* The Small Comets Frequently Asked Questions List *
Version 3.0 April 9, 1999
On this page we answer the most commonly asked questions about small comets. This FAQ will be regularly updated. More information on the small comets is available in the original discovery papers and the Replies to Comments that appeared in Geophysical Research Letters, and in The Big Splash by Louis A. Frank with Patrick Huyghe, published in 1990 by Birch Lane Press.
What is the difference between these small comets and the large comets like Hale-Bopp and Halley's?
The small comets are a million times smaller than these more famous comets. The small comets also contain little dust and lack the iron and other metals necessary to make them glow brightly and produce a tail like the larger comets. But what they have in common--and the reason they were dubbed "small comets" in the first place--is that they are both largely made of water.
Why haven't the space shuttle and our satellites been hit by these small comets?
In low Earth orbit, where the space shuttle flies, astronauts can expect to run into the cometary water clouds from the small comets once in every 200 orbits. At the shuttle's altitude a small comet has already disintegrated in a cloud; it is no longer a solid object and the collision with a cloud is benign. So the astronauts have probably flown through these things and not known it. But at high altitudes, an impact of a spacecraft with a small comet would be disastrous. Since these comets are small and the collision frequency is low, an average-sized spacecraft would only be struck once in every 50,000 years or so. This means that one spacecraft in every thousand will be struck in high Earth orbit every 50 years. Has it happened yet? No one knows. But some spacecraft have been lost and no one knows why.
Why hasn't the Spacewatch Telescope seen the small comets?
It has. In 1988, Clayne Yeates, the late Jet Propulsion Laboratory physicist and science manager for the Galileo project, used the Spacewatch Telescope in a "skeet shooting" mode to obtain some stunning optical images of very faint streaks from the small comets. The objects he photographed had the same motion in orbit, the same speed, and were about the size, darkness, and frequency as the atmospheric holes themselves, or could be deduced from the known characteristics of atmospheric holes. [ L.A. Frank, J.B. Sigwarth, and C.M. Yeates, "A Search for Small Solar-System Bodies Near the Earth Using a Ground-Based Telescope: Technique and Observations," Astronomy & Astrophysics, 228, 522, February 1990.]
How long have the small comets been bombarding the Earth?
We do not know. But if the present influx of small comets is assumed to
be true for the past 4.5 billion years as well, then the small comets may
be responsible for all the water in the oceans and in our atmosphere.
In the spring of 1999 some scientists concluded that the Earth's water probably did not come from comets. So how could the small comets be responsible for the water in the Earth's oceans?
The possibility that the water in our oceans is due to an influx of large comets during the early history of our planet has been quite popular among many scientists until recently. But things have changed now that we can remotely determine the amount of deuterium, or heavy hydrogen, in these well-known large comets. These remarkable measurements have shown that the fraction of deuterium relative to that for hydrogen in the large comets is inconsistently high relative to that in our oceans. That is, the large comets cannot be the source of our oceans because this hydrogen "fingerprint" does not match. And because some scientists view the small comets as simply miniature versions of the large comets , they have concluded that the hydrogen fingerprint of the small comets is similarly inconsistent as the source of water in our oceans. But this conclusion is not necessarily correct because the small comets have already been shown to be much different in composition than the large comets. Measurements by the Polar spacecraft have shown that there is little dust and sodium in the small comets compared to the large comets. Thus the contents of the small comets greatly differ from those of large comets and there is no reason to conclude that the hydrogen fingerprints of these two classes of solar system objects are the same. Tom Donahue, a well-known atmospheric scientist at the University of Michigan, has proposed that the question of origins of our oceans can be resolved by measurements of the hydrogen fingerprint in the upper atmosphere because some of the contents of the small comets are continually deposited there. This is a difficult measurement but it would be decisive in establishing the small comets as the source of the ocean's waters. To date such an instrument has not been proposed for launching on a small rocket or for remote sensing from an orbiting spacecraft.
How do we know that these objects are depositing water in our atmosphere?
This startling conclusion comes from trying to account for the presence in the images of the "atmospheric holes," those dark spots where the ultraviolet dayglow has been absorbed over areas of 50 to 100 km in diameter. This is a large area and requires a lot of material. For the wavelength range viewed by the Polar and Dynamics Explorer cameras, water is the only common gaseous substance in the solar system that can efficiently absorb the dayglow along the line-of-sight of the cameras. No one has ever offered an alternative mechanism or substance. The absorption cross section of the water molecule is large and very well known. The total water cloud mass is still large, in the range of 20 to 40 tons. In addition, one of the Polar cameras for visible wavelengths was used to independently verify that the objects contained large amounts of water by viewing the intensities of OH radical emissions at 308.5 nm, which is the standard proxy for water in the studies of large comets. The OH is produced by the dissociation of water molecules in the sun's light and the OH radical fluoresces very brightly in the sunlight. This finding is a great achievement and is beyond the capabilities of any other camera flown to date. There is a large amount of water in these cometary gas clouds. The final closure was provided by the remarkable fact that the frequency of the OH trails is very similar to the occurrence frequency of atmospheric holes.
Why do the small comets break up and turn into clouds of water vapor?
The small comets are giant, loosely packed "snowballs" with some kind of thin shell, made perhaps of carbon, that holds them together as they travel through interstellar space. But as they approach the electrically charged Earth, the electrostatic stress on these objects causes them to break up at an altitude of about 800 miles above Earth. Rapid electrostatic erosion appears to be the mechanism responsible for stripping the thin protective mantle from the water-snow core of a small comet. By the time the fragments of the comet have descended to about 600 miles, the "snowball" fragments have been vaporized by the Sun's rays.
How much water do the small comets add to the Earth's surface?
At a rate of one 20-to-40 ton comet every three seconds, this influx of
small comets into the atmosphere would add about one inch of water to the
Earth's surface every 20,000 years or so. The implications of this added
water for long range global climate, global warming, and pollution
mitigation will need to be examined by the experts in those fields.
Is there any geological evidence to support the need for such an "outside" source of water as the small comets?
There is indeed. In 1999, David Deming, a geologist at the University of Oklahoma, published a refereed paper [Palaeo, 146, 33-51, 1999] which has attracted the attention of many scientists. His work points out that recent investigations of the movement of oceanic continental plates into the mantle, known as subduction, show that the loss rates for the water on this planet are very large as the plates carry the water deep below the surface. So unless there is an influx of water to our planet on time scales much shorter than its age of 4 billion years or so, our planet would be presently "dry as a bone." Remarkably the necessary influx of water from interplanetary space agrees quite well with what the small comets are calculated to be bringing to the Earth.
The amount of water added to the atmosphere by the small comets seems to conflict with well-established evidence that the stratosphere is extremely dry. How can you explain this?
The influx of water into the stratosphere from the small comets is insufficient to provide a "wet" stratosphere. The problems lie in the lower thermosphere and upper mesosphere. Simple models of water transport by eddy diffusion could not support the cometary water influxes if the upper boundary were taken above these regions. But the small comet's momentum carries the water into the mesosphere and thus provides a low percentage of water vapor in the atmosphere. This effect could accommodate the cometary water influx into the atmosphere without exceeding the known densities. To date no one to my knowledge has used such a source term in the standard atmospheric models. Below the mesopause at about 50 miles there is a general pattern of atmospheric circulation that extends into the troposphere. The cometary water would be carried in this circulation pattern. The stratosphere is dry because the "cold finger" near the tropopause precipitates the water into the troposphere. This cometary "rainfall" is insignificant relative to the rest of the water being transported at these altitudes.
Are noctilucent clouds produced by small comets?
The influx of small comets into Earth's atmosphere may help explain the source of water needed to form noctilucent clouds. These strange and quite beautiful clouds can be seen over the polar regions during the summer months. They are thin clouds, wavy or banded, colored silver or bluish white. They form at an altitude of about 55 miles, in the coldest part of the upper atmosphere, a relatively unexplored boundary known as the mesopause. No other cloud occurs so high in the sky. They are called noctilucent clouds because they can only be seen against a dark sky when illuminated by the setting sun. These clouds require considerably more water vapor than can be expected at that altitude from ocean evaporation. No one thoroughly understands why these clouds exist. But rocket-borne experiments sent up by aeronomers--those who explore the upper atmosphere--to probe these clouds have shown that the clouds are composed of ice crystals formed around meteoric dust particles--a finding that suggests small comets might indeed be responsible.
Do the small comets also impact the Moon? If so, where are these impacts and why don't we see dust clouds on the moon when the comets hit? Why didn't the Apollo seismometers record their impacts? Where is all the water on the Moon?
If you remember that the small comets are like fluffy snowballs--not rocks--the Moon does not present a problem to the existence of small comets. It's the difference between throwing a rock at your car and a snowball; one will leave a permanent mark, the other will not. Because the Moon is one thirteenth as large as the Earth it should receive about thirteen times fewer objects than the Earth. But the seismometers that were set up on the Moon during the Apollo missions recorded only about 2,000 events a year. How to account for this apparent discrepancy? The small comets do impact the Moon, but the seismometers were calibrated by looking at the seismic signature of everything from nuclear explosions to bullets shot into loose sand. No one ever worked out what effect a large snowball would have on the lunar surface. The small comets that strike the Moon will not make impact craters;they probably kick up some lunar dust and produce strange glows, and indeed these kinds of anomalous events have been reported by lunar observers for centuries. It is the seismometers' lack of sensitivity to the impact of small comets that accounts for the discrepancy in the low number of large objects detected on the Moon relative to the number of such objects that are seen falling into Earth's atmosphere. But if small comets strike the Moon, where is all the water then? The lunar gravity is such that practically all the water vapor from the impact of small comets simply flies off, though some of the water molecules may wander around and eventually condense in the crevices near the poles--exactly where it has been reported of late.
Can the small comets help resolve the long standing controversy about the difference in impact rates on the Moon and into the Earth's atmosphere?
Yes, there is a well known discrepancy between the number of objects of a given mass which are impacting Earth's atmosphere as inferred from fireballs in the atmosphere and the number of objects of similar mass as detected by the Apollo seismic network. Even taking in account the fact that the Moon is smaller than the Earth, the number of objects impacting the Moon has been found to be considerably less than those in our atmosphere. This major discrepancy has never been resolved, but the flux of small comets provides the solution to this problem. Because there is no dust in these small comets, their glow in the atmosphere must be estimated from the heat they produce when they hit the atmosphere at supersonic speeds. We have roughly estimated the visual magnitudes of the impacting small comets and find them to be in the range of -2 to -4. Remember, of course, that solar radiation is not available on the nightside of Earth to produce a large water vapor cloud as it does on the dayside where the atmospheric holes are observed. The number of fireballs in Earth's atmosphere with a visual magnitude of -2 is in the range of about 10,000 to 100,000 for each 24 hour period, according to D.W. McKinley, in Meteor Science and Engineering (McGraw Hill, 1961). And so the small comets do help explain the difference in the number of observed impacts on the Moon and in the Earth's atmosphere.
If the small comets are hitting Earth and the Moon, shouldn't they also be impacting the other planets in the solar system?
They do. But few small comets will survive inside the Earth's orbit because they will be destroyed by the Sun's heat. So there will be no small comets for Mercury, and maybe just a few for Venus. But the rest of the planets and their moons do get pelted by the small comets. While Earth gets about 10 million smallcomets a year, Mars receives less than a million and a half, Jupiter gets 16 billion,Saturn gets 4 billion, Uranus gets 260 million, Neptune gets 300 million, and Pluto only about 500 thousand a year. If the ice is not visible on the surface, as is it for many planetary moons, then the water and ice from the small comets probably lies beneath the planet's surface.
Where do the small comets come from?
The small comets do not come from the Oort cloud located far beyond the orbits of the planets, but from an inner belt of cometary material beginning just beyond the orbit of Neptune. To explain the constant bombardment of the Earth by small comets, a large, dark, as-yet-undiscovered planet must be regularly passing through the outer part of this comet belt where the small comets are thought to be located. The eccentric orbit of this dark planet is speculated to cross the comet belt once every 26 million years or so, sending swarms of small comets streaming into the inner solar system and toward the Earth itself.
Are all the small comets the same size? Is there any variation in their flux at the Earth?
The size of the "small comets" no doubt varies somewhat. Most
are thought to be in the 20-40 ton range, but there will also be some
even smaller comets--and some occasional larger ones. Some of these
larger ones may be responsible for such things as anomalous ice falls
that have been reported in the literature. And just as there are
variations in the sizes of these objects, there have probably also been
peaks and valleys in the influx of small comets on Earth over time.
Is there a seasonal variation in the observed influx of small comets?
Three sets of data for the period November through January point to a very pronounced seasonal variation. Recent data from the Polar spacecraft show that the influx of small comets into the Earth's atmosphere is 10 times greater in early November than in mid-January, when the small comet rate diminishes dramatically. This is the same seasonal variation discovered in the 1980s in images from a different camera aboard a different spacecraft, Dynamics Explorer-1, which traveled a different orbit than the Polar spacecraft. The oldest data set showing the influx of small comets into the Earth's atmosphere dates back to 1955. Using forward scatter radar, two Canadian scientists, E. L. Vogan and L. L. Campbell, found exactly the same seasonal variation, a November high and January low, in their non-shower, or sporadic, radar meteor rate. Why the atmospheric hole rate should correlate so well with the meteor rate measured by forward scatter radar is no mystery. After all, small comets are just a part of the meteoric dust and debris that orbits the Sun and falls into the Earth's atmosphere on a daily basis. Because the weakly bound small comets and mantle debris are expected to produce ionization at higher altitudes than stony or iron meteoroids, forward scatter radar--which is much more sensitive to ionization at higher altitudes than backscatter radar--is ideally suited to record the infall of small comets. (Backscatter radar events, on the other hand, are dominated by the infall of iron and stony meteoroids.)
Why is there a period in January when the small comets don't seem to be running into us?
The seasonal variations of the small comet fluxes are due to events in the distant disk of comets which lies generally parallel to the orbital planes of the planets, including that of Earth. Passing stars or a rogue dark orbiting planet cause local disturbances in the distant disk of comets which send some of them into the inner solar system. The position of a given disturbance would provide a corresponding stream of small comets at a particular position of the Earth's orbit around the Sun, that is, at a given time in the year. During the course of a year our planet will intercept the composite of these showers which accounts for the features in the atmospheric hole rates. For example, the minimum during January would correspond to a position in the distant comet disk for which there was no local disturbance. In future years, telescopes should be able to determine the orbits of the small comets and hence the general location of the corresponding disturbances in the enormous comet disk which lies beyond the planets.
Is there a daily variation in the observed influx of small comets?
Yes, there is. The maximum rate of atmospheric holes is observed from about 10 a.m. to 11 a.m. This maximum is two to three times greater than the event rate at 6 p.m. There is a good reason for this. First consider a uniform stream of small comets directed parallel to the Earth's orbital motion and travelling 10 km/s relative to Earth. Of course, the small comets are all influenced by the Earth's gravitational field when they are close to our planet. In the evening the small comet trajectories are more-or-less parallel to the gravitational force. The comets speed up but they are not deflected very much as they plunge toward the atmosphere. On the other hand, trajectories passing over noon are directed almost perpendicular to the gravitational field and they will be significantly bent toward the atmosphere and thus "gravitationally focused." So the impacts on the dayside atmosphere, including the effects of gravitational focusing, will be confined to local times at the equator extending from local evening at 6 p.m. to about 10 a.m. In the absence of gravitational focusing this impact zone is confined to local times at 6 p.m. to local noon. Then realize that for the trajectories of the small comets just above Earth's atmosphere the path length, and hence the duration, of a given atmospheric hole is substantially longer for the comet trajectories which graze the atmosphere at late morning hours relative to the direct plunging of the evening cometary water clouds. What this means then is that the late morning comets have a higher probability of being recorded by the camera. The small comets also have a range of perihelia, although not as far in as the orbit of Venus, and a limited range of inclinations, which will act to widen the maximum in the late local morning hours. This daily variation in the small comet influx is a fundamental feature associated with the fact that the comets are moving in a stream past the Earth. If the small comets were moving in random directions relative to Earth, there would be no such daily variation.
Can the small comets be seen by the naked eye?
You cannot see an intact small comet with the naked eye, but if you have a lot of patience--and a little luck--you might be able to see a small comet immediately after it breaks apart in the atmosphere. To see the flash produced by the disruption of a small comet you must stand out on a clear dark night, looking up at a 40 degree angle, until you see a short streak that quickly snuffs out. It will be about the brightness of Venus for about two seconds before it vanishes. But you will have to be out there for a hundred hours or so to see one. A hundred hours of clear night viewing does not happen often in the average lifetime.
How can amateur astronomers spot the small comets?
Amateur astronomers whose telescopes have mirrors or lenses measuring 12
inches or larger should be able to sight the small comets before they
disrupt in the atmosphere. During the course of a day there are two
times for observation, each about one or two hours long. One ends about
45 minutes before sunrise; the other begins about 45 minutes after
sunset. The small comets will be seen at a distance about 4,000-7,200 km
(2,500 to 4,500 miles) from the observer, so the telescope should be
pointed in such a way that it is looking for them at these distances,
just outside the Earth's shadow. Inside the shadow the objects are not
illuminated by the Sun and are invisible. Every hour or so a small,
quite dim object will slowly move across your view, as long as your field
of view is about four times the size of the Moon. The object will move at
a distance equal to the Moon's diameter every five seconds or so. (For
more details, see How to Search for Small
Comets.) Several amateur astronomers have reported seeing such
Do the small comets contain organic material that may be responsible for seeding life on Earth?
The small comets may contain organic materials, though this is only
speculation at the moment. If they do, they would seem to be ideal
vehicles for carrying organics safely through the atmosphere; they do not
burn up the way meteors do, and their icy interiors may protect the
organics just long enough to slip safety to Earth on a cushion of water
Could the water vapor from the small comets account for the "fireflies" that John Glenn and other astronauts saw on the early orbital missions?
No. By the time of Scott Carpenter's flight three months later, NASA had determined that those brilliant little specks floating around outside the spacecraft were caused by tiny ice crystals fluttering out from beneath the rippled heat shingles of the Mercury capsules.
How do the new results from NASA's Polar satellite confirm the original Dynamics Explorer images from a decade ago showing "holes" in the atmosphere?
There is no question that the Polar images confirm the previous Dynamics Explorer observations of atmospheric holes. This includes the dimensions of the holes, their frequency of appearance over the sunlit atmosphere, and their east-to-west motion across the sunlit atmosphere. The Polar detections are approximately several thousand per day and, accounting for viewing and image accumulation times, give a global rate in the range of 5 to 20 per minute. The database consists of 50,000 to 100,000 direct detections per month as clusters of darkened pixels. In many cases the holes are detected in consecutive frames, most are moving from east to west, and the effects of the camera platform motion (double vision) are present when the instrument computers do not compensate for this latter effect. The verification of the existence of atmospheric holes is completely secure.
The spectacular small comet streak acquired on Sept. 26, 1996 at UT 2228 and shown on the "front page" of the small comet site is obviously a processed image. What does the original "raw" Earth camera image look like?
The Near Real Time images available on our Visible Imaging System web site as "Current Image" or "Past Current Images" are actually a "stack" of five consecutive "raw" images all with some cosmetic processing to remove cosmic ray hits, nightglow backgrounds, flat-field optical normalizing, distortion removal, etc. These same corrections have been applied to the "streak" image in question. On the right is the "raw" streak image of Sept. 26, 1996.
How are the altitudes of the small comet trails in the Polar images calculated?
The approximate altitudes of the trails are determined by the apparent lengths of the trails between shutter closings of the camera and the fact that the apparent speed of the objects is about 10 km/s. Generally, the shorter the trail, then the greater distance between the trail and the Polar spacecraft.
Do you maintain a catalog of small comet sightings by the Polar cameras?
Yes, we do. The Catalog of Atmospheric Holes associated with the impact of small comets into Earth's atmosphere is is available for each day of the year starting April 20, 1997. In addition, the current image from the Polar spacecraft is available live.
Now that the existence of the small comets has been confirmed by the Polar spacecraft, what's next?
What we have to do now is go up there and meet the small comets at 600 miles out. Polar sees these objects with great resolution but from a great distance. Now we have to get up close and see these objects in detail. And that's just what a group of us--Sigwarth and myself, along with some of my former critics, including Thomas Donahue and Michael Combi at the University of Michigan; Paul Feldman at John Hopkins University; Robert Meier, George Carruthers and Charles Brown at the Naval Research Laboratory; and Ralph Bohlin at the Space Telescope Science Institute--have proposed. This proposed spacecraft is the first step in doing more sophisticated studies on these objects. Its two imagers will not only be more powerful and sensitive than those on Polar, but they will be able to look at the emissions coming from these objects. We are going to be looking for carbon, oxygen and simple organic gases. Maybe later we will be able to send a major mission after these objects and bring back samples.
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