Hopping on an asteroid Kamal Lodaya In the last issue of {Jantar Mantar}, we learnt about the Japanese space agency Jaxa's {Hayabusa2} spacecraft which reached 20 kilometres from the diamond-shaped asteroid Ryugu in the end of June. From there it is supposed to do a "touchdown" to sample material from the surface. The article mentioned one {Mascot} and several {Minerva} landers which the spacecraft will gently throw down onto the asteroid from a height of about 60 metres. Two of those, {Minerva2-1A} and {Minerva2-1B}, were indeed thrown down on 21st September. And {Mascot} follows on 2nd October. By the middle of October {Hayabusa2} will itself go down and try to sample some material from the asteroid using a long "arm", as we described in the last {JM}. What is the rush? A scientist from the American space agency Nasa described the Japanese missions as "sporty" compared to Nasa's much more slow and stolid mission {Osiris-Rex} to the asteroid Bennu, which it will reach in December and leisurely survey before attempting a sampling from the surface, well into 2019. The difficulty is that Ryugu is almost on the opposite side of the Sun from the Earth. Very soon it will go behind the Sun, as seen by us. So for most of November, December and January, it will be difficult to receive any communication from {Hayabusa2}. So Jaxa wants to use the current window of time to do as many experiments as possible. In 2019 after Ryugu emerges again from behind the Sun, the next set of experiments will get done. ------------------------------- Box: {Hayabusa1} and {Minerva1} [pic http://planetary.s3.amazonaws.com/assets/images/9-small-bodies/2011/20130212 _Hayabusa_minerva-release_large-111405.jpg ] In case you are wondering why {Hayabusa2} and {Minerva2}, the Japanese space agency Jaxa sent the {Hayabusa1} spacecraft to sample another asteroid, Itokawa, and it sent down a lander called {Minerva1}, way back on 12th November 2005. The dot inside the circle in the picture is this lander {Minerva1}, photographed from {Hayabusa1}, and inside the square is a tiny magnification. This is one of the most tragic pictures of all space missions. As you can see from the picture, it does not look like the lander will land on Itokawa which you can see above in the image. It did not, it missed Itokawa and disappeared into space. That is the last picture we have of {Minerva1}! The problem is that, although you do not see that in the picture, the asteroid Itokawa has a bent potato shape (with a thin middle) and it was difficult to calculate the gravity exerted by such a shape at different points in space. At the point inside the circle, even though it looks prettyclose to Itokawa, the initial acceleration of the spacecraft meant that the Sun's gravity was more powerful than Itokawa's, even though the Sun was around 20 crore km away. Even with computers, the calculation of gravity for irregularly shaped objects in the presence of another object like the Sun is difficult. END OF BOX--------------------------------- [pic of Ryugu] Having learnt from their previous experience with {Hayabusa1} (see box), before attempting a landing, Jaxa sent {Hayabusa2} down in a free fall towards Ryugu and measured the gravity it exerted on the way down. > From this they learnt that Ryugu was less dense than expected (1.27 grams per cubic centimetre, remember that water is 1 gram per c.c.). It was guessed earlier, but now we know with more certainty, that Ryugu is most likely made from a "rubble-pile" of planetary material, and its surface must be porous, which is true if it is covered with dust. Now Jaxa could) calculate better at what height to release {Minerva2}. Why so happy? Yuichi Tsuda, project manager of Hayabusa2 at Jaxa, said he could not find words to express how happy he was. One of the reasons was the success of Minerva2 coming after the tragedy of Minerva1's failure (see box). Another reason was that the two Minerva2 spacecraft are hoppers. That is, they have small motors inside which make the spacecraft jump. These jumps are calculated carefully so that it jumps about 15 metres, but not so much that it can fly off into space! Ryugu is just 800 metres in diameter, it has very little gravity, and if you do not get things right Minerva2 could find itself sharing the future of Minerva1. The other thing is that Ryugu is full of boulders all over. Here sending two hoppers helped. One of them could get stuck, say because it crashed into a boulder. In fact one of the hoppers is not moving on Ryugu, and we still do not know why. But the second one succeeded. It has been hopping about on Ryugu for at least one full day now. Hence this picture from that hopper, even though it may not please a traditional photographer trying to take your picture, led to a lot of joy at Jaxa. It shows the hopper is hopping. Ryugu is to the left, at center is the shadow of a large boulder, the Sun is the white splotch to the right. Since there is no atmosphere on Ryugu, sunlight is very bright. [pic http://www.hayabusa2.jaxa.jp/en/topics/20180922e/img/Fig3.jpg] Thus Japan has discovered a new way to explore a small body like an asteroid. New pics: 1 When Hayabusa2 descended towards Ryugu for the MINERVA-II1 deployment operation, the ONC-T (Optical Navigation Camera - Telescopic) captured images at the highest resolution to date. mage of Ryugu captured by the ONC-T at an altitude of about 64m. Image was taken on September 21, 2018 at around 13:04 JST.This is the highest resolution photograph obtained of the surface of Ryugu. Bottom left is a large boulder. 2. September 23, 2018 at 09:48 JST: surface image taken from Rover-1A. MINERVA-II1 successfully captured the shadow of its own antenna and pin. The pins on the MINERVA-II rovers have three roles: 1. To increase friction when hopping 2. Protect the solar cells during landing 3. A few of the pins also have a temperature sensor, so surface temperature can be measured directly. The MINERVA-II1 rovers were deployed on September 21 to explore the surface of asteroid Ryugu. Here is the second report on their activities, following our preliminary article at the start of this week. We end this report with a video taken by one of the rovers that shows the Sun moving across the sky as seen from the surface of Ryugu. Please take a moment to enjoy “standing” on this new world. 1. Rover-1B hop [Open in another window (Left)]　 [(Center)]　 [(Right)] Figure 1. Images taken by Rover-1B. September 23, 2018: confirmation of Rover-1B hop. Observation time (JST): (Left) 2018-09-23 09:50, (Center) 2018-09-23 09:55, (Right) 2018-09-23 10:00 (Image credit: JAXA) 2. Image captured immediately before hop of Rover-1B Figure 2. September 23, 2018: image captured immediately before hop of Rover-1B. 2018-9-23 09:46 (JST). (Image credit: JAXA) fig1. he bottom of the Hayabusa2 spacecraft. (Image credit: JAXA) fig3: : Illustration of Rover-1A (back) and Rover-1B (foreground) from MINERVA-II1 as they explore the surface of Ryugu. (Image credit: JAXA)