Fire and Ice

The solar system’s farthest known planetary outpost is closer to getting its first visitor. NASA gave its go-ahead to start full development of the first mission to Pluto and the Kuiper Belt, while continuing its plans also to orbit our solar system’s innermost planet, Mercury. Interestingly despite being billions of miles separated, both these destinations hold some promise of having water ice, an always intriguing ingredient for astrobiologists to explore.

The Pluto mission, called the New Horizons spacecraft, is scheduled to launch in January 2006, swing past Jupiter for a gravity boost and scientific studies in 2007, and reach Pluto and its moon, Charon, as early as summer 2015. The arrival date depends on the launch vehicle NASA selects for the mission this summer – either a Boeing Delta 4 or Lockheed Martin Atlas 5. Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.

Solar System panorama, not to scale
Credit: NASA

After a 6-month encounter with Pluto-Charon – during which New Horizons will characterize Pluto’s and Charon’s global geology and geomorphology, map their surface compositions and temperatures, and examine Pluto’s complex atmosphere – the spacecraft will head deeper in to the Kuiper Belt to study one or more of the icy mini-worlds in that vast region. The Kuiper belt, an icy debris field of comet-like bodies, extends 7 billion miles beyond Neptune’s orbit. Over the past decade more than 500 icy worlds have been found in the Kuiper belt. With a few exceptions all have been significantly smaller than Pluto.

There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects (also known as Trans-Neptunian Objects). The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). Pluto’s composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Neptune’s moon Triton.

Found just 73 years ago, Pluto was discovered in 1930 in the course of a 15-year search for trans-Neptunian planets. It wasn’t realized until much later that Pluto actually was the largest of the known Kuiper belt objects. The Kuiper belt wasn’t theorized until 1950, after comet orbits provided telltale evidence of a vast nesting ground for comets just beyond Neptune. The first recognized Kuiper belt objects were not discovered until the early 1990s.

Other worlds with thin atmospheres include Pluto, Neptune’s moon Triton or Jupiter’s moon Io. Pluto’s 10 to 20 microbar nitrogen atmosphere is anticipated to have very calm winds. NASA mission planners want to arrive at Pluto while the atmosphere is still seasonally unfrozen.

Pluto’s only moon, Charon, was discovered only in 1978. Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth’s Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. Its surface seems to be covered with water ice.

Cutting Metal

"We’ve designed the mission, the spacecraft and the instruments, and we’re ready to start cutting metal," says New Horizons Project Manager Thomas Coughlin, of the Applied Physics Laboratory (APL), Laurel, Md. "This is the time in a mission when things really start rolling toward launch. We have less than three years to go and there is a lot to do between now and then – and we’re excited to get moving on it."

Artist's rendition of Kuiper belt
Artist’s rendition of Kuiper belt.
Credit: David Jewitt

Dr. Alan Stern, director of the Southwest Research Institute’s Department of Space Studies in Boulder, Colo., is the mission’s principal investigator and leads an unprecedented science effort. "This is exploration at its greatest, as only the U.S. space program can do," Stern says. "New Horizons will reconnoiter the great, unexplored ‘third zone’ of our solar system and make a historic flyby of the outermost known planet."

NASA tapped the APL-SwRI team to conduct its Pluto-Kuiper Belt mission in November 2001, and preliminary design work began in January 2002.

"The systems and instruments have all been on the drawing board and we’ve gone over many details," says David Kusnierkiewicz, New Horizons mission systems engineer at APL. "Now we’ve honed in on specific designs and we’re ready to start putting systems and instruments together."

Assembly has already started on New Horizons’ scientific instruments and the team will begin fabricating parts of the spacecraft’s structure next month. Baseline plans for the New Horizons mission include use of a radioisotope thermoelectric generator (RTG), which could supply over 200 watts of electrical power for the spacecraft. The mission’s next major milestone is a critical design review in early August; if that goes as expected, spacecraft integration and testing would begin in May 2004.

At the Ends of the Solar System

That date will come two months after the scheduled launch of another APL spacecraft – the MErcury Surface Space Environment, GEochemistry and Ranging (MESSENGER) satellite – which is set to become the first spacecraft to orbit Mercury. MESSENGER, now under construction, will launch aboard a Delta 2 rocket from Cape Canaveral Air Force Station, Fla., in March 2004 and begin a yearlong orbit study of Mercury in April 2009. Because it moves so quickly across the sky, Mercury was named for the Greek ‘winged messenger’.

"We have the unique opportunity to complete the exploration of the planets, while traveling to the solar system’s extremes," says Dr. Stamatios Krimigis, head of the APL Space Department. "Before the end of the decade we are going to visit the largely unexplored innermost planet, where surface temperatures are near 845 degrees Fahrenheit, and the thermal environment for our spacecraft will be rather demanding. And we’re leading a mission to the outermost planet, where estimated temperatures are minus 390 degrees Fahrenheit. It’s an incredible challenge and a chance to make history."

Mercury is nearly entirely made of a dense iron core, with a very thin crust. Mercury is the second densest major body in the solar system, after Earth . Amazingly given its enormously high daily temperatures, radar observations of Mercury’s north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

What’s Next

If Pluto is the only planet never visited by a spacecraft, it is not surprising to discover how many basic questions still need answering about its seasons and make-up. It along with the far distant icy-worlds in its extended neighborhood, have been characterized as a third zone in our solar system, to supplement the inner and outer planets, and thus take account of its unique geology. What are the dark areas on Pluto composed of? What happens to its thin atmosphere during seasons and Pluto’s long year?

While Mercury was visited by a mapping mission, many mysteries remain as well. In appearance Mercury resembles our own Moon, but given its dense iron core, has a suprising lack of surface iron. Researchers would like to discover if it has some unique geology that strongly differentiates its crust and core. This striation becomes particularly unusual given its smooth plains, which disguise the upheavals typical of its polar craters and whatever rocks might be transported to the surface during these impacts.

APL manages the mission for NASA and will design, build and operate the New Horizons spacecraft. This week NASA gave approval to The Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute and their partners for the New Horizons spacecraft. The European Space Agency (ESA) will also send a spacecraft to orbit Mercury called BepiColombo.