Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun. Early in Earths history, comets may have brought water and other substances to Earth during collisions. Comet tails form the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the comet vaporizes and forms a glowing coma, which reflects light from the Sun. Radiation and particles streaming from the Sun push this gas and dust into a long tail that always points away from the Sun (Figure 1.1). Comets appear for only a short time when they are near the Sun, then seem to disappear again as they move back to the outer solar system. Comet Hale-Bopp, also called the Great Comet of 1997, shone brightly for several months in 1997. The comet has two visible tails: a bright, curved dust tail and a fainter, straight tail of ions (charged atoms) pointing directly away from the Sun. The time between one appearance of a comet and the next is called the comets period. Halleys comet, with a period of 75 years, will next be seen in 2061. The first mention of the comet in historical records may go back as much as two millennia.


where comets come from

Short-period comets, with periods of about 200 years or less, come from a region beyond the orbit of Neptune called the Kuiper belt (pronounced KI-per). It contains not only comets, but also asteroids and at least two dwarf planets. Comets with periods as long as thousands or even millions of years come from a very distant region of the solar system called the Oort cloud, about 50,000 100,000 AU from the Sun (50,000 - 100,000 times the distance from the Sun to Earth). Click image to the left or use the URL below. URL:

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when do comets have tails?

a) when they are close to colliding with an asteroid

-->  b) when they are close to the sun

c) when stars get in their way

d) when they pass earth

what causes a comets tail to form?

-->  a) ice from the outer layer of the comet vaporizes and reflects sunlight.

b) nuclear fusion in the comet leaves a trail of radiation.

c) the comet lights on fire near the sun and leaves a trail of fiery particles behind it.

d) none of these.

possibly the most famous comet, halleys, has a period of _ and will next be seen in _.

a) 25 years; 2035

b) 50 years; 2053

-->  c) 75 years; 2061

d) 100 years; 2082

where do long-period comets come from?

a) the asteroid belt

-->  b) the oort cloud

c) the kuiper belt

d) the comet belt

where do short-period comets come from?

a) the asteroid belt

b) the oort cloud

-->  c) the kuiper belt

d) the comet belt

in the early solar system, comets that struck earth may have brought in

-->  a) water.

b) microbes.

c) plankton.

d) all of the above.

comets may be visible with the naked eye

a) during their entire journey around the sun.

-->  b) for a short time when they are near the sun.

c) never.

d) none of these.

without its tail, a comet looks like a ball of ice.

-->  a) true

b) false

the oort cloud is about 50,000-100,000 au from the sun.

-->  a) true

b) false

the kuiper belt is home to

a) short-period comets

b) asteroids

c) dwarf planets

-->  d) all of the above

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