Like a diamond in the sky by Kamal Lodaya [large space missions map by olaf frohn, can be used as centrespread, http://planetary.s3.amazonaws.com/assets/images/charts-diagrams/2018/20180531_solar-system-missions2018-06_f840.png] On 27 June, the Japanese spacecraft {Hayabusa2} reached within 20 kilometres of a potentially hazardous asteroid called {Ryugu}. Asteroids are thought to be leftovers from 4.5 billion years ago when the solar system was formed from a nebula of gas and dust. Perhaps there are so many between the orbits of Mars and Jupiter because they did not have enough material to form another planet. The largest among them, the dwarf planet Ceres, was visited by the US spacecraft {Dawn} 2015. {Dawn} is still in orbit around Ceres, taking more pictures. ---- Box 1: Asteroids, comets, meteors If you are getting confused by all these names, a {meteor} is an object which burns up when it enters the Earth's atmosphere. We see bright meteors as "shooting stars". A {meteorite} is a meteor which reaches the ground instead of being completely vaporized in the atmosphere. The largest meteorite of the Chelyabinsk meteor, about half a metre in size, was found at the bottom of a lake near the city. Most meteors are pieces of rock or dust which have been strewn by a comet or an asteroid as it goes around the Sun, which Earth encounters when it crosses their orbit. An {asteroid} is a small planet (usually very small) which goes around the Sun. {Comets} also go around the Sun, but they are composed more of dust and ice. When they come close to the Sun they leave behind a dust tail which is lit up by sunlight and can sometimes be seen for crores of kilometres. Asteroids are composed of rock and minerals. It is not thought that asteroids leave behind trails, the rocks coming from them could have been thrown out by a collision with other asteroids or the gravitational effect of a larger planet. ---- Box 2: Asteroids which come close {Near-Earth objects} are those whose closest distance to the Sun ({perihelion}) is within 130 per cent of Earth's (around 20 crore km). There around twenty thousand of them. Among them are about two thousand {potentially hazardous asteroids} (PHA), which cross the Earth's orbit within 20 Moon distances (around 75 lakh km), and are thought to be at least 140 metres in size. A PHA comes very close to Earth and, if it hits us, can cause damage over a large region. About two thousand PHA are known. The closest calculated approach in the 21st century is by the 350-metre PHA {Apophis}, which will pass by Earth on 13 April 2029. It will not hit us and it will not hit the Moon but it may come to within 31,000 km (0.1 Moon distance). At its closest it will be visible with the naked eye as a faint "star" moving in the sky. It is estimated that there may be three to four times this number of such asteroids still to be discovered. Because they are very small, these PHA are quite faint. They can be seen well only when they come quite close to Earth. There are many telescopes and hundreds of amateur astronomers who help in looking out for unknown asteroids and comets. ---- Asteroid falls on Earth Apart from this large "belt" between Mars and Jupiter, asteroids appear to be all over the solar system. There are tens of thousands of asteroids between the orbits of Earth and Mars, many of them classified as "near-Earth objects". Thousands are also called "potentially hazardous asteroids". The last one which we know hit Earth vaporized over the Tunguska river forests of Russia in 1908. The explosion felled 8 crore trees over tens of kilometres. Many animals were killed but luckily, people in a few villages of the remote area escaped with their lives, although their huts were destroyed by the falling trees. The photo taken in 1927 shows trees knocked over by the Tunguska blast. Sometimes these hazardous asteroids may not be seen at all! On the dawn of 15 February 2013, a 20-metre asteroid was seen as a meteor brighter than the Sun, exploding over the city of Chelyabinsk in Russia as it disintegrated because of the heat created by the friction with the Earth's atmosphere. It broke glasses in 7,000 buildings in six Russian towns. Many people were injured, luckily no one died. The asteroid was not caught earlier by any telescope since it came from a direction close to the Sun's position in the sky. A PHA is several times larger than the Chelyabinsk meteor. Just a little earlier in December 2012, the Chinese spacecraft {Chang'e-2} had reached the potentially hazardous two-kilometre asteroid {Toutatis}, which passed by us at 4 Moon distances in 2004. It was found that its orbit is so disturbed by its path between Earth and Jupiter (it approaches close to both of them), that it is impossible to predict for long periods of time. Toutatis was disovered in 1934 and then lost because of its chaotic orbit, then again found in 1989. Both discoveries were by French astronomers. The name is popular in a series of French comic books. Most countries which have an active space programme scrambled at this reminder of what could happen to Earth. Japan's {Hayabusa 2} spacecraft set off for Ryugu, just under a kilometre in size. The United States sent {Osiris-rex} to another potentially hazardous half-kilometre asteroid called {Bennu} which had passed not far from Earth in 1999, 2005 and 2011. It was then observed by radar, showing some details of its shape. Bennu will be reached by December this year. 2018 and 2019 will be years in which we will learn a lot about asteroids. The first surprise on coming close to Ryugu was that it does not have the round or elliptical shape that we know larger planets to have. We already saw in 2014 that the nucleus of comet Churyumov-Gerasimenko, visited by the European spacecraft {Rosetta}, has quite an irregular duck-like shape. It is thought to be formed by the collision of two objects. Also, a comet is losing material all the time. {Rosetta}'s estimate is that during its closest approach to the Sun in 2015, C-G's nucleus lost as much as 10 to 20 metres of its surface. Ryugu appears to be quite stable. Its orbit is elliptical but not so elliptical as comet C-G. Yet it is diamond-shaped: angular, not spherical. Compared to the faster Bennu, it rotates about itself in 7.5 hours. All along its equator, defined by being at an angle of 90 degrees from its axis of rotation, there is a large hilly ridge. Such an equatorial ridge was earlier seen in radar observations of Bennu. (Ryugu was never seen close enough to Earth to be spotted by radar.) This raises a question: will Bennu also turn out to be similar in shape to Ryugu? Both the asteroids are called {carbonaceous chondrites}. The name comes from meteorites which are made up of organic compounds, silicates, oxides and sulphides. Ryugu's composition is supposed to have more of silicate clay- or mica-like minerals (also seen on Ceres), whereas Bennu is supposed to have dryer silicates, otherwise their composition is similar. Do small asteroids with this kind of composition have this kind of shape? <67p.jpg> The European {Rosetta} visiting comet C-G had a small laboratory which performed impressive analyses of the dust grains which fell on it from the comet. Its lander {Philae} did a chemical analysis of the atomosphere near the nucleus surface. Both {Hayabusa2} and {Osiris-rex} will try to collect samples of their asteroids and bring them back to Earth, so that they can be analyzed in more detail by our laboratories. Let us look at what the Japapnese {Hayabusa2} will do on Ryugu. In another article later in {JM} we will look at the American spacecraft's plans. Shooting at an asteroid {Hayabusa2}'s "home station" is to remain 20 kilometres from Ryugu. But it can come much closer to within metres of the surface, to do a "touchdown": a {sampling horn} hangs down from the spacecraft and touches the surface. A bullet is fired into Ryugu so that it raises a lot of material, some of which will fall into the horn. Then the horn is lifted up and the sample is stowed away in the spacecraft. Before doing the sampling {Hayabusa2} will map the asteroid's surface to find interesting points from where samples can be picked up. It also has a 2.5-kg copper "cannonball" inside explosive material. When this is fired at Ryugu it is expected to make a {crater} where material from inside the asteroid comes out. Then this can be picked up by the sampler. So there will be samples of the surface materials and also of material which is deeper inside. {Hayabusa2} also has several landers which will be gently thrown down from about 60 metres. Ryugu is small and has very little gravity so they will hit the surface at a speed of about 30 centimetres per second. {Rosetta}'s lander {Philae} famously bounced on its comet and got wedged between rocks on its surface. These landers have the capability to hop around a bit on the surface and relocate, so that if they land in an awkward place they can get out of it. One of the landers is designed by the same European team which designed {Philae}, and they have tried to make sure that all the flaws of the earlier mission are corrected. This reminds us of the American spacecraft {Deep Impact}, which fired its 100-kg "impactor" at the comet {Tempel 1} in 2005 to create a huge dust cloud which was seen by telescopes back on Earth. A Russian writer on astrology, Marina Bai, took the space agency NASA to court saying that the Americans were upsetting the balance of forces in the universe. The court sided with the NASA scientists who said that the effect on the comet nucleus's mass would be miniscule and its gravity would not be affected. Mining and moving an asteroid How times have changed... The Europeans are now planning to send their {Hera} spacecraft to a potentially hazardous double asteroid, the kilometre-size {Didymos} which has a 150-metre satellite, popularly called {Didymoon} (that is not an official name). Didymoon is roughly the size of Egypt's great pyramid of Giza, or about twice as much as the Taj Mahal. The Europeans' aim is to use an impactor to try to {change} the orbit of Didymoon! Their argument is that suppose we suddenly find a small asteroid headed for Earth, and we can predict that it will cause a lot of damage here. In 2008 the orbit of asteroid Apophis was not known as well as it is today, and it was thought that Apophis might actually hit Earth in 2036. Its path over Earth would be across Russia, the Pacific ocean, Costa Rica, Colombia, Venezuela and the Atlantic ocean. Caracas, the capital of Venezuela, a city with 70 lakh people (comparable to Hyderabad), was close to this path. With better calculations of its orbit later, it was found that Apophis will not hit Earth. Should we not try to change the orbit of such an asteroid so that it does not hit Earth? If you agree with this idea then the next step is to try out this technology on an asteroid when it is far away from Earth, and that is the {Hera} plan. It was earlier called the "Don Quixote plan". If you have read Spanish writer Miguel de Cervantes's book of that name, you might guess why. The Americans were also part of this plan earlier, they have now backed out. Do all these samples and impactors remind you of the construction and mining operations that we do here on Earth? Then it will not surprise you to know that private space companies are interested in these operations. Because these small asteroids can have some minerals which are rare on Earth, not very deep and easily mineable. A space company has estimated that the value of such minerals on asteroid Ryugu is about 87 billion US dollars. The {Hayabusa2} mission costs around half a billion US dollars, but the technology tested by a few such missions can be thought of as an investment which can yield a profit decades later. Apart from the Chinese, Russians, Japanese, European and American space agencies, their industries are following in their lead to earn those profits. --Images from various sources