Download Comets

Solar System: Small Bodies
Name______________________________
Date______________
Comets:
A comet, Latin for “hairy star,” is a large ball of ice and rock often described as a “dirty snowball.”
Most comets are hidden from our eyes, far from the center of the solar system in one of two reservoirs:
the Kuiper Belt or the Oort Cloud.
The Kuiper Belt
The Kuiper Belt lies outside the orbit of Neptune, at a distance from the Sun of 30 AU to 100 AU, and
contains an estimated 35,000 objects with diameters greater than 100 kilometers (60 miles). The first
Kuiper Belt object was detected in 1992. Most periodic comets—that is, those comets that orbit the Sun
on a regular schedule—come from the Kuiper Belt.
The Oort Cloud
The Oort Cloud is much farther away, up to 50,000 AU from the Sun. There are likely billions of
comets in the Oort Cloud, but they are too small and too far away for us to detect directly. The Oort
Cloud is the source of non-periodic comets. Non-periodic comets are comets that swing around the
Sun once and are flung off into space, never to return. Some of these comets might in fact turn out to
be periodic, but with cycles of a million years or more!
A Tale of Two Tails:
Occasionally, a comet is deflected into a new orbit that takes it close to the Sun, and it becomes
visible to observers on Earth. The closer the comet gets to the Sun, the brighter it becomes. The Sun’s
heat vaporizes the comet’s ice and the solar wind blows the vaporized gas and dust into tails that can
stretch for hundreds of millions of kilometers. The longest tail yet discovered measured more than
500 million kilometers (300 million miles).
Comets have two tails—one made of gas, the other of dust. The gas tail is straight and points directly
away from the Sun, while the dust tail can be curved. The gas tail is usually longer.
Comets lose up to 1% of their ice on each trip around the Sun. If the Earth’s orbit passes through this
cometary debris, a meteor shower takes place.
It’s always an exciting surprise when a new bright comet is discovered.In the past, most comets
were discovered by (and named after) amateur astronomers, but now, the Lincoln Near Earth
Asteroid research project finds most new comets. This project was set up to detect near-earth
asteroids that could threaten Earth. Comets detected by this project bear the label LINEAR.
An Historic “Hairy Star”
This famous comet was the first to be recognized as a periodic or returning comet. The astronomer
Edmond Halley deduced that the bright comets seen in 1531, 1607, and 1682 were all the same comet
and predicted its return in 1758.
Once the period of Comet Halley had been determined, historians found evidence for past visits
dating back to 240 BC. The Bayeux Tapestry that illustrates the Battle of Hastings in 1066 clearly
shows a bright comet, now known to be Comet Halley.
Comet Halley passes close to Earth and the Sun approximately every 76 years, although the exact
period has varied from 75 to 79 years. The comet’s brightness on each pass varies significantly.
Probably the best appearance was in 1837, when Halley’s Comet reached a brightness of magnitude -3
(almost as bright as Venus), and its tail covered more than 60 degrees in the sky. Halley’s last
appearance in 1986 was relatively disappointing, because its brightness only reached magnitude 2.6.
The Giotto spacecraft visited Comet Halley and took close-up images of the nucleus during its 1986
flyby. Two minor meteor showers, the Eta Aquarids and the Orionids, are caused by Earth’s passing
through debris left behind by past visits of Halley’s Comet.
A Recent Visitor
Astronomers Alan Hale and Thomas Bopp separately discovered Comet Hale-Bopp on July 23, 1995.
Both astronomers were observing the globular cluster M70 when they noticed the mysterious new
object, the most recent great comet to pass close to Earth.
When Comet Hale-Bopp was discovered, it was still 7.15 AU from Earth, halfway between the orbits
of Jupiter and Saturn. Comets aren’t usually discovered until they’re much closer, causing scientists
to predict that Hale-Bopp was an unusually large comet and one that would become quite bright
during its journey into the inner solar system.
Over the next two years, Hale-Bopp more than lived up to expectations. At its brightest in
early 1997, it reached magnitude -1. A straight gas tail and a curved dust tail both stretched across 20
degrees of sky. By the time Hale-Bopp finally faded away near the end of 1997, it had been visible
to the naked eye for 18 months.
Some 4,000 years from now, this mighty comet will return again.
Solar System: Small Bodies
Comets:
#1 Watch this comet as it enters the inner solar system.
a) Describe what the comet looks like.
b) List two things that happen to the comet’s tail as it passes the Sun.
#2 Most people have heard of Halley’s comet. It has visited the Earth many times and was
recorded as far back as 1066 AD.
In the main window you can see the orbit of Halley’s comet. The comet itself is much too small
to be seen. Several planets’ orbits are shown for comparison.
a) Describe the size and shape of the comet’s orbit.
(Note that the comet is as far from the Sun as it can get.)
b) What is the date? (Only the year is important)__________________________
c) Does the comet maintain its speed as it orbits the Sun? Describe and explain any differences.
d) How long does it take Comet Halley do go once around the Sun?_____________________
e) Predict when comet Halley will be close to the Sun again.__________________________
Solar System: Small Bodies
Comets:
#3 As a comet orbits the Sun, many small bits and pieces break off. These pieces are spread around
the comet’s orbit. When the Earth comes near the comet’s orbit, we see these pieces burning
up in the Earth’s atmosphere. We call them “shooting stars”. Scientists call them meteor showers.
In the main window, the blue line represents comet Halley’s orbit. Notice where it meets the
Earth’s orbit (the green line).
a) On what dates (approximately) can we expect a meteor shower?__________________________
b) Does comet Halley have to be near the Earth to give us a meteor shower? Explain
#4 Compare similarities and differences between asteroids and comets by completing the table:
Characteristic
size
appearance
composition
Orbit location
Orbital shape
Inclination of orbit
compared to plane of
the solar system
Length of orbital or
period of orbit
Asteroids
Comets