Landing on a comet

Kamal Lodaya, The Institute of Mathematical Sciences, Chennai

Does that sound like a science fiction story? If all goes well it will happen in November this year. First the story.

The comet

(picture - churyumov, esa director, gerasimenko)

On September 11, 1969, just a couple of months after Neil Armstrong and Edwin Aldrin landed on the Moon, astronomer Klim Churyumov from Kiev in the Ukraine examined a photograph of a faint comet, taken by his colleague Svetlana Gerasimenko at the Alma Ata observatory in the USSR. (Since then Alma Ata has become Almaty, the capital of Kazakhstan.) The picture shows them with European Science Agency
director-general Jean-Jacques Dordain.

There was a fuzzy object at the corner of the photograph. After checking all the images, Churyumov realized that this could not be the comet they were looking for. So these two astronomers had discovered a new comet, now named Churyumov-Gerasimenko.

The comet is closest to the Sun (at {perihelion}, next in August 2015) a little outside the orbit of the Earth. At its farthest from the Sun it reaches the orbit of Jupiter. It is thought to have been in the Kuiper belt of comets beyond the orbit of Neptune, but was pushed into the inner solar system by the gravity of Jupiter. Now it takes six and a half years to go around the Sun. It is not a spectacular comet, even at its brightest it requires quite a large telescope to see it.
In July this year comet Churyumov-Gerasimenko was approaching the Sun, but still was outside the orbit of Mars. Rosetta has already found at the end of April, around 60 crore km from the Sun, that the ices on the comet have started sublimating and it has formed a {coma}.

The spacecraft
(picture - rosetta)
Why are we interested in this uninteresting comet? Because in May the {Rosetta} space probe of the European Space Agency has reached this comet. (That is why the director-general of ESA was in the picture with the two astronomers.) {Rosetta} was launched in 2004, and it has followed a complicated path using slingshot effects from the gravity of Earth, then of Mars, then of Earth once again, to get it to reach the comet. The path was so complicated that in 2007 some astronomers discovered a faint new ``asteroid'', until someone realized that this new asteroid was nothing but the {Rosetta} spacecraft.
Kepler's laws say that objects closer to the Sun go around it faster. Since {Rosetta} is approaching the comet from inside its orbit it will be slowly overtaking the comet. But as it approaches, {Rosetta}'s engines will fire and it will go into {orbit} around the comet! Then it will stay with the comet until well after the comet reaches perihelion. So the idea is that the spacecraft will see what happens to a typical comet as it approaches the Sun, how its ices vaporize, how its tail develops, and so on.

The rendezvous
(picture - halley's nucleus)
This is not without danger. When some ices under a comet's crust vaporize their pressure can break the crust of the nucleus and the gas come out like a geyser or a jet. It is the gas and dust from these jets which form the {coma} (head) of the comet. The pressure of energetic particles from the Sun (protons, electrons---this is called the {solar wind}) pushes these gases behind the comet to form its {tail}. The picture shows the jets on the nucleus of comet Halley at its last visit in 1986, seen from the European spacecraft {Giotto}. ({Giotto} flew by Halley, its closest distance was 600 kilometres.)
If one of these jets hits {Rosetta} it is likely to be destroyed! If a large bunch of dustgrains hits its solar panels {Rosetta}'s energy sources might deplete! The idea is to maneuver the spacecraft so that it avoids all these dangers, but to keep photographing the comet all the time. 
If all goes well, in November {Rosetta} will launch a {lander} (named {Philae}). This lander will approach the comet very slowly, its relative speed will be something like walking towards the comet. So that when it touches down, the lander will hopefully not destroy any part of the comet. Two harpoons will be fired into the comet to prevent the lander from bouncing off. Then the lander will start digging into the comet as the Moon and Mars rovers have done, and find out the chemical composition of its surface. 

(picture - artist's impression of landing)
We did not know what Churyumov-Gerasimenko's surface looks like. All the details, including the landing site, will be decided only after looking at photographs taken by the spacecraft when it reaches the comet in May. The entire programming will be done by people sitting on Earth. If you thought jumping safely on a surface full of rubble is difficult, think of what the {Rosetta} mission scientists are doing, with a probe which costs about Rs 5000 crores! The picture shows an artist's impression of the landing.
 
Box: Hitting a comet

(picture - deep impact)
In 2005 NASA's {Deep Impact} spacecraft undertook a different strategy to approach a comet. Its lander smashed straight onto the comet Tempel. As you see in the picture, a large and bright dust cloud was seen on the comet from the spacecraft, but not much else! NASA used amateur astronomers to observe the comet from Earth. For example, school students in Hawaii, working with other scientists, operated a telescope over the internet and obtained photographs of the comet at the time of impact from the Earth. For a short time after impact the comet, seen by us as just a ``star'', was six times brighter than before.

Box: Losing a rocket
On 11 August 2013, NASA {lost} the {Deep Impact} spacecraft which cost about Rs 104 crores. {Deep Impact}'s main mission was to send a lander smashing onto comet Tempel in 2005, which it successfully accomplished. It was then used to observe the faint comet Hartley 2 in 2010, comet Garradd in 2012 and comet ISON in early 2013. From 2011 it was headed for a faint asteroid which it would reach in the year 2020. Presumably it is still doing so. But from 11 August 2013 onwards, the computers onboard are continuously rebooting themselves, which makes them not listen to any commands sent from Earth. Why this date? The computers measure time every 0.1 second using 32 bit words. On this date the number of such seconds elapsed since the year 2000 was 2 raised to the power 32. So something like a Y2K software bug is there in the programs for these computers!