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Comets: Visitors from Deep Space
Comets are like old friends. We can see them several times in our lives if we bother to look up, and we don't fear them, but throughout much of history they were taken as great portents of change that destroyed kingdoms, caused dramatic changes in political fortunes, caused thousands of deaths, though none of it directly. Interpretations by Astrologers, religious figures, and leaders largely seemed to be that comets were warnings of misfortune coming; that, as described above, kingdoms were going to fall to conquerors; people were going to die; “gods” were going to smite people, whether themselves or enemies. We now know that comets are just chucks of dust and rock that pass through the inner solar system with some regularity.
Comets are thought to be leftover material from the formation of the solar system. They are primarily composed of ices such as water, methane, ammonia, carbon dioxide, carbon monoxide, and chunks of rock and dust. On the scale of the solar system, the central part of a comet, called the nucleus, may be as small as a few mile across to as large as tens of miles across.
Their old title of dirty snowball, which was coined by astronomer Fred Whipple, has been modified to a snowy dirtball. Through recent observations and encounters with space probes it has been determined that they’re covered in dark dusty shells and only leak gases where there are holes to expose the subterranean ices. Most have an albedo (the ability to reflect light) of four percent, which is less than a chunk of coal, which has an albedo of seven percent.
With all those volatile ices, comets tend to outgas, forming a coma when they get to within 4 AU of the Sun (Astronomical Units are the Earth/Sun mean distance, about 93.5 million miles/150,000,000 km). The force of the Solar wind pushes the cometary tail away in a long curve, so that it points somewhat away from the sun. The amount of outgassing may not be trivial. Most active comets tend to have tenuous atmospheric comas tens of thousands of miles across and sometimes more than a million miles (1.6 million km) in diameter.
Nearly all comets are faint so that we cannot see them with the naked eye, but they are measurable with telescopes and sensitive instruments. Comets have a second tail that is straight and points directly away from the sun. This tail is called the ion tail. It is caused by ultraviolet radiation ionizing components of the coma, which then react with the charges of the Solar wind, fleeing directly away from the onslaught. Regular particulate tails, called the dust tail, is made of the dusty portions that get blown off the surface by the outgassing, generally form a more irregular curved tail.
The Oort Cloud
Well beyond the orbit of Pluto there are vast clouds of material which is the birthplace of comets, this far edge of our solar system is call the Oort cloud. The Oort Cloud’s outer surface defines the edge of the Sun’s gravitational influence. The icy material at the cloud’s edge is held so tenuously that passing stars can knock chunks out of orbit to disappear forever into deep space, or send them plummeting towards the inner solar system to zip around the Sun as a long period comet. They may crash into the Sun; or before they even get close, they could be redirected to the Guardians of the Outer Reaches that protect the inner system where we live… Jupiter, Saturn, Uranus, and Neptune – without those four protectors with their immense gravity wells, we’d be in the middle of a shooting gallery.
The Oort Cloud is about 0.8 light years in radius, stretching about one quarter of the way to Proxima Centauri, our nearest stellar neighbor. Some speculate it is even larger. It’s about how far we would have to travel to be in free space, virtually unaffected by any star’s gravity. Think in terms of 50,000 AU. To paraphrase Douglas Adams “You may think it’s a long walk down to the local drugstore, but that’s nothing compared to space”.
A Comet Crashes into Jupiter
You might recall or be aware of a news-making incident of our Jovian pal as he kept us safe. The comet Shoemaker-Levy 9, back in July 1994, was safely swept up by the big planet. It had been captured into Jupiter’s orbit in the early-70s, or maybe even the mid-60s, and held in a highly elliptical path that slowly deteriorated until it fell inside the Roche Limit and was torn apart by Jupiter’s gravity.
Astronomers Carolyn and Eugene M. Shoemaker, and David Levy, spotted it on March 24, 1993 and calculations showed that it would impact Jupiter soon. Many telescopes, including the Hubble Space Telescope, were focused upon it at the appointed time. The fragments were labelled Fragments A through W. The impacts of the comet fragments into the giant gas planet Jupiter were phenomenal. The first one went screaming through the atmosphere at Mach 233 or 134,000 miles per hour (216,000 km/h), threw up an impact plume 1,900 miles (3,000 km) high, and left a huge dark spot 3,700 miles (6,000 km) wide, or about half the width of the Earth. The impacts continued regularly over the next six days. The largest was from Fragment G, leaving a dark spot big enough to contain the entire Earth—“G” was estimated to be only 1.2 miles (2km) wide. The energy released by Fragment G was estimated to be 6,000,000,000,000 tons of TNT, or 600 times greater that the entire nuclear arsenal capability of every atomic bomb on Earth. Two more of similar size followed over the next few days… Thank Jupiter we’re still here.
Depending on the nature of a comets orbit around the Sun they can be one-time visitors to the inner solar system or they can orbit the sun on a recurrent basis. Comets with orbital periods of the less than 200 years are called short-period comets. Probably the best know short-period comet is Halley’s comet which drops by about every 75-76 years to put on a show and then vanishes for another three quarters of a century. The last time it passed by the Earth was in 1986. Due to the position of the comet, the Earth, and the sun the 1986 apparition of the comet was less than spectacular.
Humans have been keeping track of Halley since at least 240 BC. It has been well-recorded by the Chinese, Babylonian, and numerous medieval European Astronomers/Astrologers but none of them recognized it as the same comet appearing over and over again. In 1705, the English astronomer Edmond Halley finally figured out that this comet was returning to be visible on the earth every 76 years. Incidentally, Edmond Halley’s last name was known variously throughout his life as Hailey, Haley, Hayley, Halley, Hawley, and Hawly.
Our Neighbor Mars is getting a Visitor
Mars will be in close association with comet C/2013 A1 (Siding Spring) on October 19th, 2014. It will pass close enough that they’ll bump atmospheres, which will give MOM and MAVEN (two Mars atmospheric probes that just arrived in Mars’ orbit) a huge treat in terms of free science! The main tail of Siding Spring will swish within ten planetary diameters, tossing lots of gas molecules and dusty bits in the direction of our ruddy neighbor. MAVEN and MOM will have a field-day sorting through all the perturbations and learn more in a few hours than possibly over the entire course of their planned missions. What incredible luck. This encounter may be visible to amateur astronomers if you happen to have an eight inch (20cm) telescope or larger. The brightness of comets is highly variable and they can suddenly brighten or dim in a matter of a few hours.
Space Documentary - National Geographic: Comet Mysteries
There is a potential danger to the Earth from comets
We actually should be wary of comets and asteroids. Not for their perceived magical effects, but for the possibility that they might actually hit our planet. They have come before and they’ll come again. There is no doubt. Sure, it’s a long-shot – but remember that a 1.2 mile long comet chunk made a hole in Jupiter’s atmosphere big enough to swallow the Earth. If something like that were to strike the Earth, the consequences would be devastating. We are not at the point in our technological growth that we are able to prevent such a thing from happening. I hope that in the not to distant future, we will have technology in place to destroy or at least change the orbit of dangerous comet or asteroids so that it does not strike the Earth.