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New Horizons, our mission to explore Pluto

Dr Amanda Doyle, Feature writer
Feb 16, 2012, 8:00 UTC, Updated Oct 5, 2014, 22:19 UTC

Sen—NASA’s New Horizons spacecraft set out on its long journey to Pluto in 2006, and it should reach its destination in July 2015, making it the first craft ever to visit the mysterious dwarf planet.

New Horizons is the first mission in NASA’s New Frontiers Program, and its main goals are to unravel the mysteries of Pluto and its moons along with investigating other Kuiper Belt Objects (KBOs) at the edge of the Solar System.

Are we there yet?
New Horizons is the fastest spacecraft ever launched from Earth, owing in part to an Atlas V-551 launch vehicle.

New Horizons was also given an extra kick of power by the Boeing solid propellant third stage motor. After the craft separated from this motor, a speed of 16 kilometres per second was reached. 

“It was a perfect combination - a small, relatively lightweight spacecraft launched atop one of NASA's most powerful rockets,” says Michael Buckley from the Johns Hopkins University Applied Physics Laboratory.

New Horizons only took 9 hours to reach lunar orbit, compared to the three days it took Apollo astronauts. Additional speed was gained in 2007 due to a flyby of Jupiter.

All this speed is necessary in order to reach Pluto in a reasonable amount of time, and thus lessen the chances of something going wrong with the craft while en route.

Pluto’s distance from the Earth varies because it is in an elliptical orbit, but in July 2015 Pluto will be 4.92 billion kilometres away. 

Even the Hubble Space Telescope struggles to study Pluto as it is 50,000 times fainter than Mars. By the time New Horizons is three months and 100 million kilometres from Pluto, New Horizons will be able to image the dwarf planet better than HST.

Pluto and Charon

Pluto and Charon imaged by the Hubble Space Telescope in 1994. Image credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA

It will take a 9 hour round trip for New Horizons to communicate with Earth. The signals will be received by the Deep Space Network’s largest antennas, which are 70 metres across. Three deep-space facilities are spread across the globe at 120 degrees apart, so that a signal can always be received.

“Generally, New Horizons is designed with that distance in mind - the spacecraft is programmed to carry out most operations well in advance, given the difficulty of sending commands in real time,” explains Buckley. “The spacecraft will not send data back to Earth during the actual Pluto flyby - rather, it will be gathering and storing the data on its solid-state digital recorders. The team will ‘play back’ and downlink the entire sequence over the following nine months.”

From planet to dwarf planet
In 1930, the “ninth planet” of our Solar System was discovered by Clyde Tombaugh. The moon Charon was discovered in 1978 and is nearly half of Pluto’s size, which actually makes the system a double planet. Also orbiting Pluto are four other satellites; Nix, Hydra, Kerberos, and Styx.

The diameter of Pluto is about two thirds that of the Moon and the surface gravity is only 6 per cent that of the Earth. The surface temperature is estimated to be a nippy minus 233 degrees Celsius, and Pluto is prone to seasonal changes. The dwarf planet has an atmosphere comprised mainly of nitrogen but also including carbon monoxide, methane and water ice. In contrast, Charon has no detected atmosphere. 

In 1992 it was discovered that Pluto and Charon weren’t alone in this region of the Solar System; the area is teeming with smaller objects dubbed Kuiper Belt Objects. The Kuiper Belt is filled with thousands of small icy objects which are left over from the formation of the Solar System.

In 2006 Pluto was demoted from planet to dwarf planet by the International Astronomical Union, however a change in definition doesn’t make Pluto any less interesting and it still deserves an intense scientific investigation.

Scientific objectives
There are three main scientific goals for New Horizons. The first is to map the geology and morphology of both Pluto and Charon with a resolution of one kilometre, as opposed to the 500 kilometre resolution of Hubble. Next on the list is mapping the surface composition of the duo. The final necessary goal is to determine the composition of Pluto’s atmosphere, along with its escape rate. 

It is believed that Pluto’s atmosphere is being lost to space in a process known as hydrodynamic escape, where the thermal energy of molecules in the upper atmosphere is high enough for the molecules to break free of Pluto’s gravity. It is thought that hydrodynamic escape occurred early in the Earth’s history, resulting in a loss of hydrogen. Studying the atmospheric escape of Pluto may help fill in some of the blanks of Earth’s early history. 

There are many additional scientific experiments that can be performed. New Horizons will awaken from its hibernation around a few months prior to closest approach to start imaging its target. By comparing images taken over this time period, weather patterns can be observed on Pluto.

When New Horizons is facing the night side of Pluto, radio signals sent from Earth will be monitored as they make their way through Pluto’s atmosphere. Measuring how these signals are refracted, i.e. how they are “bent”, will allow astronomers to measure the temperature and density of the atmosphere all the way down to the surface. The same thing will also be done with UV light from the Sun travelling through the atmosphere. Facing the night side is also the best place to look for any rings that might exist around Pluto. 

Craters are expected to be found adorning the surface of Pluto, and studying them will help to understand the layers of the dwarf planet. The craters might even reveal if a subsurface ocean is present. If the Pluto-Charon system formed via an impact, then a combination of heat from the impact and heat from the tidal forces induced by Charon could be enough to sustain an ocean. Heat flow through the interior will influence the morphology of craters present on the surface.

New Horizons should also have time to take a peek at the other moons in the system, as Project Scientist Hal Weaver tells Sen. "The New Horizons mission has added systematic investigations of Nix and Hydra to its timeline of science activities. We’ll be observing Kerberos and Styx, but we won’t be getting nearly as much data on those objects as we will on Nix and Hydra. We have one observation specifically targeted at Kerberos, but none for Styx. We will, however, get observations of Kerberos and Styx serendipitously during our 'deep searches' for new satellites during the approach phase."

The craft
The outer Solar System is a very cold place, so New Horizons has been designed like a thermos bottle in order to retain heat. Lightweight, multilayered blankets surround the craft to keep it operating between 10 and 30 degrees Celsius. An automated system ensures that enough power is being sent to the electronics to keep them at a sufficient temperature, and small heaters are activated if the temperature drops too low. 

Sixteen hydrazine propellant thrusters are used to make course corrections and point the craft in the right direction. They are not needed for speed as this was provided by the launch vehicle. As New Horizons isn’t actually going to enter orbit around Pluto and Charon, it doesn’t need to carry much propellant.

New Horizons hibernates for most of its journey in order to save power. It is periodically woken up in order to ensure that the systems are still fully functioning. The craft has now entered hibernation for the final time, and will reawaken on 7 December 2014 for the final approach to Pluto. 

New Horizons is powered by a radioisotope thermoelectric generator (RTG) which uses the natural decay of plutonium to turn heat into power. RTGs are successful in providing power when a craft is too far from the Sun for a solar panel to work. 

The tools
New Horizons comes equipped with seven science instruments which send their data back to Earth via the 2.1 metre antenna aboard the craft. The piano-sized probe impressively powers all these instruments with less power than two 100-watt light bulbs. 

Alice is an ultraviolet imaging spectrometer whose purpose is to study the atmospheric structure and composition of Pluto. Alice will be responsible for searching for an ionosphere on Pluto, and also for any evidence of an atmosphere on Charon. Alice can operate either in “airglow” mode, where it views UV atmospheric emission directly, or in “occultation” mode, where the Sun is used as a background light source to detect the amount of light absorbed by the dwarf planet’s atmosphere. 

Ralph constitutes the “eyes” of New Horizons and its purpose is to map the geology and morphology of its targets. Ralph’s Multispectral Visible Imaging Camera (MVIC) consists of three panoramic black and white imagers, along with four colour imagers. These are incredibly sensitive as they have to deal with extremely low light conditions due to the Sun’s light being severely diminished at this distance. 

Ralph will take stereo images to map the topography and will seek out clouds in Pluto’s atmosphere. It will also hunt down any additional moons and it will photograph Pluto’s night side, which will be illuminated by light reflected from Charon. 

Ralph also contains the Linear Etalon Imaging Spectral Array (LEISA) which is a spectrometer that will measure the composition of materials on the surfaces of Pluto and Charon. LEISA will also be able to map the surface temperatures. 

The Radio Science Experiment (REX) will measure the temperature and pressure of the atmosphere of Pluto and give Alice a helping hand in searching for an atmosphere on Charon. REX will also measure the masses of the Pluto and Charon accurately by enabling scientists to track any changes to the path of New Horizons, as the gravity of the double planet will influence the craft slightly. In addition, REX can time radio occultations in order to measure the radii precisely, something that is currently only known to within tens of kilometres for Pluto.

The Long Range Reconnaissance Imager (LORRI) will complement Ralph’s imaging capabilities with a high magnification imager. LORRI will enable the scientists to observe details such as craters on the surface, and it will also search for any possible geysers. 

LORRI first resolved Charon in July 2013, and snapped images of a full revolution of Charon around Pluto in July 2014. These images assist with optical navigation. The observations by LORRI in July 2014 also revealed the moon Hydra, which astronomers didn't expect to resolve until January 2015.

The Solar Wind at Pluto (SWAP) instrument will monitor how Pluto’s escaping atmosphere reacts with the solar wind, and this should help to determine the rate of atmospheric escape. 

The Pluto Energetic Particle Spectrometer Science Investigation, or PEPSSI, is an energetic particle spectrometer that can measure more energetic particles than SWAP. PEPSSI will study a stream of material emanating from Pluto which is carried off by the solar wind, akin to a comets tail. 

The Student Dust Counter (SDC) was designed by students at the University of Colorado at Boulder. It operates during hibernation to detect tiny dust grains produced by collisions of asteroid, comets and KBOs. This will give information on the collision rates in this region of the Solar System. No dust collector has ever gone beyond the orbit of Uranus, so this will give a first look at dust in the outer reaches of the Solar System. 

Both SWAP and PEPSSI have also been operating during hibernation in order to measure particles travelling in the solar wind. This information can be used to study how coronal mass ejections and flares from the Sun influence deep space.


The seven science instruments of New Horizons. Image credit: The Boeing Company

To Pluto, and beyond!
After leaving Pluto and Charon behind, New Horizons will pay a visit to some KBOs, which will be selected just as New Horizons nears Pluto. As there are many undiscovered KBOs lurking about, trying to find a good scientific target is a difficult task. A citizen science project known as Ice Hunters relied on the public to sift through images and identify potential KBOs. This project has now been competed, and while several KBOs were discovered, none were suitable for the spacecraft to visit. The Hubble Space Telescope is currently searching for any additional KBOs that could be potential targets for New Horizons.

New Horizons will eventually leave the Solar System and could stay operational up until 2037.