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Comets: Visitors from Deep Space

Updated on June 16, 2021
dougwest1 profile image

I am a retired engineer and small business owner who has authored over 60 books on history and various topics.

This color photograph of the comet Kohoutek (C/1973 E1) was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona, at the Catalina observatory with a 35mm camera on January 11, 1974.
This color photograph of the comet Kohoutek (C/1973 E1) was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona, at the Catalina observatory with a 35mm camera on January 11, 1974. | Source


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 bright 1997 comet Hale-Bopp showing the blue ion tail and the curved white dust tail.
The bright 1997 comet Hale-Bopp showing the blue ion tail and the curved white dust tail. | Source

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”.

Drawing showing the relative size of the solar system when compared to the Kuiper Belt and the Oort Cloud.
Drawing showing the relative size of the solar system when compared to the Kuiper Belt and the Oort Cloud. | Source

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.

Ultraviolet image of Jupiter taken by the Wide Field Camera of the Hubble Space Telescope. The image shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9.
Ultraviolet image of Jupiter taken by the Wide Field Camera of the Hubble Space Telescope. The image shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9. | Source

Periodic Comets

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.

Christmas Comet of 2018

Every few years a comet becomes bright enough to be seen by the naked eye. In 2018, the periodic comet 46P/Wirtanen dubbed the “Christmas Comet” can be seen with the naked eye around Christmas time. The periodic 46P/Wirtanen was named after the astronomer Carl Wirtanen, who discover it in 1948. The comet orbits the sun once very 5.4 years, passing by the Earth about every eleven years. The comet in its 2018 orbit will be closest to the Earth on December 16, 2018. Don’t worry it isn’t going to hit the Earth, at its closest approach, the comet will still be around 30 times further from the Earth than the moon (7.1 million miles). The comet will appear as a faint white-grey patch, maybe green with a telescope, in the southeastern sky after dark in North America during the middle of December in the constellation of Taurus. You may be disappointed with the view unless you live in a place with a dark clear sky as may be rather faint and hard to find for the light polluted skies of the city.

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.

Video on Comets

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.


  • Caron, Christina. “Fuzzy, green ‘Christmas comet’ to pass by Earth” The Kansas City Star. December 16, 2018.
  • Pasachoff, Jay M. Astronomy: From the Earth to the Universe, Fourth Edition. Saunders College Publishing, 1993.
  • Wilkening, Laurel L. Comets. The University of Arizona Press. 1983.

© 2014 Doug West


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    • CatherineGiordano profile image

      Catherine Giordano 

      6 years ago from Orlando Florida

      I just saw "Interstellar." I imagine the comet being like the ice planet they landed on.

    • dougwest1 profile imageAUTHOR

      Doug West 

      6 years ago from Missouri


      It is amazing that we were able to land on a comet and get pictures and data. From the pictures I saw, that comet doesn't look like a place I want to visit anytime soon.

    • CatherineGiordano profile image

      Catherine Giordano 

      6 years ago from Orlando Florida

      You must be very excited about the Philae landing. I find all things astronomical fascinating.

    • m abdullah javed profile image

      muhammad abdullah javed 

      6 years ago

      Very interesting hub Doug West Sir. Thanks for sharing. With all the details, write up ends with a hope that the presence of comet might be a potential threat but the human intellect will surely have command over the invisible threat. I do agree, earth is for humans to live anything that is a threat, the nature ensures one or the other way of its annihilation. We were child when the news of a Skylab hitting the earth spread like a wild fire, but it didn't. We should hope and stay positive about the human success in ensuring precautionary measures. But one question arises for how long this will happen? And from which of the potentials threats? There are innumerable threats, beneath and above the earth and still above in the heavens. The geologists say we are living on a physical hell because of the lava flowing down under whereas the scientists say the universe will attain the state of Big Crunch and ultimately we will have a cold death. Death for human and an end to the universe is sure, apart from scientific advancement for the prevention we should run towards our merciful Creator to ensure success in both the worlds.


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