2's view of Neptune, 57,000,000 kilometers (35 million
miles) away (Click on the image for a larger view)
summer of 1989, NASA's Voyager 2 became the first spacecraft
to observe the planet Neptune, its final planetary target.
Passing about 4,950 kilometers (3,000 miles) above Neptune's
north pole, Voyager 2 made its closest approach to any planet
12 years after leaving Earth in 1977. Five hours later, Voyager
2 passed about 40,000 kilometers (25,000 miles) from Neptune's
largest moon, Triton, the last solid body the spacecraft
will have an opportunity to study.
is one of the class of planets -- all of them beyond the
asteroid belt -- known as gas giants; the others in this
class are Jupiter, Saturn and Uranus. These planets are
about 4 to 12 times greater in diameter than Earth. They
have no solid surfaces but possess massive atmospheres that
contain substantial amounts of hydrogen and helium with
traces of other gases.
1, launched September 5, 1977, visited Jupiter in 1979 and
Saturn in 1980. It is now leaving the solar system, rising
above the ecliptic plane at an angle of about 35 degrees,
at a rate of about 520 million kilometers a year.
2, launched August 20, 1977, visited Jupiter in 1979, Saturn
in 1981 and Uranus in 1986 before making its closest approach
to Neptune on August 25, 1989. Voyager 2 traveled 12 years
at an average velocity of 19 kilometers a second (about
42,000 miles an hour) to reach Neptune, which is 30 times
farther from the Sun than Earth is. Voyager observed Neptune
almost continuously from June to October 1989. Now Voyager
2 is also headed out of the solar system, diving below the
ecliptic plane at an angle of about 48 degrees and a rate
of about 470 million kilometers a year.
spacecraft will continue to study ultraviolet sources among
the stars, and their fields and particles detectors will
continue to search for the boundary between the Sun's influence
and interstellar space. If all goes well, we will be able
to communicate with the two spacecraft for another 20 years,
until their radioactive power sources can no longer supply
enough electrical energy to power critical subsystems.