This image may show an oxygen trail created when a small comet was disrupted as it approached our planet on September 15, 1996. This image was taken by the Polar spacecraft's Earth Camera in ultraviolet wavelengths. The oxygen trail has been superposed upon a "Face of the Earth" map of our planet. Click on image for full size (26K GIF) Courtesy of Dr. Louis A. Frank, The University of Iowa and NASA

These snowballs have been labelled small comets. The hypothesis holds that these small comets are millions of times smaller than larger comets like Halley's or Linear, but they are mainly made of water like these larger comets. They lack dust and iron though and so they do not glow or produce a bright tail. These small comets don't come from the Oort cloud like 'traditional comets', but from a belt of cometary material just beyond the orbit of Neptune.

Scientists aren't sure how long these small comets have been crashing into the Earth's atmosphere -- possibly for the last 4.5 billion years! If that's true, then some or all of the Earth's water probably did come from these small comets.

It's been estimated that one 20-40 ton comet hits the Earth every three seconds. Small comets are not a danger to humans on Earth. They are loosely packed "snowballs" that get torn apart by electrostatic stress. So, they crumble apart at about 800 miles above the Earth and are vaporized by the Sun by about 600 miles above the Earth.

Small comets also impact the Moon and other planets, though there are hardly any hitting Mercury and Venus because the Sun's heat destroys the comets at that distance.

The Polar spacecraft may have confirmed the existence of these small comets originally found by Louis A. Frank of the University of Iowa. But, it sees the small comets at a great distance. The next step in investigating these snowballs is to send a spacecraft to see the small comets up close! That will help us to know whether or not this snowball hypothesis is correct!

This small comets hypothesis is very controversial. Some scientists believe the comets exist, while others are very doubtful about their existence. They think that Dr. Frank is indeed seing something, but that several lines of evidence show that what he sees cannot possibly be comets. The debate in the scientific community about the small comet hypothesis is still going on!

This image may show an oxygen trail created when a small comet was disrupted as it approached our planet on September 15, 1996. This image was taken by the Polar spacecraft's Earth Camera in ultraviolet wavelengths. The oxygen trail has been superposed upon a "Face of the Earth" map of our planet. Click on image for full size (26K GIF) Courtesy of Dr. Louis A. Frank, The University of Iowa and NASA

These snowballs may really be small comets. If the hypothesis is correct, these snowballs are millions of times smaller than comets like Halley's or Linear, but they are mainly made of water like these larger comets. They lack dust and iron though and so they do not glow or produce a bright tail.

These small comets may have been crashing into the Earth for the last 4.5 billion years! If that's true, then some or all of the Earth's water probably did come from these small comets. It's been estimated that one small comet hits the Earth every three seconds. But, don't worry about getting hit by one of these snowballs! Small comets are not a danger to humans on Earth. They get torn apart at about 800 miles above the Earth and are vaporized by the Sun by about 600 miles above the Earth.

This may sound like something out of a cartoon show, but the Polar spacecraft may have confirmed the existence of these small comets originally found by Louis A. Frank of the University of Iowa. The Polar spacecraft sees the small comets from really far away. So, the next step is to send a spacecraft to see the small comets up close! That will help us to know whether or not this snowball hypothesis is correct!

Some scientists do not believe that Dr. Frank is actually seeing small comets. They do not believe the small comets hypothesis is correct. They think there is evidence that whatever Dr. Frank has discovered, it cannot be comets. Scientists are still debating whether the small comets hypothesis is true or not.

This image may show an oxygen trail created when a small comet was disrupted as it approached our planet on September 15, 1996. This image was taken by the Polar spacecraft's Earth Camera in ultraviolet wavelengths. The oxygen trail has been superposed upon a "Face of the Earth" map of our planet. Click on image for full size (26K GIF) Courtesy of Dr. Louis A. Frank, The University of Iowa and NASA

These snowballs may be really small comets. They are millions of times smaller than comets like Halley's or Linear.

Scientists think that the comets are made mostly of water...that's why they were originally called snowballs! But, don't worry about getting hit by one of these snowballs! Even though one small comet probably hits the Earth's atmosphere every three seconds, they are not dangerous to humans or animals, because they break up in the atmosphere.

Scientists want to take measurements from close-up. So, the next step is to send a spacecraft to see the small comets up close! That will help us to know whether or not this snowball hypothesis is correct! (Remember, theories have to be proven over and over again for them to be considered true!)

Some scientists do not believe in the small comets hypothesis. They think that there must be some other explanation of the data. Scientists are still having a debate about the small comets idea. Some say it is true, but others think it is false. We still don't know for sure!