Tracking a comet

Print edition : October 22, 2004

The ESA's Rosetta spacecraft is on a 10-year mission to Comet Churyumov-Gerasimenko in a bid to unravel the secrets of comets and their place in the evolution of the solar system.

A COMET presents one of the most spectacular sights in the sky. Often unheralded, it appears first as a small source of light, somewhat fuzzy and often resembling a nebula or a distant galaxy. As it moves closer to the sun, however, a vaporous tail shoots out of its head; this tail always extends away from the sun, growing as it approaches the sun and diminishing as it recedes from the sun. Comets presented a fearsome spectacle to ancient observers and their appearance was interpreted as a sign of terrible events to come, such as the death of a king. The tail was thought to contain poisonous gases that could endanger the earth's inhabitants. There is a tale about one enterprising person at a gathering of comet watchers who provided, for a fee, his fellow viewers pills to protect them from such dangerous gases.

Halley's Comet, photographed by a ground-based telescope.-GAMMA

When a comet is far away from the sun, it consists of only a nucleus that is alternately called a "dirty snowball". Such a description implies a mixture of dust-like particles bound together by water ice and frozen gases. As the nucleus approaches the sun, within about 3 A.U. (1 Astronomical Unit = the average distance of the earth from the sun = 1.496 x 10{+8} km), the solar radiation causes the outer layer of frozen material to vapourise (sublimate), thus producing a large gaseous region called the coma surrounding the nucleus . The thin nature of the coma is revealed by the stars seen shining through it. Although the nucleus is very small - often only a few kilometres in diameter - the coma usually grows to a size several times that of the earth's diameter. When a comet passes quite near the sun, its temperature may rise to several thousand degrees.

The tail of a comet usually develops only after it comes within 2 A.U. from the sun. Both gas (ions) and dust particles are released as the icy material of the nucleus sublimates, and these two constituents of the coma respond differently to forces from the sun, thus actually forming two tails. The gas tail consists of ions, or charged atoms, which are carried outward from the sun by the solar wind (itself a flow of charged particles). The gas tail usually appears quite straight and always points directly away from the sun. This is true even when the nucleus of the comet is receding from the sun; then the tail actually leads the comet. The gas tail emits its own light by a process of downward electron transitions.

Dust particles are driven outward by another force, the influence that sunlight exerts on them. This is often referred to as radiation pressure. Because the dust particles are driven outward more slowly than the ions of the gas tail, they tend to retain more of their forward motion and the dust tail is usually more curved. Whereas the gas tail emits its own light, the dust tail shines by reflected sunlight. The length of the average comet tail is only a few million kilometres. However, the tails of some comets have reached a length of more than 160 million km, a distance greater than that between the sun and the earth. The tail is so tenuous that stars may easily be seen through it. Faint comets are common; a number of them are discovered every year. But a truly bright comet appears only about once a decade. Comet Hyakutake in 1996 and Comet Hale-Bopp in 1997 were both dramatic, but the Hale-Bopp was so bright that it can be classed with other great comets such as Halley's comet of 1910.

The current interest of astronomers in comets comes from the belief that these are made of material that has been safely tucked away in the deep freeze of the outer solar system for billions of years. Today we recognise comets as the best preserved and most primitive material available in the solar system. Stored in the deep freeze of space, these icy objects are messengers from the distant past, providing us unique access to the initial material from which planets formed 4.5 billion years ago. Thus by studying comets, one learns about the pristine material left over from the origin of our solar system. This will help us to understand our origins. For that reason, studying cometary nuclei is of prime importance to planetary scientists.

Comet nuclei are too small for their surface details to be resolved by telescopes on the ground. At the centre of a comet's head is its nucleus. The central nucleus of a comet is a loose mixture of cosmic dust and water snow. The average diameter, for all the comets that have been recorded throughout history, is around 3,000 metres. From earth we can use our largest telescopes to detect the nucleus, by collecting the light that it reflects. But it is far too small, and distant, for any details to be distinguished. The tail of gas and dust and coma that appear around the comet as it approaches the sun, further obscures the view of its centre.

At the European Space Operations Centre in Darmstadt, Germany. A file picture.-GAMMA

Therefore the only way to study the heart of a comet is to reach them through space probes. In 1986, the European Space Agency's (ESA) Giotto mission flew within 600 km of Comet Halley, revealing its nucleus as an irregular-shaped chunk of ice. And in 2001, Deep Space 1 probe of the National Aeronautics and Space Administration (NASA) of the U.S. followed up, passing within 2,000 km of Comet Borrelly. But scientists think they can do better - which is why the ESA came up with the Rosetta spacecraft to probe Comet Churyumov-Gerasimenko.

The ESA was to launch this comet interceptor on January 12, 2003. After over a year of delays, the mission finally soared to space on March 2, 2004. An Ariane 5 launcher propelled the spacecraft on its 10-year course to Comet Churyumov-Gerasimenko. Rosetta's epic voyage will help scientists gain new insights into comets and the history of our solar system. It will take a circuitous route through the solar system, using flybys of earth and Mars to receive gravity sling-shots for the comet rendezvous in 2014.

Rosetta is truly an international enterprise, involving more than 50 industrial contractors from 14 European countries and the U.S. The spacecraft consists of a three-tonne orbiter equipped with 165 kg of scientific instruments, and a 100-kg lander, provided by a consortium led by the German Space Agency and the ESA.

SUMMER 2014. In the dark depths of the solar system, far beyond the asteroid belt, a robotic emissary from earth prepares for a historic encounter with a cosmic iceberg. After 10 years of bouncing between planets, the Rosetta spacecraft with its cameras, spectrometers and other sophisticated instruments, is about to have a rendezvous with Comet Churyumov-Gerasimenko. Rosetta's 5-billion-km space trek has not been in vain. Just as it was predicted more than a decade earlier, the periodic comet is right where it should be (farthest from the sun), hurtling through space at over 130,000 km an hour as it accelerates towards the sun. The cameras on board Rosetta reveal a dim, distant object, a primordial remnant of the debris from which planets formed some 4.6 billion years ago. In this frigid region, where solar radiation is 16 times lower than in the earth's vicinity, the icy nucleus - the heart of the comet - remains dormant. Only later, as energy from the sun warms its surface, will the ice erupt, ejecting plumes of gas and dust.

Each day, new and clearer images appear on the computer screens at the European Space Operations Centre in Darmstadt, Germany. As the black nucleus emerges from the darkness, the controllers refine their calculations of the comet's position and orbit, as well as its size, shape and rotation. Eventually, the relative velocities of the spacecraft and the comet slow down to a mere 2 metres a second. The thrusters fire to insert the spacecraft into orbit just 35 km above the irregular nucleus. With their relative velocity now down to a few centimetres a second, Rosetta is free to execute a leisurely reconnaissance of the pristine surface, mapping every hollow, hill, crater and boulder.

By November 2014, the number of potential landing sites has been narrowed down to one. Now skimming just 1 km above the dark terrain, Rosetta is ordered to release its tiny three-legged lander named Philae. Over the next half an hour, the box-shaped craft creeps away from its mother ship, its snail-like progress recorded by an onboard camera. After a nail-biting wait from the helpless mission team back on earth, sensors on the lander record the historic moment of arrival. Barely influenced by the almost negligible gravitational pull from the fluffy nucleus - some 150,00 times weaker than what one experiences on earth - the lander touches down at no more than walking pace. Philae will snap high-resolution pictures and acquire data about the comet's organic crust and molecules. The instrument suite even includes a tiny drill that can bore into the comet for subsurface investigations.

Together the orbiter and lander will observe the traits and changes the comet goes through as it approaches the sun. This will be the first ever chance to be there, first hand, so to speak, as a comet comes to life. Rosetta will be orbiting around Comet Churyumov-Gerasimenko for two years, until the comet reaches its closet point to the sun and travels away from it.

During an epic journey lasting more than 10 years, Rosetta will have achieved many historic landmarks: The first spacecraft to orbit a comet's nucleus; the first spacecraft to fly alongside a comet as it heads towards the inner solar system; the first spacecraft to examine from close range how a frozen comet is transformed by the warmth of the sun; the first controlled touchdown on a comet nucleus; the first images from a comet's surface and the first in situ analysis to find out what it is made of; the first flybys of the main belt asteroids Siwa and Otawara; and the first spacecraft ever to fly close to Jupiter's orbit using solar cells as its main source of power.

Meanwhile, scientists will be ploughing eagerly through the enormous treasure trove of data in an effort to unravel the secrets of comets and their place in the evolution of our solar system. Results from previous studies by ESA's Giotto spacecraft and ground-based observatories have shown that comets contain various ices as well as complex organic molecules - compounds that are rich in carbon, hydrogen, oxygen and nitrogen. Intriguingly, these are the elements that make up nucleic acids and amino acids, essential ingredients of life as we know it. Did life on earth begin with the help of comet seeding? Did comets supply the water for earth's oceans? Rosetta may help us find the answers to these fundamental questions.

The ESA's Rosetta mission is one of the most ambitious and complex projects undertaken during four decades of robotic space exploration. Many challenges will have to be overcome before the orbiter and its piggy-backed lander are able to rewrite textbooks and reveal for the first time the true nature of the `hairy stars' that have fascinated mankind since time immemorial.

Prof. Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.

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