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The launch of the New Horizons probe has been extensively covered in the blogosphere, but few, if any, of the blogs have focused on the mission itself. Sure, it’s the first mission to Pluto, and yes, it’s the fastest thing humanity has put up in space, but there’s more to this mission than the glitzy firsts and fastests. You may not be aware that, like the Voyager probes before it (and unlike Galileo and Cassini), the New Horizons craft will not enter into orbit at the Pluto system. Instead it will cruise beyond Pluto after a several month long flyby and head out towards the ever-expanding list of Kuiper Belt Objects. So, while we wait for our children to grow up so they can process the data from the probe once it reaches Pluto, you might enjoy learning a bit about the mission, its instruments, and its history. (image credit: NASA)

The Mission that Almost Wasn’t
A mission to Pluto this century was in 2000 by no means a sure thing. The Pluto-Kuiper Express was scheduled to launch as early as 2004 but was canceled in 2000 due to budget overruns that more than doubled the projected cost of the mission. Protests from the scientific community, congressional leaders, and the public led to the issuing of a fresh set of proposals in 2001 for a much lower-cost mission. Before the proposals had even been reviewed, a fresh round of budget cuts at NASA again threatened the probe’s funding. The New Horizons probe, headed by the Applied Physics Lab at Johns Hopkins was selected, and planning and construction began immediately for a probe that would launch in just over four years. A few years later, with work at the Los Alamos Nuclear Labs halted due to security breaches and sensitive data mishandling, the supply of RTGs (explanation below) that fuel the craft was threatened. However, a few months later, the Department of Energy announced that it could supply the bare minimum of fuel required to power New Horizon’s instruments.

A Receding Opportunity
The reason for the outcry against cancellation of a Pluto mission was because the fact that Pluto’s atmosphere is rapidly freezing out and evaporating into space. Pluto is in a highly eccentric orbit, which means that every 250 years or so it is much closer to the sun than during the rest of its orbit. During that time, Pluto’s surface ices volatilize and create a temporary atmosphere on a planet that is otherwise much too small and cold during most of its existence. Pluto’s latest perihelion (occasion of closest approach) occurred in 1989 and is now steadily receding away from the Sun. Twin measurements of Pluto’s atmosphere in 1988 and 2002 (by stellar occultation; the rate of dimming of the brightness of a star as it passes behind Pluto gives the density of its atmosphere) revealed that it had already thinned in those 14 years. Projections and models predict that near total freeze-out and escape would occur by 2020. The New Horizons probe will reach Pluto in July of 2015, thus giving it a chance to observe the remaining atmosphere.

The Fast Little Craft
Much has been made of the fact that the New Horizons probe is the fastest vehicle ever launched by mankind. As I write this, New Horizons is at T+7 hrs, so in just over 2 hours it will fly beyond the Moon (though not exactly past it). It will reach Jupiter in March of 2007, just 14 months after it was launched. In contrast, Cassini required 38 months to accomplish the same feat (now, the comparison is not exactly fair because Cassini required two Venus flybys to get the speed needed to reach Jupiter and then finally Saturn). How might this be you ask? Here’s the basic principle: find the biggest gun you can find, and load it with the smallest bullet. That’s basically the case here, New Horizons is (or was, actually) attached to a kick-stage loaded with several thousand kilograms of propellant. Now, the entire weight of the craft is under 500 kg (1000 lbs). As I mentioned, it is headed directly towards Jupiter (in an astrodynamical sense, you never fly straight towards where you’re headed) where it will use a gravitational assist (or slingshot) maneuver to achieve an additional speed boost (by the way, the explanation for why this works is terrible on Wikipedia, if you know of a better one, please suggest it to me, I could try my hand as well, but I’m not completely familiar with the details). (image credit: JHUAPL)

The Piano’s Instruments
The size of a grand piano, New Horizons is covered entirely by its instruments and communications equipment. It has seven scientific instruments, one if which was built by students at Colorado University. Below is a list of those instruments along with a brief description of their functions, but first I want to discuss the Radioisotope Thermalelectric Generators (RTGs) that New Horizons uses for power. Inside the RTGs, heat released by naturally decaying plutonium (i.e., no near-critical fission) is directly converted into electricity. New Horizons uses about 10 kg of plutonium inside of an RTG to provide it with more than 250 watts of energy. It received many of its RTGs as leftovers from the Cassini mission, but the fabrication of the new plutonium nearly delayed the launch of the mission (as mentioned above). This plutonium is also what was cause for concern with the approximately 30 protestors before New Horizon’s launch. I will not discuss the irrationality of these fears here, as you can find a discussion on the safety of RTGs in the Wikipedia article.(image credit: JHUAPL)

• LORRI: High resolution monochromatic visual imaging through an 8-inch telescope
• Ralph: A visible camera and near-infrared imaging spectrometer capable of producing full-color images of the planet along with compositions of atmospheric gases
• Alice: The ultraviolet counterpart to Ralph’s IR imaging spectrometer, it produces both images and spectrum in the ultraviolet portion of the spectrum
• SWAP: A plasma spectrometer that produces a spectrum of plasma energies. This will be used to investigate the interaction of the solar wind with Pluto’s atmosphere, as well as to study its magnetosphere.
• PEPSSI: a complement instrument to SWAP that detects other ranges of ion energies, as well as electrons
• REX: The radio science experiment that is integrated with the communications package and will be used to measure portions of the radio emission of Pluto, as well as being used during the artificial occultations of New Horizon’s radio signal as it passes behind Pluto and Charon with respect to the Earth. This will provide more detailed information on the density and temperature profiles of Pluto’s (and perhaps Charon’s) atmosphere
• SDC: The student dust counter, this little probe counts collisions with dust grains greater than 10^-12 grams. It will give a profile of dust density far beyond the current record of 18 AU (1 AU is the distance between the Sun and the Earth).

When to Tune Back In
After New Horizons flies through the Jupiter system next March, it will be on an interplanetary cruise phase for about 8 years. During that time, many of its instruments will be active, but no significant discoveries are expected. So, sometime in the few months before July 2015, Pluto will approach close enough for the LORRI camera to begin surveying its surface and that of its three (and perhaps more) moons. As it approaches still closer, the other experiments will begin collecting data and imaging the entirety of Pluto and Charon in the infrared through the ultraviolet. After close approach, at 1:09:47 PM on July 14th, 2015 (how’s that for precise scheduling?) New Horizons will image the occultation of the Sun by Pluto), at 1:10:47 New Horizons will fly behind Pluto as seen from Earth, at 2:35:37 PM, Charon will occult the Sun, and at 2:37:44 New Horizons will fly behind Charon (if that was all a little fast for you, check out the mission page for more information). Then, over the next few years it will expend its remaining propellant in order to send it near enough to study a few Kuiper Belt Objects.

Voyager 3
New Horizons is following in the footsteps of the two Voyager craft launched three decades ago. It will give us our first views of a planet within our solar system (we can debate whether Pluto is a planet or not later) and provide us our first glimpses of those enigmatic icy bodies in the Kuiper Belt. At the left is a “true color” image composite using the highest resolution photos to date, New Horizons will help us to understand why there is so much contrast between the equatorial and polar regions (image credit: NASA). Just like the Voyagers, the expectation of the unknown is exciting and irresistible. On the morning of July 14th, the entire world will tune in to see pictures of our furthest planet and its little moon. We will stand in awe of their strange beauty and wonder at the process that have brought them to be. But, as stated by mission principle investigator Alan R. Stern in a presentation about the mission on his personal website: The most exciting discoveries will likely be the ones we don’t anticipate.