Descent Imager/Spectral Radiometer (DISR)
The Huygens Probe
In 1990 ESA initiated plans to send a probe to the largest moon of Saturn, Titan. ESA's Huygens probe was carried to Titan by NASA's Cassini spacecraft. Cassini was launched on 15 October 1997 and entered orbit around Saturn on 1 July 2004. After completing three orbits of Saturn, the Huygens probe was released on 25 December 2004 and entered the atmosphere of Titan on 14 January 2005. The mission was a remarkable success, returning the first images from the surface of Titan.
Huygens is an atmospheric probe designed to study the composition and structure of an atmosphere which may be similar to that of early Earth. There are several major experiments onboard Huygens. The PIGs were involved in the DISR experiment - the main imaging system.
The Probe Itself
30 kg support equipment which stays on Cassini
318 kg Probe which enters atmosphere comprising
Front shield 79 kg
Back cover 16 kg
Inner structure 41 kg
Power supply system (inc. batteries) 45 kg
Command and data management system 23 kg
Instruments 46 kg
+ other items.
Start of building phase in January 1991.
Flight model delivered to JPL on May 1, 1997.
The DISR Instrument
DISR was led by M. G. Tomasko of the Lunar and Planetary Lab. of the University of Arizona. The experiment takes light from 3 apertures and uses fibre-optic cable to bring the light onto one CCD detector. Huygens spins slowly as it descends to the surface of Titan. Using the three viewing directions, combined with the rotation of the probe, DISR obtains a panorama of the full downward looking hemisphere. Colour information is obtained by using a spectrometer channel. Light is imaged onto a grism to produce a spectrum which is also taken by fibre-optic cable to the same CCD. Both upward looking and downward looking spectra can be acquired. In addition, there are four upward looking channels which view the solar aureole. This allows us to place constraints on the particle size distribution and its variation with height in the atmosphere. Two infrared channels (again upward and downward looking) provide further information on the composition of the aerosols and the surface. A photometer operating at near-UV wavelengths assists in determining the energy deposition in the atmosphere as a function of altitude.
The original DISR comprised researchers from
- the Lunar and Planetary Lab. of the University of Arizona
- the Max-Planck-Institut für Aeronomie (now MPS) in Germany
- the Observatoire de Paris in Meudon, France, and
- the Technical University of Braunschweig, Germany.
The PIG Involvement
The PIG involvement in this experiment is small. We were involved in defining the position of the images on the CCD (which turns out not to be trivial because the CCD is a rapid frame transfer device and it is important to minimize cross-talk between the different channels). We also worked on the data analysis software. We are now using the laboratory goniometer experiment to study the reflecting properties of materials which might form analogues for Titan surface materials. We are also studying the rock/ice block distribution on the surface. The data can be seen at the University of Arizona web site and press releases with respect to the mission can be seen at www.esa.int.
Despite not being strongly involved in DISR, we were called by the Swiss Space Office and asked to re-activate our participation in DISR. Why? Well, it is very simple. It turned out that although Switzerland was a significant contributor to ESA and the Huygens probe, Nick Thomas was the only person in Switzerland involved in the Cassini-Huygens mission. This was difficult for the Swiss Space Office because two big industries (Contraves and APCO) contributed to the structure and separation mechanism of the Huygens probe and without them .... it wouldn't have worked!
Tomasko, M.G., Doose, L.R., Smith, P.H., West, R.A., Soderblom, L.A., Combes, M., Bézard, B., Coustenis, A., deBergh, C., Lellouch, E., Rosenqvist, J., Saint-Pé, O., Schmitt, B., Keller, H.U., Thomas, N., and Gliem, F. (1997) The Descent Imager/Spectral Radiometer (DISR) instrument aboard Huygens. In ESA SP-1177 The Huygens Probe pp. 109-137.
Tomasko, M.G., B.Archinal, T.Becker, B.Bezard, M.Bushroe, M.Combes, D.Cook, A.Coustenis, C.de Bergh, L.E.Dafoe, L.Doose, S.Doute, A.Eibl, S.Engel, F.Gliem, B.Grieger, K.Holso, E.Howington-Kraus, E.Karkoschka, H.U.Keller, R.Kirk, R.Kramm, M.Küppers, P.Lanagan, E.Lellouch, M.Lemmon, J.Lunine, E.McFarlane, J.Moores, G.M.Prout, B.Rizk, M.Rosiek, P.Rueffer, S.E.Schröder, B.Schmitt, C.See, P.Smith, L.Soderblom, N.Thomas, and R.West, (2005) Rain, winds and haze during the Huygens probe's descent to Titan's surface, Nature, 438, 765-778.