brought its valuable cargo home in 2010.
Its successor, Hayabusa-2, will spend 18 months following asteroid 1993 JU3. This is a C-type asteroid, making it a completely different beast to Itokawa. C-type asteroids have dark surfaces which don't reflect much light, and this “stealth mode” makes them very difficult to study from Earth, making it all the more important to visit one.
The main objectives of Hayabusa-2 are to study a primitive Solar System body in order to gain an insight into the organic material and water ice that existed early in the Solar System's history. Hayabusa-2 will compliment ESA's Rosetta mission to a comet by studying organic material on a different type of primitive body. Understanding these ancient relics is vital for piecing together how planets form.
Cataloguing the in situ resources on different types of asteroids and comets is also important for future human space exploration. Understanding the structure of NEOs is also a major motivation for the Hayabusa-2 mission. By studying the composition of the asteroid it will be possible to estimate what kind of damage such an object would cause if it were to impact with the Earth.
Like its predecessor, Hayabusa-2 will use an ion engine and will avail of an Earth fly-by. The craft has similar specifications to Hayabusa, but with some improvements and adjustments to account for the different type of asteroid. “The target 1999JU3 is C-type asteroid, which may contain organic material and water,” explains Hitoshi Kuninaka, project manager for Hayabusa-2. “In order to detect the water signal, the Near Infra-Red Spectrometer (NIRS) was improved to a wide detectable wave range.”
The spacecraft has a laser altimeter, LIDAR, which will be used to bounce a laser off the surface in order to measure the distance between the craft and the asteroid. This is used for navigation, and is vital for the touchdown manoeuvres. It will also benefit the science investigation by measuring the shape and mass of the asteroid, as well as surface properties. LIDAR can also detect scattered light from dust grains along the line of sight to investigate any circum-asteroid dust.
Hayabusa-2 has four passengers; one lander and three rovers. The rovers are called MINERVA-II-A1, A2, and A3. Each weighs only one kilogram, and will be used to explore the challenges of surface mobility in such a low gravity environment.
The lander is known as MASCOT and weighs ten kilograms. After an initial characterisation stage to select an ideal landing site, MASCOT will be released via a spring while Hayabusa-2 is 100 meters above the surface. The lander has no propulsion system and will simply drift towards the asteroid at a rate of 5 centimeters per second. It has no anchoring system, so it will most likely bounce before settling into a stable position and has the capability to upright itself if it lands in the wrong orientation. It can also “hop” on the asteroid to reposition itself. MASCOT doesn't have solar panels so it is limited to 15 hours battery time, which is the equivalent to two days on the asteroid.
The lander has a camera, a radiometer, a magnetometer and a microscope. It even comes equipped with LEDs so it can take images at night. By taking continuous images throughout the day and night it will be able to observe any time dependent processes on the surface.
Nanospacecraft like MASCOT and MINERVA have limited scientific payloads, but the continued improvements in making small, cost effective craft paves the way for future exploration of the Solar System.
Upon arrival at 1993 JU3, Hayabusa-2 will pick a spot 20 kilometers above the asteroid to call home. It will remain in this “home position” instead of orbiting the asteroid, and return home after each of its touch down manoeuvres and close approaches to the asteroid.
Although challenging, Kuninaka is confident about the success of these manoeuvres. “In the Hayabusa mission we have already succeeded in the touch-down operation. Because Hayabusa-2 was developed under the lessons learned from Hayabusa, there is little concern.”
The original Hayabusa mission touched down on asteroid Itokawa and collected a sample from the surface, but Hayabusa-2 will take this one step further by first blasting a small crater on the surface.
Haybusa-2 carries an impactor, known as the Small Carry-on Impactor (SCI), that will be deployed at 500 meters above the asteroid. Hayabusa-2 will also release a camera that will stay behind to watch the impact, while the craft itself flees for cover to the other side of the asteroid. Once the craft is safe, SCI will detonate and release an impactor at a speed of two kilometers per second. The 2.5 kilogram impactor will smash into the asteroid, leaving a crater several metres across, and the exact size of the crater will give a further insight as to the composition of the asteroid.
Once Hayabusa-2 returns from its hiding place, it will touch down to collect samples. Since the impact will have blown away the top layers of the surface, this sample will contain sub-surface material. This is important as this material is less likely to have been altered since its formation. By comparing the surface material with that below, it will be possible to show how the asteroid has changed over time. The results from the MASCOT lander will also help to put the returned samples in context.
It is hoped that a successor to Hayabusa-2 would explore a D-type or P-type asteroid. These are more primitive than S or C-type, and meteorites from these are very rare so we don't understand their composition very well yet. Hayabusa-2 will only scratch the surface when it comes to understanding asteroids, and there is still a lot to be explored in future missions.