Bake a chicken casserole, and you’ll probably have leftovers. Comets are like that. They’re what’s left from the dawn of the solar system 4.6 billion years ago. Ejected by gravitational interactions with the giant planets most were kicked into the outer solar system and now inhabit the Oort Cloud, an enormous, roughly spherical shell 0.03 to 0.79 light years away centered on the sun. Literally in interstellar space! Astronomers estimate there are at least a trillion of them out there. Gravity from passing stars and even the tug of the Milky Way itself can nudge a comet from the Cloud and start it on a long journey toward the sun. Thousands of years later when it finally arrives in the inner solar system, heat from the sun vaporizes some of the comet’s ice to form a tail.
We hear a lot about asteroids which are primarily made of rock — at least within the main asteroid belt between Mars and Jupiter. Asteroids can be anywhere from fist-sized all the way up to Ceres, the largest asteroid, with a diameter of 588 miles (946 km). Comets are made of ice and typically range from a half-mile to 12 miles across, about the size of a rural town. They’re so small the tug of gravity on their surfaces is trivial. If you could get a running start and jump from a comet you’d sail right into space! Hale-Bopp, a bright comet that put on a banner performance in 1997, was one of the largest with a diameter of 37 miles (60 km), one of the reasons it was so bright and dynamic.
Astronomers often refer to the comet itself as the nucleus. It’s where the action starts. Water ice makes up about three-quarters of the nucleus along with additional ices like carbon dioxide (dry ice), carbon monoxide, methane, ammonia and formaldehyde, a smelly liquid familiar to anyone who’s dissected a frog in biology class. Unlike refrigerated ice cubes comet ice is porous and fluffy, one of the reasons comets are so fragile and often break up when passing near the sun or a large planet like Jupiter.
Comets are also rich in carbon compounds and silicates. On Earth, familiar silicates include quartz, olivine and feldspar. Scientists have detected silicate-rich minerals like olivine, entstatite and forsterite on comets.
The mineral-rich particles riddle the ice similar to the dirty, compacted ice-snow mix that builds up in your wheel wells during the winter. When heated by the sun in the ultra-low pressure of outer space the ice vaporizes rather than melts, releasing water molecules (and other gas molecules) along with tons of dust and an assortment of small, rocky fragments. These form a fuzzy, glowing temporary atmosphere around the nucleus called the coma. The dust is so tiny, about the size of the particles in cigarette smoke, that the pressure of sunlight pushes them away from the coma to form a tail.
As I described in an earlier blog, the solar wind — a stream of subatomic particles forever blowing from the surface of the blazing sun — entrains gases leaving the nucleus and stretches them into a long, narrow “gas” or ion tail pointing directly away from the sun. Ultraviolet (UV) light from the sun excites carbon monoxide gas in the ion tail to glow blue. Comet NEOWISE displays both types of tails. The brighter dust tail is easily visible with the naked but you’ll need 50mm or larger binoculars to spot the fainter ion tail. Sometimes the head of a comet appears green or green-blue. Here, UV light works its magic again but this time on carbon and cyanogen, a cyanide-related gas.
While a nucleus measures only a couple miles across, a coma is typically tens of thousands of miles in diameter — up a million. NEOWISE’s coma is far larger than our planet but comprised of next to nothing, just puffs of the finest dust. Comet tails can grow to enormous lengths. I don’t know the exact expanse of NEOWISE’s dust tail, but it’s well over 10 million miles.
Comets may be dirty and oddly toxic, but they’re far away and pose no harm. Even if Earth were to pass through a comet’s tail as happened with Halley’s Comet in 1910 the gases are so thin they’re not a danger. While comets and meteors share a similar appearance, the former are many millions of miles away and orbit the sun. Meteors are primarily dust and rock particles shed by comets that Earth slams into. When they strike the atmosphere the grit heats up and creates a glowing trail in the air 62 miles high called a meteor.
Scientists estimate that between 0.1 and 1 percent of a comet’s mass is lost each time it swings around the sun. After repeated passes comets can eventually fizzle out or become so small they’re liable to crumble apart. Nothing goes to waste in nature. Earth gains some 40,000 tons of material from falling comet and asteroid dust every year.
When you look at Comet NEOWISE the next few nights consider how so little can make so much.