When is a comet not a comet?

Tuesday 20th October, 2015


In our evening talk by Prof Monica Grady from the Open University, entitled "When is a comet not a comet?", she explained that it was very difficult to define the difference between a comet and an asteroid. In general she said that asteroids are made of metallic and rocky material whereas comets contain ice and dust along with their rocky constituents. Although both types formed early in the history of the Solar System the asteroids formed much nearer the Sun, usually have smaller more circular orbits and tend to group into belts such as the Asteroid Belt located between the orbits of Mars and Jupiter.

In contrast comets are solitary bodies with large elongated orbits that take them to the far outer reaches of the Solar System. When they sweep back towards the Sun the ices they contain heat up and turn to gas forming the long tails that mark their journey around the Sun.

However, some of these bodies seem to have a split personality and show the traits of a comet at one time but then appear to be an asteroid later. For example, in 2013 the Hubble Space Telescope imaged an object in the Asteroid Belt with no less than six dust tails emanating from it. Scientists commented that it looked somewhat like a rotating lawn sprinkler.

The Rosetta spacecraft is currently in orbit about comet 67P/Churyumov-Gerasimenko with the Philae lander sitting on the comet's surface after surviving a somewhat haphazard descent. Philae managed to do some science when it first came to a halt (after bouncing a few times) on the surface. However, its final location was in the shade next to a cliff and so its solar panels could not charge properly but it managed to last 60 hours.

Further contact was established with Philae in early July 2015 but nothing has been heard from it since as increased activity from the comet as it approached the Sun meant that Rosetta had to orbit further away out of communications range. As the comet is now moving away from the Sun the flight engineers are able to decrease its orbital distance and so they are hoping to hear from Philae again.

Although comet 67P/Churyumov-Gerasimenko was the final target for Rosetta, on its way the spacecraft flew past two asteroids - Steins and Lutetia. Using the returned data scientists concluded that Steins was once part of a larger object that had broken up and due to its "rubble pile" consistency it is likely to eventually disintegrate. They think that it is diamond-shaped due to re-radiated sunlight altering its momentum and rotation rate causing material to shift towards its equator as well as infilling smaller craters. Unlike the diamond-shaped Steins, Rosetta's images showed that Lutetia was an irregular cratered object with a vast network of grooves and fractures. It has a high average density and so is most likely enriched with metallic elements such as iron.

The Rosetta mission was planned to finish at the end of this year but it has been extended to September 2016. With solar power failing as it follows the comet away from the Sun and the propellant in its tanks dwindling the plan is to gently set the spacecraft down on the comet's surface where it will join the Philae lander as all circle the Sun on a 6.5 year orbital journey.


This article was written for the club news column of the Stratford Herald. The actual lecture explained the subject at a deeper level.