From astronomical observations and previous space missions, comet dust grains are known to contain various minerals and some organic compounds, but we do not currently know the detailed chemical makeup of these minerals and compounds.

The question has been asked whether comets might contain some sort of alien life form, and if so, could there be an associated hazard. The answer is that there is no reason to expect any form of life in comets, and furthermore there is direct evidence against any hypothetical idea that comets contain anything that is hazardous to the Earth.

From all the scientific studies by biologists and ecologists, in order for life to be active, it must be in an environment in which liquid water could exist. Generally, this means that the environmental temperatures must be above the freezing point of water, the temperature of zero degrees Celsius. Some rare, extremely hardy single-celled organisms, like certain bacteria, can be active at temperatures as low as 15 degrees C below this, but not lower. Temperatures in a comet nucleus are far colder, ranging from -50 to -250 deg C, and must have remained extremely cold since the time the comet was formed, some 4.6 billions of years ago. Life could have never started on such small, frigid and airless bodies. Even if a small bacterium somehow was put into space, and eventually got onto a comet, it could not survive the harsh environmental conditions of ultrahigh vacuum, ultraviolet exposure, and the pervasive radiation from cosmic rays and solar particle events.

Furthermore, it is important to know that about 40,000 tons of dust particles from comets and asteroids fall on Earth every year. A large fraction of this is cometary material, and a small but important fraction of this will not have been heated to even as high as Stardust particles will be when they impact into aerogel at a speed of 13,000 miles per hour (6 km/sec), which will heat each particle to hundreds of degrees for a short time. This constant bombardment of Earth by the particles shed from comets has been going on for the more than three billions of years that life has existed on this planet, yet life has flourished.

Every second, a billion comet particles of the size to be collected by Stardust fall naturally to Earth. More particles of comet dust fall on your front yard every year than the Stardust mission will return to Earth. From a scientific standpoint, it is unfortunate that we cannot sort out these particles from the trillions of natural soil particles. That is why we must fly a space mission to a comet, so that we can bring back just comet dust.

The total amount of cometary material to be returned by the Stardust mission is less than a few millionths of an ounce (micrograms). Scientists will have to use the most advanced instruments they have in their laboratories just to make microscopic pictures and mineral analyses of these tiny dust grains. Studying comet dust and stardust is an essential step for learning how the planets formed and evolved to eventually create the warm, wet conditions from which life itself waseventually able to come about.

The Stardust samples are collected by a method that results in spike-heating of the sample. The planetary protection issue has been thoroughly reviewed by a special Task Group for the National Research Council, which draws from the National Academy of Sciences, National Academy of Engineering and Institute of Medicine. They have recommended:

* - From Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies, National Academy of Sciences, NRC, 1998. ISBN 0-309-06136-9. (available on the Internet at http://www.nas.edu/ssb/sssbch8.htm#con)