Page 413 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch15_377-404.indd Page 390 9/3/10 6:17 PM user-f465
tiL12214_ch15_377-404.indd Page 390 9/3/10 6:17 PM user-f465 /Users/user-f465/Desktop

TABLE 15.4

Spacecraft missions to study comets and asteroids
Date Name Owner Remark
Sep 11, ISSE 3 U.S. Studies of electric and
1985 (or ICE) magnetic fields around
Giacobini-Zinner comet
from 7,860 km (4,880 mi)
Mar 6, Vega 1 U.S.S.R. Photos and studies of
1986 nucleus of Halley’s comet
from 8,892 km (5,525 mi)
Mar 8, Suisei Japan Studied hydrogen halo
1986 of Halley’s comet from
151,000 km (93,800 mi)
Mar 9, Vega 2 U.S.S.R. Photos and studies of
1986 nucleus of Halley’s comet
from 8,034 km (4,992 mi)
Mar 11, Sakigake Japan Studied solar wind in
1986 front of Halley’s comet
7.1 million km
(4.4 million mi)
Mar 28, Giotto ESA Photos and studies of
1986 Halley’s comet from
541 km (336 mi)
FIGURE 15.13 This is a photo image of Neptune taken by
Voyager. Neptune has a turbulent atmosphere over a very cold sur- Mar 28, ISSE 3 U.S. Studies of electric and
face of frozen hydrogen and helium. 1986 (or ICE) magnetic fields around
Halley’s comet from
32 million km
(20 million mi)
Feb 17, NEAR Shoe- U.S. Studied the asteroid Eros
1996 maker
Oct 24, Deep U.S. Flyby of asteroid Braille and
Atmosphere of hydrogen 1998 Space 1 comet Borrelly
and helium with ammonia
and methane clouds Feb 7, Stardust U.S. Studied comet Wild 2 and
1999 returned a sample of cosmic
Water and water ice dust to Earth
(about 8,000 km)
Mar 2, Rosetta ESA Two probes will be launched
2004 into comet 67PChruyumov-
Rocky silicate core Gerasimenko on Nov 2014
(about one Earth mass)
Jan 12, Deep U.S. Studied comet Tempel 1 by
2005 Impact sending impact probe on
Jul 4, 2005
Sep 27, Dawn U.S. Studied the two most
2007 massive asteroids Ceres
FIGURE 15.14 The structure of Uranus and Neptune. and Vesta

The current theory of the origin of comets was developed
by the Dutch astronomer Jan Oort in 1950. According to the one-fourth that of solid water ice. As the comet nucleus moves
theory, the huge cloud and the belt of icy, dusty aggregates are toward the Sun, it warms from the increasingly intense solar
leftovers from the formation of the solar system and have been radiation. Somewhere between Jupiter and Mars, the ice and
slowly circling the solar system ever since it formed. Something, frozen gases begin to vaporize, releasing both grains of dust
perhaps a gravitational nudge from a passing star, moves one of and evaporated ices. These materials form a large, hazy head
the icy bodies enough that it is pulled toward the Sun in what around the comet called a coma. The coma grows larger with
will become an extremely elongated elliptical orbit. The icy, increased vaporization, perhaps several hundred or thousands
dusty body forms the only substantial part of a comet, and the of km across. The coma reflects sunlight as well as producing its
body is called the comet nucleus. own light, making it visible from Earth. The coma generally ap-
Observations by the Vega and Giotto spacecrafts found the pears when a comet is within about 3 AU of the Sun. It reaches
nucleus of Halley’s comet to be an elongated mass of about 8 its maximum diameter about 1.5 AU from the Sun. The nucleus
by 11 km (about 5 by 7 mi) with an overall density less than and coma together are called the head of the comet. In addition,

390 CHAPTER 15 The Solar System 15-14