Comet McNaught, seen at sunset from Chile and overlooking the Pacific Ocean. (ESA)
NASA's Swift satellite views Comet Lulin. (NASA, Swift, Univ. Leicester, DSS (STScI/AURUA), Dennis Bodewits (NASA/GSFC), et al.)
A Leonids meteor explodes in Earth's upper atmosphere on Nov. 23, 1998. (Robotic Optical Transient Search Experiment (ROTSE) team.)
› Watch the meteor explode
› NASA Worldbook: Meteors
› NASA Worldbook: Comets
› Wikipedia: Southern Delta Aquariids Summer means fireworks, and here's how to see some of Nature's best displays. Start with a few comets streaking around the solar system, leaving behind dusty trails of gas, ice, rocks and dust. Then find a comfortable nighttime seat on planet Earth as it passes through these dusty comet trails. Finally, sit back and enjoy the show as bits of leftover comet burn up in our atmosphere, creating the celestial displays that we call meteor showers.
On Thursday, July 22, astronomer Bill Cooke from NASA's Marshall Space Flight Center answered your questions about comets and meteors: their origins and cycles, their composition and colors and the best way to catch a glimpse of them in the night skies.
More About Chat Expert Bill Cooke
The head of NASA's Meteoroid Environment Office, Dr. Bill Cooke specializes in the meteoroid environment and its effects on space vehicles of all sorts. While a graduate student at the University of Florida, he worked on instruments flying onboard balloons, the Space Shuttle, Giotto (European mission to Halley's Comet), and the Long Duration Exposure Facility.
After obtaining his PhD, he came to work at Marshall Space Flight Center as a member of the Space Environments Team. When not occupied with meteor observations and shower forecasts, he dabbles as a free- lance author for magazines and is a mentor for the Team America Rocketry Challenge and NASA's Student Launch Initiative rocketry programs.
Bill: Hi everyone, Bill here. We're finishing the chat set-up and getting ready to start answering your questions at 3:00 Eastern. Thanks, and welcome to the chat!
dina1372: Hi,did you study physics in uni?
Bill: Hi Dina. Yes, I did.
ludmila: Where does the ice meteorites come from?
Bill: Ice METEORS come from comets. A piece of ice would never make it to the ground, so it can't be a meteorite! :)
michael: Sir, why are some meteorites "dancing" in the sky?
Bill: Meteorites hit the ground, but meteors are in the sky. They can leave a streak in the sky as they burn up, and they can leave a smoke trail in the sky that the wind may appear to make look wavy.
rahulr96: Why do we see so many meteor showers if the chance of impact is so rare?
Bill: Because most of the "stuff" never makes it to the atmosphere. Ice doesn't make it to the ground. Most meteor showers come from comets, so their meteors are made of ice.
JJ_Jordan10: What materials are found in a comet?
Bill: Ice, carbon dust and other organic chemicals.
sara: Hi Bill. I have a whole room of anxious children at science camp waiting to talk to you :)
Bill: Great! I'm looking forward to hearing their questions, sara!
Dell_Conagher: In what way is Jupiter influencing the Delta Aquariids? Are they becoming more or less likely to cause meteor showers?
Bill: That's a good question. The Delta Aquariids are caused by the Marsden family of comets, and Jupiter doesn't affect them much. However, it exerts a significant influence on showers like the Geminids and Perseids.
sara: How long does it take a meteor to reach Earth?
Bill: It depends on where it came from. We can tell how long a meteor is in space by measuring the time it's exposed to cosmic rays. These ages indicate meteors are in space from a few thousands to many millions - if not BILLIONS - of years.
dina1372: The chemicals of the meteor are s.th that we can find on earth?
Bill: Pretty much everything. Meteors contain no unknown elements, so they only difference is that they may possess higher concentrations of things like iridium that we find on Earth.
Dubplay: What time is best to see the shower?
Bill: Most showers are best seen after midnight and before dawn, but it depends on the shower.
sara: How big was the largest meteor to hit the Earth?
Bill: That would be the Mars-sized meteor that hit the Earth about 4.8 billion years ago and created the moon.
meteora_lp: What's the difference btween planetoids and comets?
Bill: A comet is generally considered to be an icy body with perhaps a small rocky core, whereas planetoids are mainly thought to be big rocks. BUT, there's a lot of fuzziness in that. Nature doesn't classify things neatly!
krpm1: Hi Bill! Why does a comets' tail preceed the comet? Shouldn't the tail be behind it as it races through space? That always puzzled me as a kid. Thanks!
Bill: Comet tails are always pointing away from the sun. So when the comet approaches the sun, the tail is behind it. When it moves away from the sun, the tail is in front because sunlight and solar wind push the tail particles straight away from the sun.
estamosenelfuturo: Do u think Oort cloud is generating all the comets?
Bill: The Oort cloud generates "long period comets," which have orbits of several thousand years. Comets that take 10s or 100s of years to go around the sun come from the Kuiper Belt out beyond Uranus.
MelnLily: Hi, nothing to do with the meteor, my question is why can we sometimes still see the moon during the day?? Surely when the sun is at its highest the moon should be on the other side if the earth.
Bill: Because it's in the orbit about the Earth and lit by the sun, the moon's position is sometimes simply right so it can be seen in the daytime. So during a solar eclipse, the moon blocks the light from the sun. It can even block it at noon, when the sun is at the highest. The moon can be anywhere in relation to the sun.
sara: How fast can a comet travel?
Bill: Sungrazing comets can travel up to 100 miles per second. But they have to get REALLY close to the sun to travel that fast. Comets in the vicinity of Earth can move up to 24 miles per second.
User_Name: What is the oldest known observed comet in our solar system as seen from earth?
Bill: Well, I'd say the comet observed the longest is Halley's comet. It was seen well before the birth of Christ, for several thousand years.
ysoner: With current technology, what is the longest time period that we can spot a meteor before it reaches earth?
Bill: We have never detected a meteoroid in space from the ground. We've seen small asteroids about two yards across about 36 hours before impact. That was 2008-TC3.
EuropaOrDie2020: Where is Aphosis now and how did it get there?
Bill: Try this Web site for tracking: http://neo.jpl.nasa.gov/, run by Near Earth Object office.
michael: What is the origin of some tektites that came from the space?
Bill: It's thought that many are volcanic in origin and don't come from space. We used to think they came from the moon, but now we know that lunar meteorites are extremely rare. So tektites come from Earth.
JJ_Jordan10: Where is the best place to view a meteor shower.
Bill: Away from city lights. Go to a place with a dark sky and look up. :)
estamosenelfuturo: What do you think about the Oort Cloud? Is it real?
Bill: Yes, the Oort cloud is the only way to explain comets that take more than several hundred years to go around the sun. It definitely exists.
reportingsjr: How many meteors hit the earth per year approximately?
Bill: Millions and billions, to quote Carl Sagan. Too many for us to know!
Dubplay: Will the shower show up tonight?
Bill: Yes, the Southern Delta Aquariids will be happening tonight and will peak in a couple of days, up to 20 meteors per hour. They'll appear to come from the constellation of Aquarius the Water-bearer, one of the signs of the zodiac.
dina1372: How did that Mars- sized meteor make the moon?
Bill: It took many many years for us to figure that out. Imagine the Earth, and imagine a rock the size of Mars striking the Earth a glancing blow. The pieces of that collision went into orbit around the Earth and then accreted or "stuck together" to make the moon.
estamosenelfuturo: Rhank you for answering!
Bill: You're very welcome!
reportingsjr: What is the difference between a meteor and a meteorite?
Bill: A meteor is the streak of light you see in the sky. A meteorite is a rock from space found on the ground.
Dell_Conagher: How is it that one meteor shower, such as the Southern Delta Aquariids, is produced by two comets? Were the comets companions or close together somehow?
Bill: Actually, the Southern Delta Aquariids and three other meteor showers are thought to be produced by an entire family of comets that share similar orbits. Because they share similar orbits, the "junk" they leave behind ends up at the Earth at about the same time.
meteora_lp: The Periodic Table contains 118 elements. Have you found some elements in meteors that we can't find here on Earth? If so how do they look like?
Bill: No unknown elements exist in the meteorites, so far.
sara: How many meteors would it take to destroy a state like Florida?
Bill: Only one if it's big enough. :) A meteor one mile across (an asteroid) would leave a crater about 10 miles across and would flatten every house in the state of Florida.
estamosenelfuturo: Has a collision between 2 comets ever been observed?
Bill: No, not so far. But we HAVE seen comets break apart.
Sumedha: Hi Bill. What is the best posibble method to catch a glimpse of meteors/ites? I hvent seen one all my life!
Bill: I'd say I'd go out on the night of Aug 12 on the peak of the Perseids shower, after midnight, make sure the sky is clear and away from city lights. Watch for a couple of hours. I guarantee that you'll see a meteor!
EuropaOrDie2020: Would it be possible to hitch a ride on a fast moving comet to the outer solar system? Would that be faster than technology now?
Bill: As you move away from the sun the comet would move slower and slower. I think we can do better with modern technology, or at least as good. A good example of this is the New Horizons mission to Pluto which used Jupiter's gravity to "slingshot" the craft to Pluto more quickly.
meteora_lp: Why is an orbid of a comet like an elipse and not a circle?
Bill: Because comets' orbits are generally changed by the gravity of the planets and there's no object in the solar system whose orbit is a perfect circle. Even the Earth's orbit is a slight ellipse.
ysoner: Do you just observe comets, or run models to predict their occurence and trajectories? If so, what are the initial and boundary conditions? What is the domain (i.e. solar system? milky way?)
Bill: My interest is confined to the solar system, thank you. :) I observe comets, but I leave the models of comets and their trajectories to the experts at places like JPL.
sara: What has been the oddest shaped meteorite to hit the Earth?
Bill: All meteorites have odd shapes.
estamosenelfuturo: Is there any chance that a comet or meteorite in our solar system actually comes from another star?
Bill: Some people think they have detected very small - dust-sized - meteoroids that may have come from other solar systems and stars. But that's still being disputed.
Dubplay: If meteors move per second then there would be no way to stop it if it were to come to earth?
Bill: This would be an asteroid, and you can stop it if you catch it a long way (millions of miles) from Earth. That's why NASA keeps looking for asteroids, so we can find them before they hit.
meteora_lp: Do you know any other dangerous comets like Apophis?
Bill: It's an asteroid, and for a list of potentially hazardous asteroids, you can check this Web site: http://neo.jpl.nasa.gov/.
Dell_Conagher: Is there anything unusual about how one meteor shower will behave as a stream, and another as a swarm? Or is it just on how the originating object broke up?
Bill: All meteor showers are streams whether the particles come from a break-up or just being ejected from the surface of the comet. I'm not sure what you mean by "swarm."
JJ_Jordan10: What is the Oort Cloud?
Bill: The Oort Cloud is a collection of comets that form a shell around the solar system about one light year out. This is where the long-period comets originate.
john: Of all comets and meteors, what percentage burn up in the atmosphere?
Bill: Comets are too big to burn in the atmosphere. Fortunately, they don't hit us very often. :) For meteors, over 99% burn up in the atmosphere and only a few make it to the ground. Those are pieces of asteroids.
krpm1: Do you recommend any great amateur telescopes for viewing comets? The one I had as a kid just doesn't cut it anymore.
Bill: I know the feeling! My parents bought my telescope at a department store and it didn't cut it, either. A good one costs several hundred dollars. If you can't afford that, buy a pair of binoculars.
adastragrl: How well do the particles collected from Stardust match up to particles collected on the ground?
Bill: The particles from Stardust were very small, but they do match up with some of the stuff we've found in meteorites.
aaiigght: Meteors strike the earth everyday?
Bill: Of course they strike the atmosphere, but most never make it to the ground.
meteora_lp: If so many meteors hit the Earth for so many years, does that mean our planet is getting bigger?
Bill: Great question, but no, wind, rain and erosion take care of all that.
Dell_Conagher: Are there areas in the solar system that have more meteoroids than "normal"?
Bill: Getting close to comets, since comets produce them, would reveal more meteoroids than are in free space.
reportingsjr: Has anyone ever observed meteors or comets outside of our solar system? (around a different star)?
Bill: No, not so far.
sara: Random question: How does astronaut food taste?
Bill: I've been told it's MUCH better now than the food in the Apollo program. :)
estamosenelfuturo: What's the biggest surprise you've had with meteors and comets?
Bill: The biggest surprise was in 1975 when Comet West showed up. That was an unexpected bright comet that was very easy to see for several weeks.
john: I am in Mammoth shooting astrophotography, what objects should I try to shoot? What will be the most interesting or recognizable?
Bill: Hi John. If you have a telephoto lens, the Andromeda Galaxy is good to start with. If you have a telescope, I'd start with Jupiter or the moon.
adastragrl: Any advice on setting up all-sky cameras for observing meteors and meteor showers?
Bill: I'd suggest Googling "all-sky cameras." There are a lot of great sites out there that will tell you how to do that. Google is my friend. :)
sara: How does a meteor or comet form?
Bill: Meteors come from comets, bits of ice and dust left as comet moves through space. A comet forms when ice particles stick together into a big dirty snowball that moves through the solar system.
Sumedha: Do comets have an electromagnetic field? how does it get affected by the sun?
Bill: No, no electromagnetic field. The sun heats comets and sublimates, or melts the ice, which forms the tail of the comet.
meteora_lp: When will the Perseids Meteor Shower peak?
Bill: The night of Aug. 12-13, but you can see Perseids around that time. In fact, we're already starting to see some Perseids now.
justmk10: What are meteorites generally made of?
Bill: Materials like olivine, carbon, things like that. Some are made of iron. No meteorites made of gold, unfortunately. :)
Sumedha: By what percentage does a meteor get burnt while entering the earth's atmosphere before crashing to the ground?
Bill: Most meteors totally burn up. Iron meteorites survive basically intact. Stony meteorites lose about 99% of their original mass. It really depends on the meteorite.
sara: Thank you for answering our questions. The kids enjoyed "speaking " with you :)
Bill: They're more than welcome. I enjoyed their questions!
john: Will the meteor shower tonight be visible from Northern California?
Bill: Yes. Outside between midnight and dawn, clear skies away from city lights. But it will be only about 10-15 meteors per hour.
ysoner: Can we talk about a mortality rate due to meteorites?
Bill: There have been no confirmed records of people being killed by meteorites, though a lady in Alabama was hit inside her house when one went through her roof. An SUV in Canada had its windshield broken last October. One crashed through a doctor's roof in Virginia recently. A dog in India in 1909 was killed by a Mars meteorite - that's the only record I have of a meteorite fatality.
dina1372: How much do you use math in your job? What subject do you use most?
Bill: I use math every day, every hour. A lot of my astronomy knowledge involves a great deal of physics. So I use math, physics and astronomy every day at work.
Sumedha: Can comets passing close to the earth or meteors entering our atmosphere interfere with the functioning of our satellites? if so how?
Bill: Comets don't interfere. Small meteoroids, because there are lots of them, can sometimes hit a satellite and cause damage. But that doesn't happen often, maybe once every five years or so.
Sumedha: What theory makes us believe that the particles produced fron the collision of a meteor with earth orbited earth and conglomerated to create the moon?
Bill: The most recent version of that theory was developed by Robin Canup at Southwestern Research Institute in Boulder. There's a lot of evidence that supports this theory, and it's now accepted by practically all astronomers.
pqpqpq: Scientists often site a high iridium conent as evidence of an asteroid impact. Is there a known reason that iridium is so much more abundant in the asteroid belt than here on earth?
Bill: Since most meteorites are rich in iridium and come from asteroids, that's the evidence for increased iridium content in the asteroid belt.
JJ_Jordan10: Where is the biggest impact crator still visible to people?
Bill: The one most easily visible on the ground would be Barringer Crater in Arizona, also called "Meteor Crater."
curious_george: Will those of us in the Southeastern U.S. have a good view of the Southern Delta Aquariids this weekend?
Bill: Yes, if the sky is clear and you're away from city lights with clear skies. Look after midnight and before dawn.
EuropaOrDie2020: Where do you stand on Comet/Asteroid deflection scenarios? Are we anywhere close to having the tech required?
Bill: I'm a meteor person - I deal with small rocks. For the big rocks and how to stop them, you might check in at JPL: http://neo.jpl.nasa.gov/.
ysoner: So you say a comet slows down as it gets away from the sun. Can you briefly explain the reason? What about gravity, shouldn't it pull the comet as it is closer to the sun thus slowing it down?
Bill: If you look at Keplers Laws, the closer an object is to the sun, the faster it moves. When it moves away from the sun, it loses kinetic energy, which means it slows down. Wikipedia: Kepler's laws of planetary motion
meteora_lp: So the older the comet is the more elliptical its orbit would be? If i'm right then, considering the fact that the orbit of Mars is more elliptical than the one of our planet, does that mean Mars is older that Earth?
Bill: No, the ellipticity of a comet's orbit isn't necessarily an indicator of age. Most of the time comets travel in very elliptical orbits, but a close passage by a planet can put it into a more circular orbit. It just depends on the circumstances.
r2d2: It seems like there have been a lot of bright meteors in the news lately. What's the reason we've seen so many big meteors recently?
Bill: We always see more bright meteors in the springtime. There appears to be an annual variation, but as summer progresses we should see fewer bright meteors - except for the Perseids - until we get into spring next year.
adastragrl: Have you ever recovered any meteorites after observing a fireball on your cameras?
Bill: Not yet. :)
Sumedha: How do we know the origin of a meteroite? how can we say that they are from solar system or outside? is it dependent on the physical properties? how can we stop an approaching asteroid?
Bill: Yes, meteorites have elemental abundances that are consistent with being formed around the sun. On stopping the asteroids, JPL is a good place to look at: http://neo.jpl.nasa.gov/. Or you can contact Bruce Willis. :)
ysoner: Which one is harder? Answering questions here or during an oral PhD qualifying exam? :)
Bill: I'll take answering questions here over the oral exams. The audience is friendlier. :)
rahulr96: What is the most likely meteorite or comet that in the future may collide with the earth?
Bill: We know of no comets or asteroids that are projected to impact the Earth within the next few hundred years.
meteora_lp: Have you heard of "Kokino", a megalithic observatory in acedonia? Is that a good place to watch meteors?
Bill: The setting is appropriate if it has a dark sky. Sounds like a cool place to visit!
r2d2: Can any meteor/meteoroid studies be done with rockets?
Bill: Hard to think of one. We used to put meteoroid detectors on sounding rockets, but they didn't detect much.
john: Does that mean that wind, rain and erosion remove the meteroites from the Earth by pushing these things back into space?
Bill: No, it just means it grinds it down. The amount of material delivered to Earth by meteors is insignificant compared to the mass of the Earth.
estamosenelfuturo: Can u describe ur career, like how did u end up studying meteors? How did meteors and comets attract you?
Bill: I started studying astronomy at the university, then went to graduate school. I kind of fell into meteors by accident when I worked on several spacecraft experiments. Because not many people do meteors, there's not much competition, and I like that. :)
rahulr96: Does the dust that surrounds extrasolar stars have any connection to meteorites and comets in that solar system?
Bill: No, no connection.
ysoner: So most of the meteors burnout in the atmosphere. Which layer? Do they reach the troposphere?
Bill: Depends on the composition of the meteor. Leonids burn up at about 65 miles altitude, whereas some asteroid material can get as low as 20-25 miles altitude.
dina1372: How come comets don't melt as they orbit the sun for too long?
Bill: Well, they sublimate, which means ice goes to gas. A comet loses several yards deep of its surface with every passage around the sun. So eventually, comets do lose all of their ice.
flunkyou2: I have read somewere on the web that there's a comet that will get that close to earth around the year 2078 and could hit us! Is this true?
Bill: Not as far as I know.
Bill: Asteroids come close to the Earth all the time - but thank goodness we're a small target. Jupiter has been hit twice in the past three years!
john: Thanks Bill, I have a 2,400 mm telephoto lens, but no equatorial mounth, I'll take a shot at Andromeda and Jupiter :D
Bill: Cool - good luck on getting some great images, John.
krpm1: What made you want to become an astronomer?
Bill: I was always torn between becoming and Egyptologist and an astronomer. I chose astronomy because experience with my father taught me that I didn't like digging ditches!
pqpqpq: What do you think is the biggest open question regarding comets that can be resolved by an upcoming mission to visit one?
Bill: What they're made of! But one thing we've discovered on missions to comets so far is that every comets appears to be unique. So, it's likely that the most significant discovery will be a surprise.
Dell_Conagher(Q) Does a comet or it's coma have a magnetosphere?
Bill: No, no magnetosphere.
riverpaul23: What would the first thing you would want to look at in spotting a comet or a meteor? and what would be the best time for observing
Bill: The first thing I look at in meteors is how low they got so I can decide if one had a chance to make it to the ground. I look at comets in a telescope, and I look for sudden brightening, which tells me the comet may be breaking apart.
Sumedha: What is driving the comets to move? sun's gravity? just like in case on the planets? what determines their speeds?
Bill: Yes, the sun's gravity. The speed is determined by how close they are to the sun.
Jose_Antonio: Hill Bill, Greetings from Valencia in Spain, witch is the largest Khnow meteor that is moving closer to our planet Earth?
Bill: We don't detect meteors in space. The largest asteroid that will make a close encounter is Apophis in 2029, named after the Egyptian god of chaos (because it drives astronomers crazy!)
michael: How much are meteotites radioactive?
Bill: They aren't - you're thinking about Kryptonite. :) But they are often magnetic.
brauns: Hi Bill! Can we see the meteor shower tonight in South America (brazil)?
Bill: Yes, after midnight and before dawn, clear skies, away from city lights.
adastragrl: Any favorite smallbody space missions?
Bill: Res, I like Mir-Shoemaker because it landed on Eros, and New Horizons currently booking its way to Pluto.
Dell_Conagher: Are the Southern Delta Aquariids capable of producing fireballs or bolides?
Bill: It's not noted for them, but any meteor shower can have at least one or two bright meteors.
estamosenelfuturo: Are there comets and meteorits in orbits perpendicular or at angle to the elliptical plane of our solar system? Are they from different origins than the in-plane ones?
Bill: Long-period comets can have any inclination relative to the elliptic, and even some short-period comets can have some distribution. The long-period comets are more pristine material, since they come from the Oort Cloud. Short-period comets have had their surfaces "processed" on many trips around the sun.
finalizer: Why are most meteor shows or comets usual are viewable very late mornings? And I live in the the North East (NY) are there upcoming viewable metor showers int his area?
Bill: Because that's just the luck of the draw. Some showers are best in the early evening, but those are rare. The Draconids in October are an example of that, depending on where you are on Earth. For now, the Southern Delta Aquariids are happening in the next few nights.
ysoner: Apparently not here but, do you get many questions about so called "Marduk"? Why do you think this has been a major interest? has there been any physical sign regarding this issue?
Bill: The only Marduk I know of is the ancient Babylonian god. Not sure of your question.
rahulr96: Do comets have an atmosphere?
Bill: Not in the sense that Earth has one. They do have a slight coma surrounding them, which is made of gas. but that only shows up when they approach the sun.
finalizer: Any viewable meteor showers in the North East (NY) upcoming?
Bill: Sure! The Southern Delta Aquariids are happening in the next few nights. The Perseids in middle August.
Dell_Conagher: Much obliged to you for answering all of our questions! 'Preciate it! =)
Bill: Thank you all for coming, and I was delighted to answer your questions!
michael: Thank you:-)
Bill: And thank you!
estamosenelfuturo: Thank you!!
Bill: Very welcome!
(Moderator Jason): A big thanks to Bill for answering your questions today and thanks to everyone for joining us to ask your questions. A transcript will be posted within the next few business days.