As a member of the OSIRIS Science Team I am happy to announce that our camera OSIRIS, that flies on ESA’s spacecraft Rosetta, now has imaged the target of its ten year long journey – Comet 67P/Churyumov-Gerasimenko!
Rosetta was launched in March 2004. The purpose of the spacecraft is to explore, in situ, what happens to a comet nucleus when it approaches the Sun from a very large distance, gradually is heated and therefore becomes active. Therefore, Rosetta first had to get very far out in the Solar System. The spacecraft swung by Earth three times, and Mars on one occasion, so that the gravitational perturbations from these planets gradually could make Rosetta’s orbit around the Sun wider. On its way, the spacecraft also passed near to two asteroids – (2867) Steins in September 2008 and (21) Lutetia in July 2010. I June the following year, Rosetta had come so far from the Sun that its solar panels no longer managed to generate the electric power necessary to keep the entire spacecraft up and running. Therefore, Rosetta was put in hibernation and all available power was used to heat the instruments to prevent them from break by freezing. The ground control had no contact with Rosetta at all.
In October 2012 Rosetta was farthest from the Sun, no less than 5.3 AU (one astronomical unit, 1 AU, is the mean distance between Sun and Earth, and corresponds to 150 million kilometers). It means that Rosetta was beyond the orbit of Jupiter, that is located 5.2 AU from the Sun. Two and a half years after Rosetta entered hibernation, on January 20, 2014 to be precise, it was time for the spacecraft to wake up. It was an enormous relief when the signals from Rosetta reached the ground control! After the wake-up, careful checks were made to make sure Rosetta was feeling well after its long sleep. We are now at a stage where the scientific instruments are switched on one by one, to see how they have coped with the hibernation. OSIRIS was switched on last week, and has now taken its first images of the comet – the camera works beautifully! We will therefore be ready when Rosetta reaches the comet in August this year, at a distance of about 4.5 AU from the Sun.
OSIRIS is the camera system on Rosetta. It actually consists of two different telescopes. One of them is called the Wide Angle Camera (WAC) and has a rather large field of view since it will be used to image the comet coma, the cloud of gas and dust that the comet nucleus surrounds itself with (see a previous post on comets). The camera has 14 different filters – glass plates with a special composition and surface coating that makes them transparent to light only at specific wavelengths. These filters are manufactured in Sweden and is the Swedish hardware contribution to OSIRIS. Seven of these filters are so-called narrowband filters – they are transparent only at very strict wavelength regions corresponding to the wavelength were seven different molecular fragments (radicals) emit light when they are illuminated by the Sun. These radicals are CS (a compound consisting of carbon and sulphur), the hydroxyle radical OH and the oxygen atom O (these are formed when the ultraviolet light of the Sun break down water molecules), NH and NH2 (compounds of nitrogen and hydrogen), CN (the cyano radical, consisting of carbon and nitrogen), and the sodium atom (Na), that can be outgassed by dust grains that are strongly heated by sunlight.
The dust grains in the comet coma will reflect sunlight, and some of this light will find its way through the narrowband filters. This is not good, since we will use the intensity of the light to calculate the abundances of radicals and atoms in the coma. Since the dust grains contribute with light, that does not originate from within the gas at all, the risk is that we overestimate the abundance of gas. Therefore, the WAC also has seven filters that is transparent to light just next to the wavelength regions of the narrowband filters. In this way, the contribution of the dust grains to the measured light can be estimated, and compensated for when determining the gas abundance. Four of these filters are transparent in the ultraviolet wavelength region (for example, a filter called UV375), while the others are located in the green, yellow and red wavelength regions.
The image above is really three different WAC images, taken through different filters. The red filter was used during an exposure that lasted one minute. The green filter was used during an equally long exposure. Finally, the UV375 filter was used three times with a total exposure time of nine minutes. By combining these images, the color photo above could be constructed.
The second camera is called the Narrow Angle Camera (NAC). It has a smaller field of view than the WAC, but is capable of resolving objects that are five times smaller than the ones the WAC manages to resolve. This camera will primarily be used to study the comet nucleus. This camera also has Swedish filters, but with quite different properties – a mixture of broadband filters in different parts of the visible wavelength region to make a rough characterization of the comet spectrum, and a number of filters that will be used to search for specific minerals, like pyroxene, hematite and hydrated silicates.
The figure below shows a picture taken with the NAC, and corresponds to the white square in the picture above. The strongly magnified picture shows a globular cluster called Messier 107 (or M107), as well as the comet nucleus within the small circle. It is still far too distant to be seen in detail, and is only a dot in the sky. But day by day Rosetta is closing in on the comet and soon we will be able to see how it looks like up close!