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EXPLORING COMETS, THE SCIENCE OBJECTIVES OF THE DEEP IMPACT MISSION - PART 1
This article is the first in a series on the science of the Deep Impact project. The first section of the article provides the
background needed to understand the questions for which the project seeks answers. The second section explains why
knowledge of comets is so important to astronomers. The third section describes how the mission will capture the data it needs.
Finally, the last section briefly states the questions to be answered.
Future issues of Deep Impact News will continue to explain the science and describe how the captured data will be used to
answer the questions.
Some Background About Comets
Comets are found in several regions of the solar system. The most remote ones live in the Oort cloud, a spherical shell of
comets located ten thousand times farther from the sun than the earth. It is thought that perhaps the gravity of passing stars tugs
on these comets and starts them spiraling inward toward the sun (or outward into interstellar space).
Comets (among some asteroid-like objects) also inhabit the Kuiper belt, a disc-shaped region starting beyond the orbit of
Neptune and extending out for several hundred earth-orbits.
In addition, some comets orbit the sun inside the Kuiper Belt, in the realm of the planets. Some venture so close to the sun
that they fall into it. The sun-observing satellite SOHO has sent us pictures of such a capture. See
for pictures and text. These links are courtesy of SOHO (ESA & NASA).
Three main components make up a comet: the coma, a fuzzy looking cloud of gas and dust surrounding a nucleus that is a
mixture of ice and rocks, and a tail that also consists of gas and dust.
Astronomers have found several comets that have an oblong shape. Tempel 1, our comet of interest, is one of them. About
6 kilometers long with a roughly rectangular cross section 2.4 kilometers on a side, it is shaped sort of like a cucumber. (Six
kilometers is about 3.7 miles.)
Why It Is Important to Know About Comets
Comets are a sample of the solar system taken when it was very young. To put that into time-perspective, note that the latest
estimate of the age of the universe is 13.7 billion years, that the age of the solar system is about 4.5 billion years and that comets
and planets were essentially complete roughly 4.0 billion years ago.
Although the planets formed early in the life of the solar system, they have continued to change. On Earth, for example,
continents shift, molten rock wells up through its crust, drying seas lay down layers of sedimentary rock, rivers erode the landscape,
and comets and asteroids bombard the surface. There is not a planet or moon that has escaped at least one of these.
Comets, however, are different in several ways. They don't have continents or molten rock. There are no seas or rivers.
There is no atmosphere, save the coma, and so there is no weathering, as we know it on Earth.
Comets have spent most of their lives frozen in the Oort cloud and Kuiper Belt and have only recently arrived in the inner solar
system. Astronomers conclude, therefore, that the material that formed the comet originally lies frozen and within reach just
below a relatively thin crust.
Roughly half of the mass of comets consists of ices of volatile components: water, carbon dioxide and carbon monoxide mainly
but with traces of other compounds as well. The other half of the mass consists of rocklike dust bound together by the ices.
Planets, except for Pluto, orbit the sun elliptical paths that are nearly circular. Comets, however, orbit in elliptical paths that
are squashed. The sun is closer to one end of the ellipse than it is to the other. When comets are close to the sun, the suns rays
heat them and the ices turn directly into gas. That's how the coma and tail are formed. When the comet is far from the sun the
comet is cold and the ices remain frozen.
Exploring the mostly frozen comets gives us a window into the past, provides clues regarding the origin of the solar system
and a means for testing our theories of solar system evolution.
To Explore a Comet
Humans are insatiably curious about these relics of the solar system's past. To satisfy our curiosity, we will explore Comet
Tempel 1 by positioning a 360-kilogram (about 790 pounds) mass of copper in its way and observing the results. Over time, there
have been minor changes to the weight of the impactor and other Deep Impact materials list it as 370kg or 820 lbs.
Recall now that a flyby spacecraft carries the impactor spacecraft from earth to a position near the comet. There the impactor
and the flyby separate and, during the next 24 hours, the impactor guides itself to its final destination, directly in the path of the
The impactor spacecraft contains the copper impact body, scientific instruments, guidance computer, solid rocket attitude
control thrusters and communications gear. It will plough through the comet's thin crust and on into the interior where the heat
of friction between the impactor and the comet will cause it all to vaporize. A tremendous explosion will ensue. A crater will
form, dust and gas, the ejecta, will pour forth. The flyby spacecraft will observe all this.
Both the flyby spacecraft and the impactor spacecraft carry instruments. The flyby spacecraft carries the High Resolution
Instrument (HRI), a 30 cm (12 inch) telescope. A spectrometer in the HRI can record images of in one of several narrow color
bands. The HRI can also record images in much broader bands of light by means of color filters.
The flyby spacecraft also carries a Medium Resolution Instrument (MRI), which is a 12 cm (5 inch) telescope that can take
pictures through its own eight-color filter but cannot record spectra.
The impactor spacecraft, besides carrying the 360 kg impact body, carries the Impactor Targeting Sensor (ITS), a near
duplicate of the flyby spacecraft's Medium Resolution Instrument (it has no color filters).
What Will We Learn
The Deep Impact mission to Tempel 1 focuses on seven questions. (1) What are some basic properties of the nucleus, for
example: what does it's landscape look like, how dense is it, how strongly is it held together and how massive is it? (2) How
has the comet changed during its lifetime? (3) What kinds of ice remain unchanged from the comet's early days? (4) Can the
heat of the sun finally drive all the ice out of a comet so that it becomes extinct or will it only go to sleep perhaps to wake again?
(5) Do smaller comets collide and form larger comets? (6) Are there impact craters on comets like there are on moons and
asteroids? (7) Can the course of a comet be altered to reduce the effect of, or avoid, a collision with Earth?
To Be Continued...
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