Dust Grabbers

Two years ago I have written a post, Hayabusa2 , about a Japanese spacecraft and a few months later an update, Solar System Explorers, in which I mentioned the American spacecraft Osiris-Rex.

The two spacecraft have in common that they have a similar mission: travel to an asteroid, collect some surface material from it and bring that back to Earth. That’s why I have given this post the frivolous title of Dust Grabbers.

When I wrote the two posts, both spacecraft had arrived at their respective destinations, but had not yet collected asteroid material. Now they have, so it is time for an update.

First about Hayabusa2. Here are asteroid Ryugu and the Hayabusa2 spacecraft.

After reaching Ryugu, Hayabusa2 had already successfully dropped two Minerva rovers and the Mascot lander on the surface of the asteroid (see my first report). The surface of the asteroid was much rougher than expected, here is a picture taken by one of the Minerva’s.

For the touchdown, Hayabusa2 needed a flat surface, without boulders or big rocks and it was difficult to find a suitable location. In my first report I have described the touchdown, here is a very informative animation of the process. Two screenshots, left just before the touchdown, right just after. Notice how flat the asteroid surface is in the animation.

The touchdown was originally planned for October 2018 but postponed, to give the scientists time to redesign the touchdown procedure and check if navigation could be made accurate enough to land in a very small region.

On 6 February JAXA (the Japanese equivalent of NASA) published an extensive pdf-file for the press, Asteroid explorer, Hayabusa2,reporter briefing with a detailed description of the redesigned touchdown. A location had been selected and a target marker (TM) had been dropped near the chosen location. A TM is a small (10 cm) ball of reflecting material, you can see it in the animation screenshots above, left in the foreground.

The actual touchdown finally took place on 22 February 2019. One day earlier Hayabusa2 had already descended slowly from its home position (HP) at 20 km altitude to an altitude of 45 m. Here is a time diagram of the touchdown phase. Notice that between 7:07 and 7:50 JST time, there is no communication between the spacecraft and mission control.

Also keep in mind that the touchdown is an autonomous process, because it takes 15 minutes for a signal from Earth to reach the spacecraft. All steps have been programmed by the scientists and engineers.

Try to imagine the tension in Mission Control during this “blackout” period. And the explosion of joy when the first signals from the spacecraft showed that it was still alive.

At first this is the only thing they know, that Hayabusa2 is still alive. Only later images and data were coming in. A small video camera was mounted next to the collecting horn and recorded the touchdown. Have a look at the video Here are two screenshots, before and after touchdown. I have indicated the position of the target marker, it is the white stip inside the blue circles. Also notice the surprising amount of rubble whirled up after the touchdown, mostly caused by the thrusters firing.

Originally a second touchdown was planned at a different location, but this was cancelled because of the rugged surface of Ryugu. Instead the scientists concentrated on the most ambitious part of the program. The first touchdown had collected some surface material of the asteroid. Technically called regolith . This material has the same age as the asteroid itself, but has been exposed during millions of years to solar wind and radiation.

Of course it would be very interesting to collect some asteroid material from BELOW the surface. Here is the ingenuous plan developed by the Japanese scientists:

  • shoot a projectile to Ryugu to create an artificial crater.
  • touchdown later in the crater to collect some newly exposed material.

And that’s what they did! I have described the working of the so-called SCI (Small Carry-on Impactor) in my first post. Basically it is a copper projectile (2 kg) that hits the surface of Ryugu with a speed of 2 km/s, creating a crater of several meters diameter. Here is a diagram of the operation. Lots of details, I will point out a few. The operation took place on 5 April 2019.

Hayabusa2 descends from HP to an altitude of 500 m and releases the SCI with a downward speed of 20 cm/s. Here you see a time-lapse of the deployment. It takes about 40 minutes for the SCI to reach the surface and detonate. The detonation will cause a lot of debris, so Hayabusa2 must take shelter and does that by moving horizontally away and then down into the “shadow” of Ryugu.

Before disappearing below Ryugu’s horizon Hayabusa2 deployed a camera (DCAM3) to take pictures of the explosion. Here are some images. Not that spectacular for a layman, but apparently the scientists were able to draw conclusions from the vague plumes of debris that are visible.

Because of Ryugu’s weak gravitation it takes days before the debris of the explosion settles down. Notice times and distances in the diagram, Hayabusa2 moves away horizontally for about 100 km before “climbing up” again and finally reaches HP more than 10(!) days after the deployment of the SCI.

After the successful creation of an artifical crater, Hayabusa2 descended a few times above the crater to explore the new situation. Was it feasible to touchdown in or near the crater to collect material, exposed by the explosion, without jeopardizing the success of the first touchdown? On 8 July Jaxa published a very readable report discussing the pros and cons of a second touchdown: To go or not to go . It was decided to go and have a touchdown in the region C01-Cb, not really inside the crater but on the rim. Here are two images to show the touchdown area. The left image shows the artifical crater at the bottom right, the right image gives details about the size of rocks near the touchdown.

The procedure for the second touchdown was basically the same as for the first one. During one of the descends a target marker was released and on 11 July 2019 the touchdown took place. A sample was collected successfully.

Mission accomplished, time to go home. How to deliver the two samples to Earth? Have another look at the Hayabuss2. I have indicated the Sampler Horn and the SRC, the Sample Return Capsule. In this tiny (40 cm diameter) capsule the two samples have been stored (right diagram) and it is this capsule that will be released when Hayabusa2 arrives back at Earth.

After leaving Ryugu on 13 November 2019,Hayabusa2 l will reach Earth on 6 December 2020, using its ion engines for navigation.

Here is a diagram of the SRC return. The capsule will enter the Earth atmosphere with a speed of 12 km/s, the heat shields will protect the sample container. At 10 km altitude a parachute will be deployed.

The planned landing location is the Woomera desert in Australia, about 450 km north-west of Adelaide. Expected landing 6 December between 2:47-2:57 JST. It will take time to find the capsule, hopefully within one day. Here is a photo of the Woomera desert.

JAXA is maintaining a monumental website about Hayabusa2, updated with the latest news: JAXA Hayabusa2 Project.

After this long report about Hayabusa2, I will be much shorter about the Osiris Rex mission. Here is asteroid Bennu, smaller (~500 meter) than Ryugu (~ 900 meter). Both asteroids are spinning fast (~ 4 and ~ 8 hours respectively) and that might partly explain their similar shape of a “spinning top”. Although the material of these asteroids is about 4.5 billion years old, both were probably formed after a catastrophic collision of parent asteroids, millions of years ago.

The Osiris Rex spacecraft has a similar design as the Hayabusa2, with a SRC csapsule for the collected asteroid material. But there is one big difference, instead of a sampler horn, the Osiris Rex has a robotic arm, which can be unfolded and grab regolith while the spacecraft is hovering above the surface.

This has its advantages. The Hayabusa2 has to touch the surface (through its horn) and therefore has to worry about rocks nearby. The robotic arm is longer (about 3 meter) and more flexible. Another advantage is the way of collecting regolith. The Hayabusa2 fires a tiny bullet inside the horn and catches the regolith that is swirling upwards. That will not be much, the team is hoping for 1 gram (!) of material. Only after the capsule is opened the mass of the samples can be measured.

The TAGSAM robotic arm of Osiris Rex works very differently. As soon as the sampler touches the surface, nitrogen gas is blown through the arm and regolith will be collected, almost like a vacuum cleaner. Here is a nice animation of the process.

The TAGSAM procedure took place on 20 October 2020 and was very successful, it is estimated that about 60 gram was collected. (For physicists: how can they know that now already? By using a clever trick, rotating the spacecraft and unfolded robotic arm before and after collection, a difference in the moment of inertia will be observed)

The spacecraft will leave Bennu next year and will return to Earth on 24 September 2023, deploying the SRC capsule to land in the Utah desert.

These has been close cooperation between JAXA and NASA. They will share a percentage of the collected material with each other.

TRIVIA:

Brian May, the lead guitarist of Queen, is also an astrophysicist and quite interested in the Hayabusa2 project. During a Queen tour in Japan in January 2020, he met a few project people: Meeting Brian May.

Because of the Covid19 pandemic, the Japanese Sample Collection Team had to arrive early in Australia (with special permission) and first go into 14 days compulsory quarantine . Here a team member is standing at the heliport where the search for the capsule will start.

2 thoughts on “Dust Grabbers

  1. Impressive! Inconceivable that all this is actually possible. And they did actually deliver the sample on December 6! Further… two clicks click away … it’s o so very nice to see how my hero Brian collaborates with his Japanese friends while demonstrating his stereoscopic image thing. Teo torriate!

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