Education Corner


A question I often get asked, either by email or when I give talks, is what is the difference between a comet and a meteor. Sometimes the question also includes an asteroid. The remainder of this page is dedicated to providing both descriptions and movies of each object in an attempt to help clarify the differences.


Comets

Naked Eye Appearance:

Seeing a comet with the naked eye is a somewhat rare occurrance. On the average we get a naked-eye comet once every five or six years and this includes comets that become barely visible to the naked eye. Classic comets with long tails only appear about once every 10-12 years. The motion is very difficult to detect and comparing its place with naked-eye stars over several days is the only way to see it move. In general, comets are best observed with telescopes or binoculars.

What are They?

Comets are primarily composed of ice and dust, causing some astronomers to refer to them as "dirty snowballs." They typically move through the solar system in orbits ranging from a few years to several hundred thousand years. Comets are not on fire. As they near the sun, the sun's heat melts the comet's ices and releases dust particles which are most evident as the comet's tail. Comets rarely come within a few million miles of Earth and, thus, have a slow apparent motion across our sky. Typical comets remain visible for periods of several weeks up to several months.

    Example #1: This movie of C/1997 T1 (Utsunomiya) was obtained by Tim Puckett on 1997 October 7. It was obtained with a 60-cm reflector and shows the comet's motion over a period of about 75 minutes. The field of view is 9 x 9 arc minutes or about one-third the diameter of the full moon.

    Example #2: This movie of C/1995 O1 (Hale-Bopp) was obtained by Tim Puckett on 1997 March 5. It was obtained with a 30-cm reflector and shows the comet's motion over a period of 66 minutes. The field of view is 22 x 22 arcmin or about three quarters of the diameter of the full moon.

    Example #3: This movie of C/1994 N1 (Nakamura-Nishimura-Machholz) was obtained by Dennis Luse and William McLaughlin on 1994 August. It was obtained with a 20-cm Ultima reflector with an ST5 CCD camera taking one-minute exposures every 5 minutes. It shows the comet's motion over a period of 85 minutes. The field of view is ?? x ?? arcmin. Interestingly, frame 9 shows the streak of a meteor that passed through the camera's field during the exposure.

Meteors

Naked Eye Appearance:

Meteors appear as fast-moving streaks of light in the night sky. They are frequently referred to as "falling stars" or "shooting stars." Most are white or blue-white in appearance, although other frequent colors are yellow, orange. The colors seem more related to the speed of the meteor rather than composition. Red meteors occasionally appear as very long streaks and are usually indicative of a meteor that is skimming the atmosphere. Green meteors are also occasionally seen and are usually very bright. The green color may be a result of ionized oxygen.

What are They?

Meteoroids are the smallest particles orbiting the sun, and most are no larger than grains of sand. From years of studying the evolution of meteor streams, astronomers have concluded that clouds of meteoroids orbiting the sun were produced by comets. Meteoroids can not be observed moving through space because of their small size. Over the years numerous man-made satellites recovered by manned spacecraft have shown pits in their metal skins which were caused by the impact of meteoroids.

Meteoroids become visible to observers on Earth when they enter Earth's atmosphere. They are then referred to as meteors. They become visible as a result of friction caused by air molecules slamming against the surface of the high-velocity particle. The friction typically causes meteors to glow blue or white, although other colors have been reported. Most meteors completely burn up in the atmosphere at altitudes of between 60 and 80 miles. They are rarely seen for periods of more than a few seconds.

Occasionally, a large meteor will not burn up completely as it moves through Earth's atmosphere. The subsequent pieces that fall to Earth's surface are known as meteorites.

    Example #1: This meteor was video taped on 1995 January 5/6 by members of the International Meteor Organization near Hannover, Germany. The maximum brightness reached -4.

    Example #2: This is a movie of a fireball breaking up over Peekskill, New York, on 1992 October 9. It was video taped by spectators at a football game. The fireball produced meteorites, one of which crashed through the trunk of a car.

Asteroids

Naked-Eye Appearance:

Out of more than 10 thousand numbered asteroids, only Vesta has consistently been a naked-eye object if the observer has extremely dark skies and the asteroid is moving in relatively star-poor regions of the sky. The motion of asteroids is similar to that of comets in that the position must be plotted on a star chart over two or three days for motion to be detected.

What are They?

Asteroids, or minor planets, have been described as "mountains in space." They are large rocks typically ranging from a few feet to several hundred miles across. The vast majority of asteroids move between the orbits of Mars and Jupiter in what is commonly called the "asteroid belt." They always appear starlike and their motion with respect to the stars is usually so slow that several hours may pass before any movement is noticed. Most asteroids within the asteroid belt never come closer than 100 million miles from Earth, but there are some asteroids which come close to and even cross Earth's orbit. These objects can occasionally pass within a few million miles of Earth, and even within the orbit of the moon, and then exhibit a rapid motion that is discernable after only a few minutes. Asteroids within the asteroid belt can be observed every year, while the ones passing especially close to Earth may only be visible for a few weeks or months.

    Example #1: This movie of minor planet 1997 GL3 was obtained by Gordon Garradd (Australia) on 1997 April 10. It shows the asteroid's motion over a period of 2 hours 20 minutes. The asteroid had passed less than 5 million kilometers from Earth just 6 days earlier.

    Example #2: This movie of minor planet 433 (Eros) was obtained by Gordon Garradd (Australia) on 1998 January 28. It shows the asteroid's motion over a period of 3 hours 30 minutes. The asteroid was 95 million kilometers from Earth.