The power system for the Huygens probe was selected on the basis of the mission requirements and to fit the probe's mass, cost, telecommunications and other constraints. A battery was determined to be the right power source to meet the mission requirements. The scientific objectives of the Huygens probe mission are to characterize the chemical reactions occurring in the Titan atmosphere, find the source of abundant methane in the atmosphere, determine if there are seas or lakes on the surface, and determine if more complex organic compounds and pre-biotic molecules exist on Titan. The probe was designed and instruments selected to answer these high-priority questions. The battery is designed to provide 3 hours of electrical power - more than adequate to meet the probe's electrical needs in the 2 and 1/2 hour descent. In that time, the probe should return a great wealth of observations and measurements that answer the science objectives and leave us with new questions that could be addressed by future missions. For longer-lived landed or orbital missions to Titan in the future, radioisotope power sources, and/or a space nuclear reactor under study by NASA's Prometheus program, would be among the power sources considered.

The Huygens Probe will give us our first close-up look at Saturn's largest moon, Titan. It will parachute down through Titan's hazy atmosphere for about 2.5 hours, taking images and making copious measurements of the atmosphere and surface as it drops. If it survives the fall, Huygens will continue to operate on the surface for up to an hour, analyzing the surface material until its batteries run out of power. We hope it will reveal whether Titan has lakes or oceans of liquid hydrocarbons, and other details of the moon's mysterious landscape.

The Committee on Space Research (COSPAR), an interdisciplinary committee of the International Council for Science, maintains a consensus international planetary protection policy to be followed by spacefaring nations. In 1988, the U.S. National Research Council's Space Studies Board, working under COSPAR guidelines, determined that the Cassini-Huygens mission fit into "Category II" as follows:

"Category II missions comprise all types of missions to those target bodies where there is significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration. The requirements are for simple documentation only. Preparation of a short planetary protection plan is required for these flight projects primarily to outline intended or potential impact targets, brief pre? and post-launch analyses detailing impact strategies, and a post-encounter and End-of-Mission Report which will provide the location of impact if such an event occurs. Solar system bodies considered to be classified as Category II are listed in the Appendix to this document." For original documents, see http://www.cosparhq.org/scistr/PPPPolicy.htm.

As a Category II mission, the Huygens Probe was not sterilized. For more information on this, see this European Space Agency web page on planetary protection: http://www.esa.int/export/esaCP/ESAUB676K3D_Life_2.html

For information on NASA's planetary protection policies and procedures, see http://planetaryprotection.nasa.gov/pp/index.htm.

For descriptions of the instruments aboard the Huygens Probe, see http://saturn.jpl.nasa.gov/spacecraft/instruments-huygens.cfm. You'll also find links to web pages maintained by some of the science teams responsible for the instruments, and to the European Space Agency's Huygens web site.

In addition, here are a couple of additional sites (not sponsored by NASA or ESA) where you can get some background on the Titan investigations:

http://www.es.ucl.ac.uk/research/planetaryweb/undergraduate/dom/weathering_titan/chap8.htm

http://www.universetoday.com/am/publish/simulating_titan_l_ab.html

If you are unable to find the material you require, write to: education@esa.int