Space diary
FISHING FOR WATER
by
Kamal Lodaya
{Bennu} is a half-kilometre diamond-shaped asteroid, orbiting the Sun from somewhere near Venus to somewhere near Mars. It was reached by the American agency Nasa's {Osiris-rex} spacecraft in December 2018, soon after the Japanese agency Jaxa's {Hayabusa2} reached {Ryugu}, also diamond-shaped but twice the diameter of Bennu. Bennu rotates about twice as fast as Ryugu, once every 4 hours and 20 minutes. It has a pronounced bulge on its Equator, possibly because rubble accumulates there.
As we reported in {Jantar Mantar}, in 2019 {Hayabusa2} did a couple of touchdowns on Ryugu and picked up some samples to be transported back to Earth. It left Ryugu on 13 November and will reach us in December 2020. The sample capsule will be landed in a desert region in Woomera in South Australia.
Osiris-rex had a more relaxed 2019. It went into orbit around the little Bennu. This is the smallest body that any spacecraft has ever orbited. The previous one was the duck-shaped nucleus of comet Churyumov-Gerasimenko, a few kilometres in size. >From its orbits it has mapped the entire surface of Bennu in great detail, from as close as 600 metres. This mapping has involved hundreds of volunteers who identified every boulder, seen from several different angles as the spacecraft orbited Bennu. That led to a surprising discovery (see Box 1).

[rubble pic: planetary.s3.amazonaws.com/assets/images/9-small-bodies/2019/20190319_bennus_boulder_and_limb_from_detailed_survey_201903074_f840.png]

The whole of Bennu is full of rocks, it is called a rubble-pile asteroid. Osiris-rex identified four possible sites where there is less rubble, from where a sample of the {regolith}---the rubble and dust on the surface---might be picked up. These sites are called Sandpiper, Osprey, Kingfisher and Nightingale. Nightingale was selected for sample pickup and a test approach was done on April 15 this year. In case you wondered, Bennu is named after a bird-like Egyptian deity. Osiris is also an Egyptian deity, but the name of the spacecraft comes from Origins, Spectral Identification, Resource Identification, Security, Regolith Explorer, which tells you a lot about what its job is.
Why would an asteroid mission be worried about {security} ? Earth's main interest in Bennu is that it comes uncomfortably close to us as it goes round the Sun. In the 22nd century (not far away now), there is a small chance (about 0.07 per cent) that it will hit Earth. That might seem small, but an impact of something that size can wipe out the largest city on Earth and a lot of the region around it. So we Earthlings are worried about our safety. Venusians should also be worried about Bennu, we are more worried about ourselves than about Bennu hitting Venus.

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Box 1: Do asteroids have tails?

[pic: asteroid gault with tail, wikipedia]

Comets, we know, have spectacular tails, sometimes lakhs of kilometres long. Sometimes when an asteroid is unstable, some material might be thrown out from it to form a tail. This was seen for asteroid Gault in January 2019. Later examination of images taken of it showed that this had been happening since 2013.

(A small asteroid has been caught in the process of spinning so fast it’s throwing off material, according to new data from NASA’s Hubble Space Telescope and other observatories.
Images from Hubble show two narrow, comet-like tails of dusty debris streaming from the asteroid Gault. Each tail represents an episode in which the asteroid gently shed its material — key evidence that Gault is beginning to come apart. )

[pic: www.space.com/asteroid-bennu-active-osiris-rex-discovery.html]

January 2019 also revealed to the quietly orbiting Osiris-rex, which was trying to detect if Bennu had a moon (it didn't find one), that the asteroid was throwing out material. This is too little to form a tail, one can only barely see the ejected centimeter-sized particles shining reflected in sunlight. Tracing them made it clear that some of them had enough speed to escape from the weak gravity of Bennu. But why are they being ejected?
There are possible explanations, but no one knows the answer yet. If this is happening all the time, then Bennu should leave behind it a trail of dust, and this should lead to a small meteor shower on Earth on September 25.
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[pic: en.wikipedia.org/wiki/OSIRIS-REx#/media/File:Artist's_concept_of_OSIRIS-REx_TAGSAM_in_operation.jpg]

Like Hayabusa2 did, Osiris-rex will approach close to the surface of the asteroid, and a long arm will reach down to fish the asteroid's surface in July 2020. It will release a burst of nitrogen gas which will kick dust up from the surface. This regolith will be collected in a sampling pouch. There will be three sampling attempts to ensure that at least one succeeds.

After a wait for several months for Bennu to come closer to Earth, Osiris-rex will move out from Bennu. The sample will be returned by a parachute to the Utah desert in the US in September 2023. The spacecraft will fly past the Earth and continue orbiting the Sun. It is possible that it could be reused for another mission.
One thing which interests astronomers a great deal is what asteroids like Ryugu and Bennu are made of. Both are {carbonaceous chondrites}, with rubble-like material similar to meteorites which fall on Earth, with many organic compounds, which have plenty of carbon atoms. Osiris-rex has studied the spectrum of Bennu and found that it has {hydrated} compounds, those formed in the presence of water. So when we look at the sample brought back to Earth, we might be able to see if the water in a clayey part of the sample is similar to that on Earth.
If you thought water is the same everywhere, think again! The spectrum of water on the comet Churyumov-Gerasimenko showed a different composition of Deuterium (an isotope of Hydrogen which has a neutron in addition to a proton) to that found in water on Earth, suggesting that these two have different origins. If it turns out that water on Bennu matches water on Earth, astronomers could consider the possibility that all the water on Earth might have come from asteroids like Bennu. One could even mine the water from Bennu's clays and use it on Earth.

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Box 2: Three attempts on the Moon,
Chandrayaan 2, Beresheet and Chang'e-4
In the July-August 2019 issue of {Jantar Mantar}, we followed India's {Chandrayaan 2} mission to the Moon. It is successfully in orbit around the Moon. As you may have read, its {Vikram} lander was lost while trying to land on 6 September because some of its braking equipment did not work. Tracking by a radio telescope in the Netherlands suggests that Vikram hit the surface at a speed of 50 metres per second (about 180 km per hour), rather than the desired speed of 2 metres per second. The lander has not been located in photographs.
Earlier, on 11 April 2019, Israel's {Beresheet} lander crashed on the Moon at a vertical speed of 130 metres per second and a horizontal speed several times that much. Even though the Moon's gravity is one-sixth that on Earth, it causes acceleration which is very difficult to control.
Even earlier, on 3 January 2019, the Chinese {Chang'e-4} landed on the far side of the Moon, with communications being carried out through a relay satellite {Queqiao}, as we wrote in {Jantar Mantar}. The lander used variable thrust engine braking which slowly brought it down to a height of 4 metres, at which point it fell on to the Moon. Chang'e-4 is a collaboration between the US and China. Nasa is interested in using the spacecraft and the {Queqiao} satellite on future Moon missions.
The Chang'e lander carried plants like cottonseed, potato, rapeseed, yeast and fruit fly eggs, in an attempt to see whether photosynthesis could happen on the Moon if an Earth-like temperature of 24 degrees C was preserved. Images of sprouting cottonseed were shown on 15 January. The long lunar night finished off all the experiments, the temperature could not be maintained, but some data has been obtained.
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