The Giotto mission – exploring the secrets of a comet’s nucleus
In 1986, the European probe Giotto flew past the nucleus of Halley’s Comet – at a distance of just 596 km. For the first time, a wealth of data and pictures of an active comet nucleus were sent back to Earth. Halley’s Comet has a highly eccentric orbit and passes the Sun once every 76 years. On Earth, the comet lights up the night sky with its characteristic tail. However, we cannot see the comet’s nucleus. It is surrounded by a dense layer of gas and dust called the coma, which reflects the light from the Sun. With the beginning of the Space Age came the possibility of exploring the elusive nucleus. Giotto’s pictures revealed the surface of Halley’s Comet to be pitch black: With an albedo of just 4%, comet nuclei are darker than the blackest tarmac on Earth, which has 7% reflecting power.
From dirty snowball to icy dirt ball
This was a big surprise for researchers of the 20th century. Spectroscopic analyses around the middle of the 20th century had led to the assumption that the radicals and ions found in the coma emerged from gas molecules such as water vapour, carbon monoxide and methane. These volatile gases were assumed to be frozen. Together with enclosed silicates, this ice was thought make up a comet nucleus ranging between 10 and 100 km in diameter. In 1950, American astronomer Fred Whipple devised the first model of an “icy conglomerate” (his equivalent for a nucleus) in order to explain the slight changes in some comets’ orbit that were apparently not gravity-related.
Giotto’s findings improved this model: Because ice reflects sunlight so well, it is not possible for even the dirtiest snowball to appear as black as Halley’s Comet does. Giotto’s pictures also showed that the ice does not vaporise evenly across the comet’s surface, but rather that large, singular “jets” of gas and dust particles burst out irregularly from that part of the surface that is warmed by the Sun’s rays. Both of these observations led scientists to conclude that the proportion of dust and silicates is higher than originally thought. It is therefore more appropriate to speak of icy dirt balls when describing a comet nucleus.
Artificial comets – the beginnings
The Giotto mission answered some very important questions, but it also raised new ones. What causes the jets? What are the physical characteristics of the comet’s surface? Within a year of the Giotto mission, an international team of scientists was researching answers to these questions. In the space simulation chamber in the Institute for Aerospace Simulation at the DLR in Cologne, scientists carried out research into artificial comets. They carried out a total of 14 different experiments – known as the KOSI experiments (KOmetenSImulation – comet simulation) – between 1987 and 1993 that garnered worldwide renown. The Giotto mission had shown that the comet’s surface was not made of ice dirtied by a little dust. Instead, it was more likely to be a large layer of silicates that also contained ice. However, for a coma to appear large quantities of ice were needed. This ice could be embedded just underneath the surface. Where the Sun heats the surface and the ice lying underneath, pressure will build beneath the surface due to the vaporising ice. Eventually, the pressure will be high enough for the gas to break through the surface above it and be released into the extremely high vacuum of space. During this process, minute silicate particles (dust) in the surface are also propelled into space with the gas. The jets that were observed in the Giotto mission are exactly this phenomenon.
The KOSI experiments examined the jets in some detail. For the first time, their results enabled researchers to describe the interactions between gas and dust with the help of a physical model. A comet that is passing close to the Sun loses up to 40 tonnes of its mass per second due to these jets. If a comet orbits the Sun often enough, most of the volatile material will have been vaporised and the comet becomes a small, dark, inactive rock and will never again sport a fascinating tail. Scientists assume that some known asteroids with highly eccentric orbits around the Sun are in fact inert comets.
In the course of the KOSI experiments, researchers found that the originally loose artificial comet material hardens after being exposed to conditions that resemble those in space. This process was examined in detail, the surprising result being that comet material apparently sinters. These findings were invaluable for the planning and design of the Rosetta mission.
The Rosetta mission – the first landing on a comet
The 2nd of March 2004 saw the Rosetta probe begin its long journey from the European spaceport in Kourou, French Guiana to the comet Churyumov-Gerasimenko. With it travels Philae, a small robot lander that is attached to Rosetta. In May 2014, both will rendezvous with the comet while it is still in its inactive phase and enter into orbit around it. The orbiter, Rosetta, will draw an accurate map of the comet’s surface with which scientists will select an appropriate landing site for Philae. Philae will land on the comet in November 2014 and will be securely fastened to the surface. Ideally, Rosetta and Philae will spend 17 months collecting physical and chemical data as well as photographic images and sending these to Earth. Scientists are expecting a whole range of new findings within the field of comet research, particularly with regard to the development of our solar system 4.6 billion years ago.
A milestone within European space exploration
Never before has anyone tried to land on a comet nucleus. Rosetta and Philae are expected to be able to send vital information to comet researchers during the beginning of the comet’s active phase as well. In order to acquire the data here on Earth, a special control room was set up in the DLR’s Microgravity User Support Center (MUSC) in Cologne Porz. All communication with Philae is conducted from here. Until things get busy in 2014, the control room is being used for tests and controls. For example: When Rosetta flew past Earth in March 2005 – on what was the first of three Earth swing-bys on its way to the comet – data sent by Philae was collected. This was done to make sure everything in the robot was working as planned. Results from this ambitious and pioneering mission are eagerly awaited even today.
For more information and the chance to have a look around a virtual control room, go to http://www.dlr.de/dlr-rosetta/.