Page 313 - NS-2 Textbook

308 NAUTICAL SCIENCES

what seemed to be a system of rings in the space between
the planet and its five larger moons.
Voyager 2 flew by Uranus on 24 January 1986 and ver-
ified the presence of the rings. It assessed them as com-
posed of small gravel-like rocks and stones. It also dis-
covered ten small moons, unobsen'able from Earth.
Another significant discovery about the planet was that
its magnetic axis is inclined about 65 degrees with respect
to its spin axis, which was thought to make it unique
among the planets of our solar system until Voyager 2
found a similar situation at Neptune in 1989.

NEPTUNE

Uranus and Neptune are often called the twin planets,
even though the latter is more than 1 billion miles farther
from the Sun. They are similar, though, in size (roughly
An amazing image of Saturn's small moon Mimas, taken in August
2005 by the Cassin; spacecraft as it flew nearby. The feature in the 30,000 miles in diameter) and composition.
upper right is the Herschel Crater, an 88-mile-wide crater thought to When it was discovered that Uranus did not travel in
be the result of an ancient impact. NASAfJPLlSpace Science Institute its regular orbit at all times, astronomers figured that
there had to be some object whose gravity pulled Uranus
off its path. Astronomers calculated the probable position
of such an object-and thus found the planet Neptune in
1848.
Much of what we now know about Neptune and its
satellites was discovered by Voyager 2 when it passed
about 2,900 miles above the planet's north pole on 25 Au-
gust 1989. Neptune's orbital period around the Sun is 165
years. Voyagers spectroscopy readings showed that Nep-
tune's atmosphere is mainly hydrogen, with some helium
and about 2 percent methane, which gives it a blue color.
The interior is probably rock and water ice. Violent winds
as high as 1,250 miles per hour were observed. Voyager's
magnetometer discovered that Neptune's magnetic axis
is tilted 47 degrees from the planet's rotational axis, simi-
lar to Uranus. Why these phenomena exist at both planets
remains a matter of scientific speculation.
Voyager also found three rings of dark, carbon-like
material surrounding the planet and six previously un-
known small moons circling it.
Following its encounter with Neptune, Voyager 2 went
on to pass close to its largest moon, Triton. The spacecraft
revealed even more amazing facts about this satellite. Its
surface temperahlre was found to be -391 degrees F, mak-
ing it the coldest body in our solar system, only 69 degrees
F above absolute zero. Surface features strongly suggest
the possibility of large-scale water-ice volcanism, making
it unique in this respect in the solar system.

Variations in the orbit of Uranus (perturbations) led astronomers to
believe that it was being influenced by the gravitational pull of an
unknown planet beyond it. In 1930 Clyde Tombaugh discovered the PLUTO
ninth planet after examining a series of telescope photographs. This
photo shows Uranus as viewed from the twenty-four-inch telescope The pertllrbations (variations in the regular orbit) of
at lowell Observatory, Flagstaff, Arizona. Uranus were not completely explained by the discovery