315
See also: The Oort cloud 206 ■ The composition of comets 207 ■
Exploring the solar system 260–67 ■ Exploring beyond Neptune 286–87
squeezed. By 2000, the plans had
been shelved, but Stern made the
case for sending a mission to Pluto,
the smallest, most distant planet,
which had been discovered by US
astronomer Clyde Tombaugh in 1930.
In 2003, Stern’s New Horizons
proposal was given the green
light, and the 2006 launch set the
spacecraft on a nine-year flight to
Pluto. It occurred not a moment too
soon. In August 2006, prompted
by the discovery of a possible
tenth planet beyond the orbit of
Pluto, the general assembly of the
International Astronomical Union
(IAU) gathered in Prague to discuss
issues raised by the new discovery.
The first question was whether it
was a planet at all. The IAU agreed
that the new body, to be named
Eris, was not a planet. Its gravity
was too weak to clear other bodies
from its orbit. The planets from
Mercury to Neptune are big enough
to do this, but the bodies of the
asteroid belt manifestly are not—
and nor was Pluto. However,
Pluto and Eris were not like most
asteroids. They were massive
enough to be spherical rather than
irregular chunks of rock and ice.
So the IAU created a new class of
object: that of dwarf planet. Pluto,
Eris, and several large Kuiper Belt
Objects (KBOs) were given dwarf
planet status, as was Ceres, the
largest body in the asteroid belt.
For most of these objects, this was
a promotion in the hierarchy of
the solar system, but not for Pluto.
If Pluto had been declassified as
a planet prior to New Horizons’
launch, it is uncertain whether
the mission would have happened.Long journey
Although Pluto’s orbit does bring
it closer to the sun than Neptune
for some of its 248-year revolution
around the sun, the New Horizons
probe had the longest journey to the
most distant target in the history ❯❯THE TRIUMPH OF TECHNOLOGY
The spacecraft has revealed that
Pluto’s icy structure is a completely
new kind of planetary body.Pluto is too far away
to observe details with
a telescope.The only way to
study Pluto is to send
a spacecraft.Alan Stern
Born Sol Alan Stern in New
Orleans, Louisiana, in 1957,
Stern’s fascination with
Pluto began in 1989 when
he worked with the Voyager
program. While he was there,
Stern witnessed the final
encounter of Voyager 2
as it flew past Neptune and its
moon Triton. Triton appeared
as a ball of ice, and looked
very much like the Pluto
Stern and other scientists
had imagined. (Triton is
thought to be a Kuiper Belt
Object that has been captured
by Neptune.)
In the 1990s, Stern trained
as a Space Shuttle payload
specialist (technical expert),
but he never got the chance
to fly into space. Instead, he
returned to the study of Pluto,
the Kuiper belt, and the Oort
cloud. In addition to leading
the New Horizons mission
as principal investigator,
Stern is active in developing
new instruments for space
exploration and more cost-
effective ways of putting
astronauts into orbit.Key work2005 Pluto and Charon: Ice
Worlds on the Ragged Edge
of the solar systemJust as a Chihuahua is still
a dog, these ice dwarfs
are still planetary bodies.
Alan Stern