After over nine years of travel in deep space, NASA's New Horizons spacecraft is within hours of its historic flyby of Pluto. When the unmanned nuclear-powered probe speeds past the frozen dwarf planet tomorrow at 7:49 am EDT, it will mark not only the success of one of man's most ambitious space missions, but also the completion of the first era of planetary exploration that began in 1962 when the US Mariner 2 mission flew past Venus. Here's what to expect as events unfold.

Discovered in 1930 by American astronomer Clyde Tombaugh, Pluto is the final classical planet in the solar system to be visited by an unmanned spacecraft and has been the subject of controversy since being downgraded in 2006 to a dwarf planet; a decision still under debate. Until the 21st century, it's never been more than a faint, frozen, blurry dot, and to the average person it may seem as uninteresting and pointless to visit as a random rock in the Antarctic. However, there's a great deal more to it than ice cube jokes and HP Lovecraft references, which is why NASA spent US$720 million to get there.

Why Pluto

One reason why Pluto is of such interest to scientists is that it's different from every other planet in the Solar System. It isn't a terrestrial planet like Earth, nor is it a gas giant like Jupiter. Instead, it's an ice dwarf. That is, a dwarf planet that's composed largely of ice. Also, Pluto is the largest known resident of the Kuiper belt – a band of frozen asteroids, comets, and planetoids that begins at the orbit of Pluto. Little is known of that region, but by studying Pluto, it could not only shed more light on the belt, but also provide clues as to the origins of the Solar System and perhaps even life on Earth.

As to Pluto itself, it's not much to look at at first glance. It's smaller than the Moon, with a diameter of only about about 2,380 km (1,500 mi). Its composition is 35 percent ice and 65 percent rock, and has a very low density, as is reflected by its gravity, which is surprisingly low at only six percent of Earth gravity.

What Pluto lacks in size, its surface makes up for in unpleasantness. The estimated temperature on Pluto is -233º C (-387º F) and the surface is made of frozen nitrogen, carbon monoxide, methane, and ethane. Meanwhile there's a complex atmosphere of nitrogen, with traces of methane, carbon monoxide, and hydrocarbons. However, the pressure at the moment is 50,000 times less than on Earth, which makes it pretty much a vacuum, and what's there will probably all freeze as the Plutonian winter sets in over the next few decades.

Pluto has five known moons consisting of Charon and four smaller ones named Nix, Hydra, Styx, and Kerberos. The largest is Charon with a diameter of 1,200 km (745 mi). Like our Moon, Charon is tidally locked and always shows the same face to Pluto. However, Pluto is also locked, so it always shows the same face to Charon, which is so large in comparison to Pluto that the two form the only double planet in the Solar System, with each rotating every 6.4 Earth days about a common center of gravity located in space.

Encounter with Pluto

When New Horizons awoke from hibernation six months ago, it began an intense program of study in the run-up to the flyby as well as rehearsals for tomorrow's encounter, as the spacecraft's 16 hydrazine thrusters made trajectory adjustments and ran the probe through flyby simulations.

According to NASA, part of this program was making the most of the opportunity for studying the approaching planet. However, another part involved the practical job of scouting out any hazards (such as previously unknown rings) that might require a course correction to avoid, or turning the spacecraft's main antenna to act as a shield against dust particles. In addition, the long-distance images taken were used to provide a precise fix of New Horizon's position and trajectory, so it could hit its target point about 7,750 mi (12,500 km) from Pluto’s surface.

As New Horizons drew closer to its goal, the increasingly detailed images began to reveal features that even to the Hubble Space Telescope were mere blurs. Regions of high contrast began to show, then spots and polygonal shapes. The first signs of geology, such as what may be cliffs and craters, became visible and might be clues to the planet's internal composition and history.

What to expect

On Tuesday morning at 7:49 am EDT, new Horizons will makes its closest approach to Pluto at a distance of 12,500 km (7,750 mi) and a speed of 14 km/sec (31,000 mph). It will pass over the planet's northern hemisphere, which will be in daylight. During the encounter, Pluto will be 4.77 billion km (2.96 billion mi) from Earth and radio signals will take 4 hours and 25 min to reach the spacecraft. This means that New Horizons must carry out all of its operations autonomously as it aims at a target circle only 300 km (200 mi) in diameter and a timing error of less than 100 seconds.

The time delay also means that during the actual flyby, mission control will be completely in the dark as to the status of the spacecraft. Worse, as New Horizons makes the flyby, all its instruments will be aimed at Pluto and its moons, so the probe's main antenna will not be aimed at Earth. Therefore, NASA said at today's mission press conference that no news of the success or failure will reach Earth until about 9:00 pm EDT. In addition, New Horizons has only 202 watts of power available from its radioisotope thermoelectric generator, so the amount of data that can be sent will be very limited.

With this in mind, the mission planners have programmed New Horizons to take what are called "contingency samples." The name comes from the 1969 Apollo 11 mission, when Neil Armstrong's first task after taking his first steps on the Moon was to scoop up soil samples just in case he had to hightail it back to the Lunar Module. With the same sense of caution, New Horizons is programmed to collect the best data from its recorders and transmit it back to Earth in case it's damaged or destroyed during the flyby.

NASA says that during the flyby, Horizon's suite of seven highly advanced instruments will come into play. These include cameras, spectrometers, radio science experiments, and plasma and dust detectors. As the probe closes with Pluto and its moons, it will carry out 30 scientific objectives that will include mapping, atmospheric studies, and looking for new rings and moons. These will constitute 380 separate observations, such as image captures capable of identifying objects 70 m (230 ft) across, and stereo images. These will help scientists to produce maps of Pluto and Charon with a 40-km (25-mi) resolution, as well as ultraviolet and infrared maps.

New Horizons was launched on January 19, 2006, from Launch Complex 41 at Cape Canaveral Air Force Station, Florida atop a Lockheed Martin Atlas V-551 rocket. It's the fastest spacecraft ever launched, and it reached the distance of lunar orbit in only nine hours. After a February 2007 gravity assist from the planet Jupiter, it reached a speed of 83,600 km/h (52,000 mph) relative to the Sun.

Along with other mementos, New Horizons also carries a sample of the ashes of Pluto's discoverer, Clyde Tombaugh. This will make him, in a manner of speaking, the farthest-traveled astronaut in history and the first on a spacecraft that will leave the Solar System.

After the encounter with Pluto, New Horizons' mission will be far from finished. It will continue to make observations for weeks as it recedes into deep space. Because it can only send back data to Earth at two kilobits per second, it will be transmitting the results of its flyby for the next 16 months.

The video below outlines new Horizons' science payload.

Source: NASA

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