This month, scientists will attempt the first soft landing on a comet, in search of answers to some of the biggest mysteries.
Despite centuries of study, comets are among the most enigmatic denizens of the solar system. They are thought to be key players in the history of the Earth, and perhaps in the origin of life itself.
Yet exactly what they are, and where they fit in the celestial family tree, remains mysterious.
That looks set to change.
After a decade-long journey, the European Space Agency’s (ESA) Rosetta spacecraft is in orbit around comet Churyumov-Gerasimenko, named after two Soviet astronomers who discovered the comet orbiting the Sun in 1969.
Now, mission controllers are about to instruct Rosetta to send a lander filled with scientific instruments down on to the surface of the comet to investigate its mysteries.
Simply arriving at the comet is an astounding achievement. To intercept it on its path around the Sun, Rosetta had to reach a speed of 55,000km/h, powered only with small thrusters.
Engineers worked out a solution in the form of a 6.5 billion-kilometre path of staggering complexity that included repeated fly-bys of the Earth and Mars, whose gravity fields acted as slingshots, hurling the spacecraft towards the comet.
In August, Rosetta reached its destination and went into orbit – another impressive feat given the bizarre, duck-like shape of the comet and its hideously distorted gravitational field.
Sidling up ever closer to the comet, Rosetta carefully mapped out the field, and found a region where its thrusters could maintain a fairly steady orbit. Since August, Rosetta has been swooping down to within just 10km of the comet’s surface.
The ESA hopes to pull off the biggest coup of all – the soft-landing of Rosetta’s lander, Philae.
Choosing where to land was difficult enough. Images taken during Rosetta’s approach revealed the comet to be a four-kilometre-wide chunk of crater-pitted rubble. After intensive study, mission controllers last month decided on what they hoped would be a suitable landing point for Philae, on the “head” of the duck-like object.
Usually, the biggest fear that mission controllers have with landers is that they will burn up in the atmosphere, or crash-land. The good news is that the mass of the comet is so small by cosmic standards – about 10 billion tonnes – that its gravity is about 100,000 times weaker than that of the Earth. As a result, it is unable to hold on to an atmosphere.
But it also means that the comet will struggle to hold on to Philae, which risks landing and then bouncing straight back off into space. An astronaut could literally hop off the comet, never to return.
So mission controllers are pinning their hopes on a harpoon-like anchor that Philae will fire into the comet to stop a disastrous rebound, with thrusters finally settling the lander down.
According to ESA, the US$1.7 billion (Dh6.24bn) Rosetta mission is the single most complex project that the agency has undertaken in its 40-year history. But it believes that the scientific findings will more than justify the risk and expense.
Comets originate in a vast cloud far beyond the planets, and are sent our way by the gravitational stirring of passing stars.
As such, these so-called “dirty snowballs” are crammed with pristine material from interstellar space, unchanged for billions of years.
Astronomers think that buried within them are clues to the origins of the two most distinctive features of our planet: its oceans and life forms.
The origin of the water on Earth is hotly debated by scientists. According to some, water was originally trapped in rocks within the Earth and released as the planet cooled following its formation 4.5 billion years ago. Yet attempts to estimate the resulting quantities have yielded contradictory results.
It is now thought that at least some of the water must have been supplied by comets colliding with the Earth; the question is how much.
Among the experiments aboard the Philae lander is a miniature laboratory designed to investigate that.
Although all water molecules are chemically identical, their central nuclei can be subtly different. Philae’s laboratory is able to compare the water nuclei found on the comet with those found on Earth.
If they turn out to be similar, it will support the theory that much – perhaps most – of the water in the oceans was delivered from deep space by comets.
But Philae’s creators hope it would also be able to cast light on an even deeper scientific mystery: the origin of life.
As with the water on the Earth, there are two basic theories – that life originated from chemicals already present on the Earth, or that they were delivered from elsewhere.
In the 1950s, two American scientists made headlines with laboratory experiments suggesting that conditions on the primordial Earth could lead to the production of amino acids, the so-called building blocks of life.
Serious flaws have since been found with the design of the experiments, and it is now thought that only a few types of amino acids could be made by Earth-bound processes alone.
Scientists hope that Philae will cast light on the contribution of comets, using its on-board laboratory to look for more – and more complex – amino acids.
Intriguingly, Rosetta has already detected many of the raw materials for amino acids in materials escaping from the comet: carbon dioxide, methane and ammonia.
Despite their moniker, amino acids are only part of the recipe for life on Earth; cells use them to create proteins according to the genetic instructions in DNA.
That said, they can form proteins that act like primitive versions of DNA. According to some scientists, some life forms on the early Earth might have been based solely on proteins.
As such, the discovery of complex amino acids by Philae would be a major step towards understanding the origin of life – and not only on the Earth.
Astronomers have found comets orbiting other stars, which are themselves known to have planets. We may thus be about to get the best reasons yet for believing that we are not alone in the Universe.
Robert Matthews is a visiting reader in science at Aston University, Birmingham
