Cosmic secrets

The comet Churyumov-Gerasimenko as seen by Rosetta's navigation camera on September 19 at 28.6 km from the centre of the comet. The image shows the extreme jaggedness of the comet's surface and the increased cometary activity on the neck.

COMETS are the most primitive objects in our cosmic neighbourhood. They are fragile and irregularly shaped, with diameters ranging from several hundred metres to tens of kilometres, and largely made of chunks of dust and ice containing organic molecules. Located in the farthest reaches of the Solar System, they are very cold objects and, therefore, preserve volatile constituents in the form of ice over extremely long periods of time including the pristine material left over from the formation of the Solar System 4.5 billion years ago. They are thus believed to harbour information about the initial stages of the formation of the Solar System. It is quite likely that comets impacting the earth brought with them the essential constituents of life, such as water and organic molecules. Studying the chemical composition of comets may answer some of these open questions and hence the importance of a Rosetta-like mission.

The largest reservoir of comets is the Oort Cloud, a huge spherical cloud enveloping the Solar System at a distance of 50,000-100,000 AU. (Astronomical unit 1 AU = 150 million km and is roughly equal to the earth-sun distance.) It most likely contains over a trillion objects, which are predominantly icy planetesimals, most of them long-period comets. There is a second, significantly smaller, comet reservoir called the Kuiper Belt, a flattened ring-like distribution that begins just beyond Neptune’s orbit (at a distance of about 50 AU). Comets in both the reservoirs are believed to have originated from the primordial nebula that gave birth to the Solar System and are gravitationally bound to the Solar System.

Once in a while one of these icy comets gets kicked into the inner Solar System by gravitational disturbances. As they go nearer towards the sun, they develop their characteristic “coma” and tails because of sublimation under the influence of the Sun’s radiation. As comets loop back into the outer reaches of the Solar System, they can get into highly eccentric orbits and form long-period comets. In other cases, under the strong gravitational pull of one or more of the giant planets, a comet can remain trapped in a more regular, less eccentric short-period orbit much closer to the sun.

The comet 67P/Churyumov-Gerasimenko (C-G), which is the object of target and study for the comet-chasing Rosetta mission, belongs to the nearer Kuiper Belt. It is named after the Ukranian astronomers Klim Churyumov and Svetlana Gerasimenko, who discovered it in 1969. The “67P” refers to its being the 67th short-period comet. It is trapped in an elliptical 6.45-year orbit around the sun between the orbits of Jupiter and the earth and has a rotation period of 12.4 years, and the orbit is inclined at 7° to the ecliptic. The mass of the comet has been estimated to be 10 trillion tonnes. Because of this trapped orbit under the strong gravitational influence of Jupiter, it belongs to what is called the Jupiter Family of comets.

Close encounters with Jupiter in 1840 and 1959 are believed to have reduced the comet’s perihelion (closest approach to the sun) from its earlier 600 m km to 186 m km, which now falls between the orbits of the earth and Mars. The aphelion (the farthest point) lies slightly beyond Jupiter’s orbit. The comet has now been observed from the earth on seven approaches to the sun—1969 (discovery), 1976, 1982, 1989, 1996, 2002 and 2009. After its perihelion pass in 2009, it was found that the rotational period of C-G had decreased from 12.76 hours to 12.4 hours. This change is believed to be due to sublimation-induced torque. During its perihelion in 2002/2003, C-G blew an estimated 60 kg of dust per second into space, and during the 1982/83 perihelion, the amount of ejecta is estimated to be as high as 220 kg/s. Even during the approach phase of this mission at a distance of about 4 AU from the sun, Rosetta detected faint signs of activity on the comet; from May onwards, it was observed that a coma was beginning to form around the comet’s nucleus but material losses were still small, about a few hundred grams/s.

Comets are known to have irregular and rather potato-like shapes. The first images of the comet C-G were provided by the Hubble Space Telescope in March 2003 in preparation for the Rosetta mission. The images revealed an oval-shaped body measuring 3 km x 5 km. But a detailed image could be obtained only during Rosetta’s approach in July 2014 when the spacecraft was just 14,000 km from its target, which revealed a curious shape.

The comet consists of two parts in close contact. It is a double-lobed structure with a larger “trunk” measuring about 2.5 km and a smaller “head” measuring about 1.5 km. The shape has been likened to a huge rubber duck. “It is not unlikely that comets were formed from the collision of small building blocks far away from the sun during the emergence of the Solar System,” says Ekkehard Kuehrt of the German Aerospace Centre, one of the partner institutions in the Rosetta mission. “The two blocks… collided at slow speed, stuck to each other and have since been moving together. Scientifically it is now very interesting to find out whether the two components differ in their composition. If the two parts are from different regions, their structures may also differ.”

When Rosetta reached the comet on August 6, the high-resolution images showed a jagged world. The double nucleus is connected by a distinct “neck” whose surface is strewn with numerous boulders. The “neck” appears to be the most active area on the comet at this time. Both the parts are covered by terraces and round-to-oval depressions separated by sharp ridges. Contrary to expectations, parts of the surface seem to be covered in dust. On the other hand, bare, exposed ice appears to be rare or possibly even non-existent.

R. Ramachandran