Comets have this strange duality whereby they first brought the building blocks of life to Earth some 3.9 billion years ago and subsequent cometary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. Indeed, we may owe our preeminence at the top of Earth's food chain to cometary collisions. A catastrophic cometary collision with the Earth is only likely to happen at several million year intervals on average, so we need not be overly concerned with a threat of this type. However, it is prudent to mount efforts to
discover and study these objects, to characterize their sizes, compositions and structures and to keep an eye upon their future trajectories.
As with asteroids, comets are both a potential threat and a potential resource for the colonization of the solar system in the twenty first century. Whereas asteroids are rich in the mineral raw materials required to build structures in space, the comets are rich resources for the water and carbon-based molecules necessary to sustain life. In addition, an abundant supply of cometary water ice can provide copious quantities of liquid hydrogen and oxygen, the two primary ingredients in rocket fuel. One day soon, comets may serve as watering holes for space travel and fueling stations for interplanetary spacecraft.
Why study comets?
In any active area of science, new results are always accompanied by new questions - and this is especially true for the study of comets.
1. Comets offer clues to the composition and processes that formed the giant planets in the outer solar system. For example, some of the particles from the Stardust mission were formed at very high temperatures near the sun and yet comets formed in the cold, outer regions of our solar system so there was apparently significant material flows
from the inner to the outer solar system during its formation process. By what process did this material transport
2. Comets may well have delivered to the early Earth much of the water and carbon-based molecules (organics) that allowed life to form. One of the goals of cometary science is to understand to what extent the materials in comets
(e.g., water ice, organics) are similar to those on the Earth's surface. How large is our debt to comets in terms of our own origins? Are we indeed all made of cometary stuff?
3. Infrequent comet collisions with Earth punctuated evolution and allowed only the most adaptable species to evolve further. We mammals may owe our dominant position in the animal kingdom to a comet that wiped out the then dominant large reptiles (including the dinosaurs) 65 million years ago. A belated thank you to that comet—but is there another comet with our name on it?
4. Recent observations of comets from ground-based telescopes and from space missions have refined our knowledge of the structures and compositions of these objects. In general terms, comets are now thought to be made up of early
outer solar system material that agglomerated into rather fragile matrices of jet-black, potato-shaped, icy dust-balls.
But comets have an extraordinary diversity with some (e.g., Tempel 1) having impact craters and showing evidence
for liquid water alteration of its minerals while other comets (e.g., Wild 2) showing no evidence for impact craters and
very little evidence for liquid water in its history. If comets formed in the same region of the outer solar system at
about the same time, why are they so different from one another?