The Journey of Flight.pdf - Valkyrie Cadet

Chapter Chapter 25 25 - - Our Our Solar Solar System System
System
Comet West as Seen Above Table Mountain in California
(NASA Courtesy of the Jet Propulsion Laboratory)
English astronomer Sir Edmund Halley first suggested that comets were members of our solar
system. After studying historical writings of sightings of bright objects in the sky, he predicted the
appearance of a comet in 1758. When it appeared right on schedule, it proved his theory and the comet
was named after him. Halley’s Comet continues to make regular appearances in our skies; it last
approached the Sun in 1996 and will return in 2061.
Comets originate at the very edge of the solar system and are probably simply icy material mixed
with dust that failed to come together to form true planets. The Oort Cloud is the source of long-period
comets and possibly higher-inclination intermediate comets that were pulled into shorter period orbits
by the planets, such as Halley and Swift-Tuttle. Comets can also shift their orbits as a result of jets of
gas and dust that rocket from their icy surfaces as they approach the Sun.
Although they can change their course, comets have initial orbits with widely different ranges.
Long-period comets come from the Oort Cloud; Hyakutake and Hale-Bopp are two recent examples.
The Oort Cloud itself is an immense spherical cloud surrounding the planetary system and extending
approximately three light years from the Sun. This distance is considered to be the edge of the Sun’s
range of physical, gravitational, and dynamic influence.
The cloud structure is believed to be a relatively dense core that lies near the ecliptic plane and
slowly replenishes the outer boundaries. One sixth of an estimated six trillion icy objects are in the
outer region of the cloud; the remainder are found in the core. The total mass of the Oort Cloud is
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