560 Encyclopedia of the Solar System
FIGURE 3 Spectacular image of comet Tempel 1 taken from
Deep Impact’sflyby spacecraft 67 seconds after the impactor
spacecraft’s impact. The linear spokes of light radiate away from
the impact site. Light from the collision site saturated the
camera’s detector. Compare with Figure 2D. (Courtesy of
NASA/JPL-Caltech/UMD.)
The impactor spacecraft delivered 19 GJ of kinetic en-
ergy to comet Tempel 1. The spectacular impact is shown
in Fig. 3, and a view of the ejecta plume containing∼ 106
kg of material is shown in Fig. 4. In addition to observations
fromDeep Impact, the event was extensively observed by
ground-based and space-based observatories. By 9 July the
comet had returned to its pre-impact state; the impact crater
has not been seen. The ejecta consisted of fine particles
(1–100μm) and individual species, including water,water
ice, carbon, carbon dioxide, hydrocarbons, and crystalline
silicates. The spectra of the ejecta are a good match to the
spectra of material ejected from comet Hale–Bopp and to
the dusty disk spectrum of a young stellar object.
All these images confirm the basic view of the nucleus
as a single, sublimating (direct-phase transition from the
solid to the gas state) body as proposed by F. L. Whipple.
As the solid body approaches the Sun, energy supplied by
solar radiation raises the temperature of the near-surface
layers, sublimation of ices (mostly water ice) takes place,
and the emission of gas and entrained dust produces the
large features seen in the sky.
Before leaving space missions, it is important to note that
E S A’sRosettamission to comet Churyumov–Gerasimenko
was launched on 2 March 2004 to begin its 10-year journey
to the comet. The plan is for the main spacecraft to spend
approximately 2 years in the vicinity of the comet and to
place a lander on the surface.
FIGURE 4 Image of comet Tempel 1 taken fromDeep Impact’s
flyby spacecraft 50 minutes after impact showing the plume of
ejected material. The comet’s nucleus is mostly in shadow with
the sunlit portion visible on the right-hand side. (Courtesy of
NASA/JPL-Caltech/UMD.)2. A Brief History of Comet Studies
The realization that the nucleus of a comet was a single, sub-
limating body prior to the confirmation by direct imaging
was the result of several lines of reasoning. In the 17th cen-
tury, it was known that the part of a comet’s orbit near the
Sun could often be accurately represented by a parabola
with the Sun at the focus. This idea was used by Isaac
Newton to determine a parabolic orbit for the comet of- Edmond Halley refined the calculation and showed
that an ellipse of high eccentricity very accurately repre-
sented the comet’s orbit. Comet orbits generally are ellipses
with high eccentricities. Halley continued to determine the
orbits of comets and found that the orbits of comets ob-
served in 1531, 1607, and 1682 were quite similar and had
periods of approximately 75 to 76 years. This was the basis
of his famous prediction that the comet that now bears his
name would return in 1758.
A complication in the detailed orbit calculations was that
Jupiter and Saturn would perturb the orbit through their
gravitational attraction. Halley’s comet passed perihelion
in early 1759. The successful prediction of the return of
Halley’s comet began the development of celestial mechan-
ics and the positional astronomy of comets that flourished in
the 18th and 19th centuries. But the orbit of comet Encke
presented another problem. The comet had a very short
period of 3.3 years. Many orbits were observed, and it would