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National Aeronautics and Space Administration
National Aeronautics and Space Administration
Supplemental Educational Support Materials
for Special Feature: “An Icy Traveler Sweeps Past Mars”
Discussion questions
Q1:
What is a comet, and how does it change as it gets closer to the Sun?
Answer:
A comet is, at its core, a dusty chunk of ice and rock ranging in size from
a few kilometers to tens of kilometers across. The core is called the “nucleus”
of the comet. When a comet gets closer to the Sun, the ice near its surface is
heated, releasing gas and dust. This expelled gas and dust forms a bright, cloudlike coma around the nucleus that shines brightly in reflected sunlight. The gas
and dust in the coma is swept back by the solar wind to form one or more tails.
These tails always point away from the Sun. The thicker dust tail, made of the
comet’s dust, usually has a slight curve in the direction of the comet’s orbit, but
the ion tail, made up of ionized gas, will be thinner and straight. The shape and
color of the tails tells astronomers about the composition of the comet.
Q2:
What is the difference between a short- and a long-period comet?
Answer:
Short-period comets take less than 200 years to orbit the Sun. These comets
originate in the Kuiper Belt — the region beyond Neptune’s orbit. These comets
can have their orbits disturbed by the giant planets and end up within the inner
solar system. Many short-period comets have orbits in the same region as the
asteroid belt, located between the orbits of Mars and Jupiter. There may be as
many as 100 million objects in the Kuiper Belt.
Long-period comets require more than 200 years to orbit the Sun, generally
taking many thousands to a few millions of years. These comets spend most of
their time in the vast spherical region in the outer reaches of our solar system
known as the Oort Cloud. Unlike the Kuiper Belt, which is a relatively flat belt
of material, the Oort Cloud surrounds the solar system. Trillions of long-period
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Educational Product
Educators & Students Grades 5–8
comets must reside in this region, but we only can observe the ones with
very elongated orbits that take them into the inner solar system. They can
come from any direction and are usually only observed once.
Q3:
Choose a short- and/or a long-period comet that interests you, and explain
why you chose the one (or ones) you did.
Answer:
Student answers will vary. Some short-period comets include Halley’s
Comet, Comet Holmes, Comet Encke, Comet 9P/Tempel (also known as
Temple 1) and Comet 81P/Wild (also known as Wild 2).
Famous long-period comets include Hale-Bopp, Comet Hyakutake, Comet
West, Comet McNaught, Comet Lovejoy, Comet Shoemaker-Levy 9, and
Comet ISON.
Students may choose a comet because they have heard of it, as in Comet
Halley, Comet Shoemaker-Levy 9, Comet Lovejoy, or Comet ISON. They
may have chosen a particular comet because they read about it in the Star
Witness News series, which featured Comet ISON (October 2013), Comet
Holmes (November 2007), or Comet 9P/Tempel (July 2005) and mentioned
Comet Shoemaker-Levy 9 in several stories. Students may have seen, heard,
or read about any number of comets, and will have a personal reason for
their selection.
Vocabulary words
Astronomer
A scientist who studies the universe and the celestial bodies residing in it,
including their location, motion, composition, and history. Many of the scientists at
the Space Telescope Science Institute are astronomers. Astronomers from all over
the world use the Hubble Space Telescope.
Astronomical Unit (AU)
The average distance between Earth and the Sun, which is about 150 million
kilometers (93 million miles). This unit of length is commonly used for measuring
the distances between objects within the solar system.
Comet
A ball of rock and ice, often referred to as a “dirty snowball.” Typically a few
kilometers in diameter, comets orbit the Sun in paths that either allow them to
pass by the Sun only once or that repeatedly bring them through the solar system
(as in the 76-year orbit of Halley’s Comet). A comet’s “signature” long, glowing tail
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is formed when the Sun’s heat warms the coma or nucleus, which releases vapors
into space.
Hubble Space Telescope
An orbiting telescope that collects light from celestial objects in visible, nearultraviolet, and near-infrared wavelengths. The telescope’s primary mirror is 2.4
meters (8 feet) wide. It orbits the Earth about every 96 minutes and is powered by
sunlight collected with its two solar arrays.
Observatory
A structure designed and equipped for making astronomical observations.
Observatories are located on Earth and in space.
Oort Cloud
A vast spherical region in the outer reaches of our solar system where billions
of long-period comets (those with orbital periods greater than 200 years) reside.
Comets from the Oort Cloud come from all directions, often from as far away as
50,000 astronomical units (4.6 trillion miles).
Education Standards
Common Core Standards for English Language Arts
http://www.corestandards.org/ELA-Literacy/CCRA/R/
College and Career Readiness Anchor Standard for Reading
CCSS.ELA-Literacy.CCRA.R.10
Read and comprehend complex literary and
informational texts independently and proficiently.