What is a Comet?

What is a Comet?

“…like a comet I was wonder'd at.” (William Shakespeare, Henry IV, III, i).

Comets have inspired awe and wonder for generations. Are they falling stars? Are they alien spacecraft exploring our solar system? Are they omens of disaster, as people once believed? It would be easy to see them as cosmic warning signals, or even rockets being fired at Earth from outer space! In spite of our advances in science, we still have a lot to learn from comets.

We think that comets formed about the same time as the giant planets of our solar system - Jupiter, Saturn, Uranus, and Neptune. That was about 4.6 billion years ago. Some scientists think that comets are made from the same clumps of dust and ice that erupted from our sun’s birth, creating the planets. Others think they contain grains of interstellar stuff that existed even before the birth of our solar system. Most do agree, however, that comets are roving time capsules, carrying primitive debris from long, long ago.

Do comets carry the “seeds of life”?

When scientists looked into the dust they captured from the comet “Wild 2” (Stardust mission, 2006), they found particles rich in organic matter. Could particles like these have served as the “the seeds of life” on Earth, billions of years ago?

Lindsay Keller, co-investigator for the Stardust mission, said that one of their first studies of Wild-2 samples "showed abundant hydrocarbons in many of the particles." This supports the theory that comets might have brought the these hydrocarbons, the “building blocks” of life to Earth. Of course, we still don’t know what would have “sparked” them into life, although scientists have their theories about that, too.

Cosmic dust

The comet’s coma and tail are continuously shedding dust into space. As Earth circles the sun, it plows into this dust. As the dust ignites in the Earth’s atmosphere, it creates dazzling meteor showers.

About a billion particles of comet dust fall to Earth every second, each as small as a particle of cigarette smoke. You’re probably inhaling comet dust as you read this. About 40,000 tons of “cosmic dust” fall on Earth every year. This includes dust from comets, as well as asteroids, interstellar clouds, and other sources.

Most of NASA's deep space missions - including Galileo, Cassini, and Ulysses - have carried, or are carrying now, detectors to monitor and trace the origin of dust they encounter. NASA has also used airplanes flying at high altitude to sample this cosmic dust.

What’s in a Comet?

Comets contain lots of stuff, including the following, discovered in other comets by spectrographic analysis:
Organic: C, C2, C3, CH, CN, CO, CO2, CS, HCN, CH3CN, HCO, H2CO
Inorganic: H, NH, NH2, O, OH, H2O, S, S2, NH3, NH4
Metals: Na, K, Ca, V, Mn, Fe, Co, Ni, Cu
Ions: C+, CH+, CO+, CO2+, N2+, O+, OH+, H2O+, H3O+, S+, S2+, H2S+, CS2+
Dust: silicates, organic compounds.

This is only a small fraction of the stuff that scientists expect to find in comet C-G.  Rosetta’s ROSINA, IAS, and COSIMA instruments (designed outside the U.S.) can detect a great variety of molecules, isotopes , and particles large enough to constitute a dust grain.  These instruments can directly detect molecules of up to 3,000 AMU (atomic mass units). Since a proton is one AMU, this adds up to a huge number of molecules! The goal of the Rosetta mission is to collect a complete inventory of these elements.

Rosetta's ALICE instrument (from the U.S.) and VIRTIS will also remotely detect some of these molecules, radicals, and ions in the comet C-G. ROSINA, IES, and others will directly detect them. This overlap of instruments will enable scientists to cross-check the data from one instrument against the others.