With just three weeks remaining until New Horizons speeds through the Pluto system for its historic close flyby of the distant dwarf planet and its system of moons, the robotic spacecraft has returned a series of images of both Pluto and its largest moon Charon that are the best yet, revealing tantalising new features on the surfaces of both these mysterious, distant celestial objects. And with its mission transitioning today (June 23) into its last of three approach phases toward its ultimate destination, New Horizons is set to begin its long-awaited science investigation of the Pluto-Charon binary system with its full suite of remote sensing instruments, which will culminate with the spacecraft’s close passage within 13,000 km above the surface of Pluto during the evening hours of July 14.
Following its reawakening from its long electronic slumber back in December of last year, New Horizons has been flawlessly executing a pre-scheduled, six-month-long, long-range reconnaissance of the Pluto system, which had been divided by the mission’s science team into several smaller phases, called Approach Phases 1, 2, and 3. Approach Phase 1, which lasted from Jan. 6 to April 4, was mostly devoted to long-range optical navigation imaging that has allowed ground teams to determine Pluto’s exact position with great accuracy against the fixed background stars and plan for the trajectory correction maneuvers that were needed in order for the spacecraft to maintain its proper course through space. Approach Phase 2, which lasted from April 5 to June 23, saw the spacecraft returning its first images that had the necessary resolution for scientists to begin identifying interesting surface features on Pluto, including the existence of a northern bright spot, which may be indicative of a polar ice cap.
The latest images to be beamed back by New Horizons were taken during May 29 and June 19 with the onboard Long Range Reconnaissance Imager, or LORRI, from a distance ranging between 55 and 29 million km and are the best yet, while revealing interesting new features on both Pluto and Charon as the latter more than doubled in size compared to the images taken in previous months. One of the features that really stands out in this latest batch of images is an enigmatic and quite unexpected dark area on Charon’s north pole which forms antithesis to the northern bright spot that has already been identified on Pluto. “This system is just amazing,” comments enthusiastically Dr. Alan Stern, Principal Investigator for the New Horizons mission at the Southwest Research Institute, Boulder, Colo. “The science team is just ecstatic with what we see on Pluto’s close approach hemisphere: Every terrain type we see on the planet—including both the brightest and darkest surface areas —are represented there, it’s a wonderland! And about Charon—wow—I don’t think anyone expected Charon to reveal a mystery like dark terrains at its pole. Who ordered that?”
As has become apparent from the images returned so far, both Pluto and Charon are high-contrast bodies that are both very unique in their own right, featuring a series of bright and dark regions which are indicative of very differentiated surface geologies. “The unambiguous detection of bright and dark terrain units on both Pluto and Charon indicates a wide range of diverse landscapes across the pair,” says Dr. Jeff Moore, a science team co-investigator and imaging lead for New Horizons, at NASA’s Ames Research Center, Calif. “For example, the bright fringe we see on Pluto may represent frost deposited from an evaporating polar cap, which is now in summer Sun.”
The different landscapes of both Pluto and Charon will start coming clearly into view throughout the next three weeks as Approach Phase 3 moves on, with New Horizons starting its science observations with its onboard Alice ultraviolet spectrometer, the Ralph imaging system and the Radio, Science Experiment, or REX. These remote sensing instruments will allow scientists to study Pluto’s atmospheric pressure, composition, and variability and search for any signs of clouds or hazes, create global color maps of the surfaces of both Pluto and Charon, and probe their chemical compositions. These observations will provide important insights to the overall structure and dynamics of both celestial bodies as the spacecraft gets ever closer, only to be surpassed in quality by the observations that will be conducted just two days before and after closest approach in July. “It’s getting better and better,” says Dr. Hal Weaver, project scientist for New Horizons at the Johns Hopkins University Applied Physics Laboratory, Md. “The cool thing is that we’re 20 days out and Pluto is now 20 pixels across [in the latest LORRI images]. It’s dramatically improved, we’re starting to see more and more surface features on the planet. Another 10 days and it will be twice as big as that and then roughly 3 days out it gets to be a hundred and then a thousand pixels across.”
Furthermore, Approach Phase 3 will see the continuation of the mission’s optical navigation campaign, tasked with making sure the path ahead for New Horizons all the way through the Pluto system is clear. Previous searches have been promising, by showing there are no hazardous rings, dust debris, or moonlets across the spacecraft’s trajectory, with a brightness four times less than that of Pluto’s smallest moon, Styx. “We have a couple of very important deep searches with LORRI to look for [new] satellites and dust just to make sure the coast is clear and so far everything is looking great,” says Weaver.
As to the nature of the upcoming findings on Pluto and Charon, many planetary scientists have speculated that Pluto may be similar in structure and composition to Neptune’s largest moon, Triton. The latter proved to be one of the greatest surprises of the Voyager 2 Neptune flyby in 1989, exhibiting a substantial geologic activity on its surface, including geyser-like eruptions that spewed substantial amounts of nitrogen gas high into space. Since Triton is believed to be a Kuiper Belt object that was captured by Neptune’s gravity in the distant past, and is approximately the same size as Pluto, it has been argued that the latter might exhibit a similar geologic activity on its surface.
One of the ways with which New Horizons will look for the presence of such geyser-like features is by turning its cameras back toward Pluto after the day of closest approach, by which time the dwarf planet will occult the distant Sun, allowing the spacecraft to detect any light-scattering fine particles that may be coming from its surface. “We have definitely planned to look at high-resolution for evidence of [geologic] activity in the form of geysers,” explained Stern during a recent NASA Google+ Hangout. “Both on Pluto and Charon there is some evidence in the form of crystal and water ice, which maybe shouldn’t be there on Charon unless it’s very recently emplaced and some evidence of ammoniated hydrates. There could be evidence of activity on Charon as well, so we’re gonna look on Pluto and on Charon about equally well and we’re gonna take advantage of looking back at both worlds after we pass in a special observing geometry, called forward-scattering geometry, and this is the same technique by which the Enceladus geysers [one of Saturn’s moons] were first discovered by looking at the same way backwards towards the Sun, where fine particles brighten up by scattering sunlight.”
Besides their pivotal importance to our deeper understanding of the Pluto system as well as the Kuiper Belt, New Horizons’ observations will also usher in a greater understanding of a far more familiar world—our own. Since Pluto’s atmosphere exhibits a certain loss of its hydrogen into space, it constitutes a perfect natural laboratory for studying the processes of atmospheric hydrogen loss that were also believed to be dominant on the primordial Earth, soon after it was formed. “We’ve never before been able to go to a place to see how hydrogen escape actually works,” says Stern. “It turns out Pluto is our only opportunity in the Solar System to see this in action. So, in a way, a mission to Pluto like New Horizons is a chance to travel back in time and see what the Earth was like immediately after it was formed, when it had this poisonous hydrogen-helium atmosphere that it lost in order to make room if you will, for the comfy nitrogen-dominated atmosphere that we have today.”
As has always been the case in planetary exploration, learning more about an alien world is to learn more about our own as well. During the next three weeks, the mysteries of a distant planetary frontier shrouded in enigma for almost a century will begin to be uncovered before our eyes.
It won’t be long now before the Pluto system starts surprising us with its revelations.
A video created from images that were taken by New Horizons between May 29 and June 19. Video Credit: NASA
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