The Ice Dwarf Cometh

Pluto and Charon
Artist’s concept of the New Horizons spacecraft during a planned encounter with Pluto and its moon, Charon.
Image Credit: The Johns Hopkins University Applied Physics Laboratory, New Horizons

The New Horizons mission is currently preparing for a mid-January launch to Pluto.

Pluto is one of the most mysterious objects in our solar system. Ten thousand times fainter than the eye can see, and a hundred times smaller than Mars in the night sky, Pluto has never been investigated up-close by a spacecraft. The best image of Pluto, obtained by the Hubble Space Telescope, is just a crude grouping of bright and dark pixels.

"[Pluto] raises images of something very dark, far away, and cold," says Andy Dantzler, director of NASA’s solar system division. "Actually, Pluto is a treasure trove of scientific discoveries just waiting to be uncovered. It’s different from the inner rocky planets; it’s different from the outer gaseous planets. And as such, it holds many clues as to how the solar system was formed, and also, perhaps, how other systems are formed."

After launch, the New Horizons spacecraft will speed away from Earth at 8 miles per second, reaching the moon in about 8 hours (the Apollo missions took several days to cover this same distance). In early 2007, a flyby of Jupiter will increase the spacecraft’s speed by an additional 2.5 miles per second, and also provide an opportunity to test the spacecraft instruments. Depending on the launch date, the spacecraft could reach Pluto by mid-2015.

This illustration shows the Pluto system from the surface of one of the candidate moons. Pluto is the large disk at center right. Charon is the smaller disk to the right of Pluto. The other candidate moon is the bright dot on the far left.
Credit: NASA, ESA and G. Bacon (STScI)

The spacecraft has seven science instruments — three are cameras, while three others will measure solar wind, space dust, and energetic particles. A radio instrument will measure Pluto’s surface temperatures and atmosphere.

"It’s a scientific wonderland for atmospheric scientists," says New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "[Pluto’s] atmosphere is escaping like a comet, but on a planetary scale. Its surface, which is only 40 degrees above absolute zero, is covered in exotic ices."

The spacecraft also has a CD-ROM inscribed with about half a million names of people who signed up on the Internet. A nuclear power supply will generate electricity during the long mission.

Pluto’s moon Charon is large enough that some consider them to be a binary planetary system. Astronomers recently discovered two smaller moons orbiting more than twice as far away as Charon. The goal of the New Horizons mission is to map the sunlit portions of Pluto, Charon, and these newly discovered satellites.

Pluto is embedded in the Kuiper Belt, an enormous disc-shaped region of ice and dust that encircles our solar system. The half-million icy bodies that make up this belt are thought to be over 4 billion years old. Periodically an object gets kicked out of the belt and becomes a short-period comet.

Pluto and Charon
Artist’s conception of Pluto and Charon

After visiting Pluto, Charon, and the two newly discovered moons, an extended mission would allow the New Horizons spacecraft to encounter one or two of these Kuiper Belt Objects (KBOs). Because the Kuiper Belt is so vast, the closest KBO to Pluto is about 1 AU away, the same distance between the Earth and the Sun.

There has been much debate about whether Pluto is a planet or just a large KBO. To Stern, there is no question that Pluto is a planet. He also says there could be hundreds if not thousands of such "ice dwarf" planets in the Kuiper Belt, making them the most common type of planetary body in our solar system.

"Just as a Chihuahua is still a dog, these ice dwarfs are still planetary bodies," says Stern. "The Pluto-like objects are more typical in our solar system than the nearby planets we first knew. And the opportunity is to go now and have a chance to study this most common type of planetary body in the solar system for the first time."

The New Horizons launch window begins at 1:24 pm Eastern Standard Time on January 17, and lasts until February 14. If the spacecraft is unable to launch from Cape Canaveral, Florida during this time, mission scientists will have to wait until February 2007 for a second opportunity.

Related Web Pages

Santa et al.
Charon’s Occultation
A New Class of Planet
Tenth Planet
Pluto’s Pair
Solar System Exploration Survey
Pluto-Charon System