Ten Things You Don't Know About Comets

By Phil Plait | April 20, 2010 6:00 am




Comets spawn meteor showers


So we have millions of tons of material blowing off a comet every time it passes the Sun. What happens to that junk?

There’s very little force acting on this material, so it continues to orbit the Sun in very much the same orbit as the comet itself. That stuff can happily circle (ellipse? Ellipticate?) the Sun for hundreds of millennia, but not always. Sometimes the comet’s orbit intersects the Earth (more on that not-quite-so-happy circumstance in a moment), which means the Earth can plow through this debris. Normally, the Earth hits about 100 tons of space detritus a day, but that number can jump substantially when it rams a comet’s orbit. This material burns up in our atmosphere, and we get a meteor shower.

That’s right: when you go out and observe the Perseids, the Leonids, the Geminids, the Taurids, you are seeing the dandruff from a comet (which is an appropriate metaphor, since coma means "hair" in Latin). Even a snowflake can put on quite a show when it slams into our atmosphere at 100 km/sec. That kinetic energy is converted into heat and light, and is visible as a shooting star for hundreds of kilometers. Sometimes you get a small chunk of gravel, too, and that becomes an intense fireball, bright enough to leave an afterimage on your eye (I’ve seen a few, and they’re heart-stopping).

This also means that meteor showers are associated with specific comets. Halley’s has two: the Orionids in October and the Eta Aquarids in May. The August Perseid shower is due to litter from the comet Swift-Tuttle, and the November Leonids are from the comet Tempel-Tuttle. They happen at the same time every year because that’s when the Earth is at the intersection of the two orbits. The spectacular image above is from the 2001 Leonids, seen in Australia by Jen and Vic Winters of ICStars.

I’ll add the orbits of the debris do get pushed around a bit. Solar wind, light pressure from the Sun, gravity of the planets: these can affect the debris, changing when and where the Earth hits them, so over time the meteor showers evolve. Also, if the parent comet made a recent passage, the debris cloud can be thicker, and when the Earth rams into it we get a meteor storm. The Leonids of the late 1990s produced many such intense storms, with rates as high as thousands of meteors per hour!



Comments (82)

  1. Luis

    The link is broken… :)

  2. Clive DuPort

    I just got an ad asking me if I have balls. Yes, thanks.

  3. Jeff

    One obvious one people don’t know, is that the visible comet they see is mostly just gas and dust. The “comet” itself (aka nucleus) is a city-sized ball of dirty ice which sublimes all this debris off on its periodic (or non periodic) rendevezvous with the solar neighborhood, and then returns back to its popsickle state when it moves outward.

    Their orbits are either slightly below zero total energy (bound orbits) or slightly above zero total energy (unbound trajectories), and this speaks volumes to their distant origins.

    And many people mix up meteors with comets, and don’t realize comets aren’t “shooting stars”.

  4. some readers will know some of these, and some will know all ten

    but the hard core will see if they can find an error in each one…

  5. Looking forward to follow the link once it is up and running :-). Ready to get some astonishing comet facts!

  6. I’m just getting a blank page. Does that mean I already know everything about comets?

  7. I’d say there were about 7 or 8 that I already knew. The others I kind of knew.

    The embiggenated pictures were great! I especially liked the meteor shower and the SOHO galleries (the movie of comet Hyakutake passing the sun was pure awesome!)

    For those who can’t get into the presentation, you can navigate through the pages at the bottom. Just click on page 2 and you’re set.

  8. Chip

    I didn’t know comets were sponsered by Zales Diamonds. Interesting.
    Just random teasing…Its ok Phil

  9. Petzphur

    Comets are like sex, you never forget your first time.

  10. u got it, justin… smart fella..

  11. MoonShark

    The link for “Enter 10 Things You Don’t Know About Comets” is broken from the BA homepage (404 error). If I open the page just for this post (where you are now with comments at the bottom), the same link says “Sorry, no posts matched your criteria”.

    The only way I could enter is by coming here (comments page) and clicking the #2 for the second page.

  12. Chris

    I didn’t know comets had a $499 special to Cabo! Fix the link Phil :-)

  13. mike burkhart

    I’ve only seen a few comets .Yes many people confuse comets,meteors ,and asteriods,they don’t seem to know the difrence .I think maybe school science textbooks focus little on astronomy and only tend to focus on the planets and not on the minor members of our solar system .One more thing (I’m sure Phill knows about this)a Pope in the middle ages did once declare Haleys comet an evil sprit in defence of the Church I would say in the middle ages no one knew what comets were .Since then the science of astronomy has found out what comets are and no Pope in recent times has declared comets to evil sprits .



  15. Whoops…retracted.

  16. That was weird. I have no idea why the links failed, but they work now. Hmmm. I’ll look into it to make sure that doesn’t happen with a future post.

  17. Chris

    The blue color of this ion tail is due to carbon monoxide, which scatters blue light toward us (similar to why the sky is blue).
    I have to disagree with you here. First off, since it’s an ion tail, it can’t be CO which is neutral, wouldn’t it be CO+? Also I don’t think the density is nearly enough to be enough for scattering. I think this is due to the emission spectra of the excited ions. (arXiv:astro-ph/0003122v1)

  18. Chris T.

    Model comet looks too much like a chocolate rice-crispie treat for my liking.

  19. David

    19: Are you sure it doesn’t look like a hot fudge sundae?
    (Please, somebody get the reference…)

  20. Na

    How long would it take for Comet Halley to entirely disappear?

  21. Chris (18): I thought it was fluorescence as well, but found a comet person who said scattering. I’m looking into this more…

  22. OK, found a spectrum, and it does look like line emission. I’ll have to send a note to the comet guy I found… but at least I was right in the first place! :)

  23. Old Rockin' Dave

    I can confirm the fears that the 1910 pass of Comet Halley inspired. My grandmother was twelve years old at the time and she remembered it quite clearly. She said that people sat on the rooftops of Brooklyn to see it, expecting to be gassed at any moment. They didn’t know about cyanogen, but thought it would be like the cooking gas they used in those days.
    That fear inspired two famous writers, as well. H. G. Wells wrote ” In the Days of the Comet,” (1906) and Sir Arthur Conan Doyle wrote a Professor Challenger story, “The Poison Belt” (1913).

  24. TheBlackCat

    Great article!

    Phil: I have heard that rather than “dirty snowball”, a better description of comets would be “frozen mud” (due both to the debris/ice ratio and the presence of organic molecules). What do you think of this?

    Soho is great. I have a soho widget on my desktop that gives me the latest images. I had been using the EIT304 extreme ultaviolet image (more because of the colors than anything), but I just switched to LASCO 2.

  25. Adam English

    Wait, in 1910 they were able to learn about comets and analyze them with technology?

  26. 11) Nobody ever puts the umlaut over the “m.”

  27. Trebuchet

    Off topic, but Phil, while you’re fixing broken links you need to take a look at what’s happening with http://www.badastronomy.com, which is returning a blank page for several days now. This is the link that is used from the forum banner, so you can’t get here from there.

  28. Douglas Troy

    Cool, I learned new stuff today. Thanks Phil.

  29. MoonShark

    Man, you got my hopes up that I’d know ’em all with some of the softball facts at the start. Stuff I’d known since I was a kid. But those last four gave me some new brain-wrinkles, so that’s good :)

  30. Ken (a different Ken)

    @20 David: Served up with a chocolate-coated manhole cover?

    [Edit: This may not be the exact reference – all his stuff is kinda blurring together at the moment.]

  31. Old Rockin' Dave

    Adam (#26) asks: “Wait, in 1910 they were able to learn about comets and analyze them with technology?”
    Yes, they had had spectrometers for more than half a century by then.

  32. This was an awesome read! Much better than working. Thanks, Phil!

  33. Plutonium being from Pluto

    Good write up there – even though I knew all ten & have seen most of those photos here before. :-)

    Additional fact or three (plus a few extra bits of info.) for y’all – from a powerpoint talk I gave recently on astronomical anniversaries :

    1. This year marks the 100th anniversary of the Great Comet of 1910. This Great January Comet or Great Daylight Comet of 1910 :

    * was the brightest comet of the 20th century.

    * was independently found by so many people in the southern hemisphere that no single original discoverer could be named, though the first astronomer to see it appears to have been Robert Innes (who also discovered Proxima Centauri) on January 17, 1910, at the Cape Observatory in South Africa.

    * Most observers judged the comet at maximum to be brighter than Venus, giving it a magnitude of about -5. It was much more spectacular and impressive than the apparition of Halley’s comet which took place later the same year & folks often confuse the two.

    2. This is the 33oth anniversary of the very first comet ever discovered through a telescope which is variously known as Kirch’s Comet, Newton’s Comet or the Great Comet of 1680.

    * Discovered by Gottfried Kirch on 14 November 1680, it became one of the brightest comets of the 17th century.

    * It was reputedly visible even in daytime & was noted for its spectacularly long tail. Passing only 0.4 AUs from Earth on 30 November, it sped around an incredibly close perihelion of .006 AU (898,000 km) on 18 December 1680.

    * While discovered & named for Gottfried Kirch, credit must also be given to the Jesuit, Eusebio Kino, who charted the comet’s course.. Kino’s Exposisión astronómica is among one of the earliest scientific treatises published by a European in the New World – in Mexico.

    * Although Kirch’s was an undeniably a sungrazing comet, it was probably not part of the Kreutz family.

    * Aside from its brilliance, it is probably most noted for being used by Isaac Newton to test and verify Kepler’s laws.

    & the last cometary anniversary (well that I mentioned) was

    3. 2010 is also the 40th anniversary of Comet Bennett, (C/1969 Y1) passing closest to Earth on March 26, 1970.

    * At that time this comet’s average brightness was around 0, while estimates of the tail length ranged from 5 to 10 degrees. It was a spectacular sight for some months & one of two brilliant comets to grace the 1970s, along with Comet West.

    See : http://www.cometography.com/lcomets/1969y1.html
    & http://www.daviddarling.info/encyclopedia/G/Great_Comets.html

    ..&, of course, Wikipedia has pages on all three comets too as do many comet and general astronomy books.

    Hope y’all found this interesting. :-)

  34. Steeev

    David #20

    Which falls on a Tuesdae this month!

    The whole time I was reading the article, I was thinking of how well these facts were addressed in “Lucifer’s Hammer” — except for SOHO and the robotic visits, of course.

    Who doesn’t love a good book about Death from the Skies?

  35. The Other Ian

    11) Nobody ever puts the umlaut over the “m.”

    What? Why on earth would there be an umlaut over the em?

  36. JohnW

    35. Steeev Says:
    April 20th, 2010 at 11:14 am
    David #20
    Which falls on a Tuesdae this month!

    Darnit! Beaten to the punch.

  37. Torbjörn Larsson, OM

    a lot of organic (carbon-based) compounds have been detected in comets.

    Spectroscopically perhaps, but IIRC when Stardust brought back samples they were scarce on them. Besides, Earth itself had lot of carbon from the start.

    it’s also possible that a significant fraction of the water in our oceans came from ancient comet impacts as well!

    If you by maor fraction mean something like 10 %. [Lectures in Astrobiology: Vol I.] IIRC D/H ratios preclude more.

    OTOH there was a recent paper that confirmed that most water must be from impactors, not original nebula material by looking at mantle isotope ratios. [Um, it’s googeable; I’m late.]

    So impacts (late veneer), not comets so much, likely outer main belt asteroids.

  38. himtha

    thank you yet again for another great post! You ROCK my world! getit?

  39. Tommy D

    Talking about David Lavy… I have his signature on my scope, with an happy smiley comet!

  40. Reverend J

    Comets are awesome, slide shows, not so much…

  41. Hey I knew some of those things already! I demand a partial refund!

  42. Bill Nettles

    You could put this in with “two tails” or have an 11th point: The tail can be in front of the comet.

    The tail direction is determined by the solar wind. If the comet is moving away from the sun, the tail will be pushed out in front of the path. Also, near the sun, the tail will be sideways compared to the path of the comet.

  43. jcm

    “Every time a comet comes near the Sun, it dies a little bit.”

    And, occasionally crashing into the sun.

  44. Jon Hanford

    #43 Bill Nettles,

    I think you’re referring to antitails seen with some comets. The antitail appearance is due to our viewing geometry from the Earth instead of being a tail “pushed out in front of the path”. Check out the relevant wiki page on antitails: http://en.wikipedia.org/wiki/Antitail

  45. Mike Matessa

    The picture of Tempel 1 reminded me of Emily Lakdawalla’s collection of asteroids and comets shown to scale that were visited by spacecraft: http://www.planetary.org/image/asteroids_comets_sc_0-000-075.png

  46. tudza

    Hmm, I do believe I knew all these points. Minus points for not knowing the name of SOHO, just that such a system existed.

  47. Gary B

    I was inspired by Phil’s description of the density of cometary comas to post on Reddit a proposal for a new unit of measurement for gas pressure (vacuum), to be part of the Library of Congress system. Here’s the link: http://www.reddit.com/r/funny/comments/btu52/a_new_measurement_unit_for_vacuum_gas_pressure_in/

    I proposed the “Politician’s Promise” (denoted PP) as 1nPa, which is less than most laboratory vacuums, and more than the density of interplanetary space. Of course, PP is divisive, not additive, so 2PP = 1nPa * 1nPa = 10^-12Pa.


  48. ethanol

    I strongly suggest Carl Sagan’s “Comet”, which is still pretty current and in addition to some great writing about the science and the potential of comets, also has some great science history. In particular the history of Edmond Haley, who kicked a surprising amount of ass. It turns out that his predictions of the return of his eponymous comet was one of his less significant discoveries.

  49. MadScientist

    I missed out on what was probably the most spectacular comet in my lifetime because I was in the southern hemisphere at the time. Bah. **sniff** Everyone keeps telling me how awesome the comet Hyakutake was.

  50. MadScientist

    Sorry – wrong comet. Hale-Bopp. But for me Hyakutake was a mere amorphous blob as well.

  51. @10 wrote:

    Comets are like sex, you never forget your first time.

    Or, a really great one! I guess that I have seen all the really good ones since the 90s, and even though 73/p wasn’t as spectacular as some of the brighter ones, it is the one that brings back the fondest memories. I sat outside for a couple of hours every morning… for a couple of months, in the spring of ’06, writing all of the details that I could discern in my journal. I guess it was the multiple fragments that intrigued me???

    But, I didn’t keep a journal of Holmes. Anyone know what the cluster is in the first photo?

  52. Mike J.

    Phil, you forgot the parts where comets conceal alien spacecraft and / or reanimate the dead. No biggie, but The More You Know :p

  53. Grr, I actually knew all ten facts. I don’t want a prize, just a good comet to view.

  54. Jeffersonian

    I’m trying to figure out when a typical comet starts outgassing from the sun. I mean, it wouldn’t be until it was well within our solar system, right? It is linear: it just increases in strength? And aren’t some captured by Jupiter and/or Saturn?

  55. Sauss

    Yep, my great grandmother saw Halley twice (second time with me thu a telescope in the crater of a hill). She said, even here in NZ, the first time people saw it, many hid in their cellars for days, dreading the plague/pestilence/undefined bad thing it might bring.

  56. Nate

    Mark Twain was born during a pass of Halley’s Comet and accurately predicted he would die during the next one.

    From his wiki:

    In 1909, Twain is quoted as saying:

    I came in with Halley’s Comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don’t go out with Halley’s Comet. The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’

    His prediction was accurate – Twain died of a heart attack on April 21, 1910, in Redding, Connecticut, one day after the comet’s closest approach to Earth.

  57. Fitzsy

    Actually, very active comets like Hale-Bopp have a third tail composed of neutral sodium and potassium, just like the Moon and Mercury – see the images at

  58. Chris F.

    Grrr… its done as a psuedo slide show. I hate slide shows. They’re usually used just to get people to load more advertisements to up the site’s revenue, and even when they’re not being used for that purpose (like here), I still find them very grating.

    Please, just do a continuous list next time. Pretty please?

  59. Paul

    What causes the line that extend from the sun the the lower left of the SOHO images? I enjoy checking SOHO’s site quite often and have always wondered.

  60. mln84

    Paul (61)- I believe that is the arm holding the disk that blocks out the sun. The disk is necessary or the image would be overwhelmed with direct light and you wouldn’t see the surrounding details.

  61. mln84

    On Slide #8,point 2, wouldn’t Hale-Bopp have thousands of times the mass of Toutatis, as mass would vary with radius-cubed?

  62. manaen

    Here’s my list:
    1. Ford produced Comets 1960-1977
    2. It was planned as a model in the Edsel line
    3. With the demise of the Edsel, it was sold in 1960 and 1961 as neither a Ford nor a Mercury but as an unbranded vehicle
    4. It was the high-end sibling of the better-known Ford Falcon 1962-65
    5. Ford bought the “Comet” name from Comet Coach Company, which had used the name — in possible anticipation of Hale-Bopp/Heaven’s Gate — for their line of funeral coaches, mostly Oldsmobiles (really; even I wouldn’t write a pun this bad)
    6. In 1966, the Comet named was upgraded to sibling of the Ford Fairlane
    7. In 1971, the Comet appeared as Mercury’s version of the Ford Maverick
    8. A Comet station wagon with wood trim was named the Villager, a name used earlier by an Edsel station wagon and later by Mercury’s mini-van from Ford’s joint venture with Nissan — a vehicle notable for using Nissan engines, after Henry II’s comment that he’d never source engines from another company
    9. In 1964, Ford produced 50 Comet Cyclones with 427 c.i. engines that did well on the racing circuit
    10. Comets were bumped from Mercury’s lineup in 1978 by the Zephyr
    (Wikipedia contributed much of the above information)

  63. Bill Nettles

    45. Jon Hanford

    Don’t be so literal. “Out in front” means a component of the radial vector along the tail is parallel to the velocity, not that the tail itself is perfectly lined up with the velocity vector. That said, the wiki article you pointed me to perfectly illustrates what I was talking about. Those diagrams show both the dust and ion trails are ahead of the nucleus if you split the path into 2 portions, “ahead” and “behind.”

  64. Mike Morris

    No one expected comet Hyakutake to be a powerful source of x-rays. “Astronomers using ROSAT (the European Space Agency’s Roentgen satellite) decided to look at Hyakutake and they were shocked by what they saw. ROSAT images revealed a crescent-shaped region of x-ray emission around the comet 1,000 times more intense than anyone had predicted.” Dr. Michael J. Mumma wrote, “We had no clear expectation that comets [would] shine in X-rays.” Some astronomers wondered why they would bother pointing an x-ray telescope at a comet. The x-rays were as intense as those ROSAT usually picks up from bright x-ray stars and they flickered like a fluorescent-tube on a time scale of hours. Flickering effects in plasma discharges are normal because of the non-linearity in its current carrying ability. Meanwhile another ad hoc proposal had to be dreamt up to explain the x-rays. So the Sun was made entirely responsible for the x-rays by suggesting that highly ionized atoms from the solar wind were scavenging electrons from the cometary atmosphere and the energy available from that recombination was sufficient to generate the observed x-rays. But that constitutes an electric current into the comet which is unsustainable if a comet is supposed to be electrically neutral. – WAL THORNHILL


  65. Mike Morris (#67): I hate to burst your antiscience bubble, but X-rays from comets are well understood. <a href="http://www.badastronomy.com/bad/misc/mccanney/misc.html#X-rays&quot; target="_blank"Try reading this.

  66. nazley

    10 things i dont know about comets?

  67. Update : Click on my name here to see the new comparison chart updated to include Comet Hartley 2 which the EPOXI – formerly Deep Impact space probe flew past in November 2010.

    This BA blog post is titled ‘Asteroid comparison chart, Part II’ posted on December 9th, 2010 6:50 AM & containing links and an image gallery of the Hartley 2 fly-by.

  68. SteveGinIL

    It is rather a joke that this is in Bad Astronomy, because this author seems ignorant of several things.

    In “Ten Things You Don’t Know About Comets by Phil Plait” is this statement: “The last big impact-related event was 65 million years ago.”

    Simply not true. Tunguska (1908). It was as big an explosion in the 1015 megaton range, and it was an airburst, not even a ground impact. It took out 830 sq miles of trees. Younger-Dryas (12,900 years ago). It killed all the mammoths, saber-toothed tigers, giant sloths, the American horse, Clovis man, and every large mammal in North America, except the bison. It drove the world back into a 1200 year long ice age, just as it had come out of an ice age. It was the last mass extinction, and it was only 13,000 years ago.

    Then there is Rio Cuarto, a multiple low-angle impact in Argentina, with around ten acknowledged impacts, some of them over 2 miles long and 700 yards wide. Its explosive power was 30 times as big as Tunguska. When? Opinions vary, from 1500 BC to 10,000 BC.

    He also didn’t mention the Taurids as being probably the biggest “meteor stream,” and that the Tunguska object was probably from that stream, meaning that every year when we go through it (TWICE) we run the risk of another one that size.

  69. SteveGinIL

    Oops! That was “10-15 megaton range.”

  70. Brock

    I love comets.


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