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Planetary News: New Horizons (2006)New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper BeltBy Amir Alexander16 January 2006 A towering giant of a rocket sits quietly at Launch Complex 41 in Cape Canaveral, Florida. It is 60 meters (200 feet) tall and, with five solid fuel booster rockets attached at its base, weighs 575,000 kilograms (1.26 million pounds). It is an Atlas V, NASA's mightiest launcher. At 1:24 p.m. Eastern Standard Time tomorrow, if all goes well, the sleeping giant will come to life in a deafening roar of fire and smoke. Slowly at first, and then faster and faster, it will rise into the sky, shedding its solid boosters and then its first, second, and third stages in succession. By the time the sound and fury subside, only a small spacecraft will remain, streaking silently through the emptiness of interstellar space. So will begin the 9-year voyage of New Horizons, the first spacecraft destined to visit Pluto and the Kuiper belt. Why Go to Pluto?That no spacecraft has yet visited the ninth planet of our solar system is partly a result of its enormous distance from Earth, but also a matter of bad luck. When Voyager 2 visited Uranus and Neptune in 1985 and 1989, Pluto was out in a different part of its long orbit around the Sun and could not be reached. Voyager 1, which could potentially have reached Pluto, sacrificed its chance in favor of a more extensive look at Saturn and its fascinating giant moon, Titan. And so, for the past 17 years, Pluto has remained the only unvisited planet in our solar system. During this time, however, scientific interest in Pluto has grown by leaps and bounds. A small planet with a strangely inclined orbit, Pluto was once considered an aberration in the orderly architecture of the solar system. There were the rocky planets of the inner solar system -- Mercury, Venus, Earth, and Mars -- and there were the gas and ice giants of the outer solar system -- Jupiter, Saturn, Uranus, and Neptune. Pluto did not fit either of these categories and was something of a stepchild in the family of planets.
In 1992, astronomers at the Mauna Kea Observatory in Hawaii discovered a new object orbiting in Pluto's vicinity, proving that Pluto is not alone. Since then, about 1,000 more objects have been detected and tracked in the same region, firmly establishing the existence of a belt of rocky debris orbiting the Sun in the outer reaches of the solar system. Known as the Kuiper belt after astronomer Gerard Kuiper who predicted its presence in the 1950s, this region contains more objects than all other areas of the solar system combined! Far from being an anomaly, Pluto suddenly became the star representative of an unexplored region of the solar system, sometimes referred to as the "third zone." Not only is the third zone of the solar system unexplored, but it also offers planetary scientists something nothing else in the solar system can provide: a glimpse into the past. Left out in the cold distant edges of the solar system, Kuiper belt objects (KBOs) have remained largely unchanged since the formation of the solar system 4.5 billion years ago. The first and second zones (i.e. the inner solar system and the region of the giant planets) have been transformed by the effects of solar radiation and planetary formation, and in one case at least, by life. The Kuiper belt alone remained much as it was, a preserved fossil of the protoplanetary cloud of gas and debris from which the solar system emerged. Scientists, furthermore, believe that the organic compounds that made possible the emergence of life were brought to the Earth a by barrage of objects from the Kuiper belt. Traveling there is a journey to the very origins of our world and ourselves, and could help us learn more about how and why we came to be. Pluto is fascinating to scientists for other reasons as well. With its giant moon Charon, it is, for one, the only case of a "binary planet" in the solar system, in which both objects orbit a center of gravity, which lies in the space between them. And while there are many known binary asteroids and KBOs, Pluto is the only binary planet. Charon, furthermore, is the largest moon in the solar system in proportion to its planet, with the Earth's own moon a distant second. Scientists believe that both moons were formed as a result of a catastrophic impact of the planet with a large space body. This sent large amounts of debris into orbit, which eventually coalesced to form the moons we see today. By studying the Pluto-Charon system, scientists expect to learn a great deal about the Earth-Moon system as well. Then there is the matter of the appearance of the two large components of the Pluto system (which also includes two small moons discovered in 2005). When viewed from Earth-bound telescopes or with the Hubble Space Telescope, Pluto appears highly reflective and with distinct regional markings; Charon, in contrast, is darker, and fairly uniform in appearance. What does this contrast signify, and how did it come to be? And which of the two is more characteristic of other KBOs orbiting in the vicinity? The question is further compounded when scientists consider Triton -- Neptune's large and volcanically active moon -- which to all appearances seems like Pluto's twin. Will Pluto also show signs of volcanic activity? And are Pluto and Triton representative of a wider class of planet-like objects beyond the orbit of Neptune?
Pluto's remarkable atmosphere also offers scientists much to chew on. Although more than 7,000 times less dense than the Earth's atmosphere, it nevertheless contains three different gasses that regularly transition between gas and solid states: nitrogen, carbon monoxide, and methane (the Earth's atmosphere, by contrast, contains only one -- water vapor). Many scientists believe that as Pluto moves away from the Sun on its highly eccentric orbit, temperatures on the planet will drop far enough for the atmosphere to freeze and fall to the ground. In fact, it is not at all clear in what state New Horizons will find the atmosphere when it visits the planet 9 years from now. Furthermore, Pluto's atmosphere is unique in that it seems to "bleed" into space at an exceedingly fast rate, comparable to the gas and dust coming off a comet. While this process, known as "hydrodynamic escape," does not occur in the Earth's atmosphere, it may have been responsible for our atmosphere's rapid loss of hydrogen when our home planet was young. By studying Pluto, planetary scientists hope to learn more about the mechanisms of hydrodynamic escape and, by extension, more about our own planet's history. For all these reasons and others, the National Research Council in 2001 ranked a mission to Pluto and the Kuiper belt at the very top of its list of priorities for planetary missions. From the Pluto Underground to New HorizonsFired by the exploding scientific interest in Pluto and its environs, a group of young planetary scientists calling itself "The Pluto Underground" joined forces in 1989 to campaign for a mission to the ninth planet. But even with the weight of scientific opinion behind them, the project of sending a mission to Pluto faced almost insurmountable obstacles. In a time of shrinking budgets for space exploration, a risky reconnaissance mission whose journey time would be in the order of decades was a hard sell to NASA officials whose future careers did not seem to extend that long. Five different missions were proposed in succession, considered, and then cancelled without any of them ever leaving the drawing board. In the summer of 2000, NASA Administrator for Space Science Ed Weiler went so far as to declare the Pluto mission "over, cancelled, dead." Despite it all, the vision survived. By the end of 2000, NASA announced an open competition for Pluto mission proposals, and by the end of the following year, a winner was selected: New Horizons, led by Principal Investigator Alan Stern of the Southwest Research Institute, with the Applied Physics Laboratory (APL) at the Johns Hopkins University in Baltimore, Maryland as the chief spacecraft builder. Over the following years, New Horizons still faced a bumpy ride -- its budget repeatedly slashed by Congressional budget committees. Time after time, The Planetary Society joined forces with the New Horizons team in grassroots letter-writing campaigns in support of the Pluto mission, and each time the funding was restored. Now, after 17 years of intensive effort, New Horizons is finally set to launch on its long journey to the far edges of the solar system.
New Horizons is a relatively small spacecraft. At 478 kilograms (1,054 pounds) it is but a fraction of the mass of the giant Cassini, which weighed in at 5,712 kilograms (12,593 pounds) for its launch, but comparable in size to recent probes like Stardust (380 kilograms or 838 pounds) and the lost Mars Polar Lander (576 kilograms or 1,270 pounds). Its scientific payload weighs a modest 30 kilograms (66 pounds), and it will require a mere 30 Watts to operate. The source of this energy is a nuclear powered radioisotope thermoelectric generator (RTG), which provides New Horizons with 200 Watts throughout the mission. New Horizons' scientific package contains seven instruments, designed to map, scan, and gather data on the Pluto system and its neighborhood, the Kuiper belt. The instruments include "Ralph" -- a four-color mapping camera combined with an infrared spectrometer, designed to map the surface composition and temperature of Pluto and Charon, and "Alice" -- an ultraviolet spectrometer that will study the structure and composition of Pluto's upper atmosphere. LORRI, the "Long Range Reconnaissance Imager," is a high resolution camera fed by a telescope, which will observe Pluto during the final months of New Horizons' voyage to the planet and take high resolution images during the flyby. PEPSSI and SWAP are plasma sensors, designed to record and measure the escape of Pluto's atmosphere. REX is a radio science experiment, which will receive a strong signal from Earth via NASA's Deep Space Network, just as the spacecraft is passing "behind" Pluto as seen from Earth. The signal's refraction in Pluto's atmosphere will enable REX to determine the density and temperature of Pluto's lower atmosphere. Finally, the Student Dust Counter (SDC) will measure the density of interplanetary dust in the outer reaches of the solar system and the Kuiper belt. The JourneyA little more than a year after its launch, in March 2007, New Horizons will undergo the first major test of its long journey -- a flyby of Jupiter. The pass is necessary to provide the spacecraft with the gravity assist that will speed it on its way to Pluto in a (relatively) short 9 years. But the flyby is also an opportunity to learn more about our giant neighbor and test New Horizons' systems in the harsh conditions of outer space. During its closest encounter, the spacecraft will close in to a distance of only 30 Jovian radii from the planet -- four times closer than Cassini when that spacecraft flew by Jupiter on its way to Saturn. Then, after leaving Jupiter behind, New Horizons will settle down for an 8-year journey through the emptiness of the outer solar system. During this long stretch, the spacecraft's systems will be mostly in hibernation mode, awaking for periods of 60 days each year for maintenance and system check-ups. Six months prior to New Horizons' summer encounter with Pluto, the spacecraft will wake up and begin systematic monitoring of the planet. First, images approaching planet will be taken with LORRI through its telescopic lens, followed by Ralph. At the same time, SWAP and PEPSSI will begin their measurements of Pluto's atmosphere and SDC will monitor impacts from dust grains. The closest approach to Pluto will take place around July or August 2015, a time when Pluto will be diametrically opposed to the Sun when viewed from the Earth. This will reduce the interference in communication -- which could be caused by the solar wind -- and facilitate the REX experiment, which requires a strong radio signal to be sent from Earth. Alice, the ultraviolet spectrometer, will study the composition and varying density of Pluto's atmosphere by observing the Sun through the atmospheric layers.
The closest approach will be timed to coincide with Charon passing "behind" Pluto when viewed from the Sun. New Horizons will pass between Pluto and Charon, possibly as close as 3,000 kilometers (less than 2,000 miles) above the planet's surface, which will be illuminated by a soft glow of reflected light from its moon. This will enable LORRI and Ralph to take pictures of the planet with a resolution as high as 25 meters per pixel. During the hours before closest approach, New Horizons' instruments will obtain detailed maps of the topography and surface composition of Pluto and Charon. Once the spacecraft emerges from the shadow of Pluto, it will begin transmitting the data gathered back to Earth. Its primary mission complete, New Horizons will then set course for its next target -- a Kuiper belt object which has yet to be selected. This journey, too, may take years, but its rewards could be as great as those of the Pluto encounter itself. Today, New Horizons sits atop an Atlas V rocket -- a tiny appendage to NASA's largest and most powerful booster. The journey that brought it there lasted 17 long years of persistence, determination, and vision. The journey that will take it 6 billion kilometers, to the outer edges of the solar system, is about to begin.
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