In the desert of South Australia on Sunday, December 6 at around 4:30 a.m. local time, a container of dust and rock samples from asteroid 162173 Ryugu will touch down on the Woomera test site. In Central Europe it will still be around 7 p.m. Saturday. Hayabusa 2, the probe carrying the samples, has been on its way back to Earth for a little more than a year. 

Just 13 hours before the landing of the valuable cargo, satellite experts from the Japan Aerospace Exploration Agency (JAXA) will enter the phase of the landing operation concerning the separation of the landing capsule from the space probe. At the point of ejection, the Hayabusa2 (the "Falcon 2")  will be about 220,000 kilometers from Earth — just over half as far away as the moon. 

The detached, heat-protected landing capsule will reach the Earth via parachute, which is similar to the way manned space capsules return — only it is much smaller.  

Two landings led to success

The collection of the asteroid rock samples was a difficult operation. On February 22, 2019, the probe landed on Ryugu for the first time and loosened dust and rocks. In April 2019, the probe fired a copper projectile at Ryugu's surface to create a crater in order to expose even more dust. Thereafter, on July 11, 2019 Hayabusa2 landed on the asteroid a second time to collect the debris samples. In that moment, the probe was about 300 million kilometers away from Earth.

After the probe successfully collected its samples, it circled and measured the asteroid and collected more data with its remote sensing instruments until November 13. At that point, Hayabusa2 finally began its return flight to Earth, because the constellation of orbits was favorable. 

The components of Hayabusa2's mission — an orbiter and a lander —  may seem to resemble the journey of Rosetta and Philae to the comet Tschuri (2004-2016), but only at first glance. One marked difference between the missions is that the Rosetta spacecraft did not send a capsule with rock samples back to Earth.

Low gravity as a challenge

A major challenge for the mission was Ryugu's low gravity. For this reason, the landing maneuvers had to be initiated carefully, as there was a danger that the probe would bounce off the asteroid's surface and tumble uncontrollably. 

However, this is not the first time JAXA has brought asteroid dust to Earth. The probe Hayabusa succeeded in doing so for the first time in 2010. 

A long journey

Hayabusa2 has been travelling for more than six years — since December 3, 2014 to be exact. On June 27, 2018, the probe reached the asteroid that bears the name of a mystical Japanese underwater castle: Ryugu. 

Asteroids, like comets, are original celestial bodies. They allow us to look back into the history of the origin of the solar system more than four billion years ago. This makes them interesting for researchers. But asteroids are also in focus because they are a potential threat to us.

Ryugu's trajectory may be far away from Earth for the next few centuries, but similar asteroids near Earth could one day become quite dangerous. Therefore, planetary researchers want to learn as much as possible about these types of celestial bodies.

A German-Japanese-French lander

Besides the complicated sample collection mission, Hayabusa2 did more than just land on Ryugu twice. Before that, it dropped three other landers on Ryugu. First the probe launched two Japanese Minerva II mini-landing robots  on September 22, 2018. On October 3, Hayabusa2 then successfully dropped the Franco-German Mobile Asteroid Surface Scout, or "MASCOT"  for short.  

The mission is a complete success so far 

MASCOT weighs 9.6 kilograms. It was built by the German Aerospace Center (DLR) and the French space agency (CNES).

MASCOT's swing arm allowed it to bounce on the asteroid's surface, which allows it to prove and change its position in the event that it fell in an unfavorable position. It was equipped with a camera, a radiometer, a spectral microscope and a magnetometer. 

The use of MASCOT lasted only a few hours, but under extreme weather conditions. Temperatures on site ranged from minus 47 to 63 degrees Celsius. Afterwards the lander remained on the asteroid. 

Hayabusa2 had an infrared spectrometer to measure the mineral and water content and a thermal imaging camera to study the temperature and thermal inertia of the asteroid. 

Currently, NASA's Osiris-REx probe is also exploring the asteroid 101955 Bennu, also known as 1999RQ. It has taken soil samples, which it expects to bring back to Earth in 2023. Bennu is more dangerous than Ryugu, and the probability that it will hit the Earth during the last quarter of the 22nd century stands at 1:2700.