Up till now, 2015 has been a year of historic, ever bigger revelations regarding some of the Solar System’s “lesser bodies”, with ESA’s Rosetta spacecraft returning breath-taking close-up images of comet 67P/Churyumov-Gerasimenko and NASA’s Dawn mission providing stunning views of dwarf planet Ceres in the vast expanses of the asteroid belt between Mars and Jupiter. More than 5 billion km further out from the Sun, as if not wanting to be left out from all the buzz, the New Horizons spacecraft has beamed back the first focused images of Pluto, finally transitioning this last unexplored planetary body of the Solar System from an astronomical object to a geological one.
The New Horizons mission is currently in the middle of a six month-long, long-range reconnaissance of Pluto, in preparation for its historic and eagerly awaited first close fly by of the distant dwarf planet in July 14. Throughout this time the spacecraft has returned tantalising glimpses of the entire Pluto system from afar, which have revealed the graceful orbital ballet between the dwarf planet and its menagerie of five known moons. In most of the images that New Horizons has beamed back during its approach phase, Pluto has occupied just a couple of pixels with each pixel covering an area more than 2,000 km wide, which is almost the diameter of the planet itself, resulting in poor quality images where the planet was barely resolved at all. This started to change in mid-May, when New Horizons while being at a distance of approximately 77 million kilometers away from Pluto, imaged the latter with almost double the resolution, revealing a series of finer details of the distant planet’s surface features for the first time. Whereas the previous set of images that had been taken by the spacecraft one month earlier had shown a series of interesting surface brightness variations on Pluto which indicate the possible existence of a broad northern highly reflective polar ice cap, in the latest ones the distant world appears almost 50 percent bigger, allowing scientists to discern finer details throughout most of the planet’s surface during the latter’s week-long rotation along its axis, including what appears to be as a series of fascinating bright and dark areas. “These new images show us that Pluto’s differing faces are each distinct; likely hinting at what may be very complex surface geology or variations in surface composition from place to place,” explains Dr. Alan Stern, Principal Investigator for the New Horizons mission at the Southwest Research Institute in Boulder, Co. “These images also continue to support the hypothesis that Pluto has a polar cap whose extent varies with longitude; we’ll be able to make a definitive determination of the polar bright region’s iciness when we get compositional spectroscopy of that region in July.”
Despite this increase in image quality however, these latest raw images that were taken with the onboard Long Range Reconnaissance Imager, or LORRI, had to be processed with the use of a technique called image deconvolution in order for the additional details to come out, in a similar way to the one that had been previously used to sharpen the images of the Hubble Space Telescope in the early 1990’s prior to its first servicing mission, when it was determined that the primary mirror of the space observatory suffered from spherical aberration which greatly reduced its observing capabilities. Nevertheless, the resolution in this latest batch of images of Pluto from New Horizons is already exceeding those that have been previously taken by Hubble and are only bound to get better from now on as the spacecraft approaches closer to its destination by about 1.2 million kilometers with each passing day. “As New Horizons closes in on Pluto, it is transforming it from a point of light to a planetary object of intense interest,” says Dr. Jim Green, Director of Planetary Science at NASA’s Headquarters in Washington, DC. “We’re in for an exciting ride for the next seven weeks.”
Another very importance task for the New Horizons mission that was successfully completed during this past month’s imaging session, has been the first in a series of searches for any as-of-yet undetected hazardous moonlets, rings and dust debris that may lie along the spacecraft’s planned trajectory through the Pluto system. The results of the analysis of the images that were taken in mid-May, has revealed no such hazardous objects so far. New Horizons is close enough to Pluto now that its onboard cameras have the ability to detect any object with a brightness half that of Styx, Pluto’s smallest and faintest known moon, that could pose a threat to the mission. This means that if any such objects lie ahead they should be less than 10 to 20 km-wide, which is the estimated size for Styx.
New Horizons’ next observing session of the Pluto system is scheduled to begin today (May 29), by which time the image resolution will be almost double that of the ones that were taken in mid-May, allowing for even more crisp views of the mysterious distant world in the edges of the Solar System. The reason for these long gaps between the mission’s observing sessions, is the agonizingly slow data rates of 1 kilobit per second with which the spacecraft transmits its data back to Earth. At its current distance of more than 5 billion km away, New Horizons’ signals are so weak that can only be picked up by the largest, 70-m-wide antennas of NASA’s Deep Space Network in the US, Spain and Australia. Furthermore, the spacecraft has to periodically empty its data recorders in order to be able to collect as much data as possible during its closest approach of Pluto in July. As a result, the mission’s team have devised a timetable during which New Horizons stops observing Pluto in order for it to transmit all of the data that it has accumulated up to that point back to Earth. With the very slow data rates that the spacecraft can achieve at its current distance of more than 30 Astronomical Units away, it takes weeks before New Horizons has cleared its data recorders and begin observations again.
This long hiatus means that the data that are being returned with each observing session, are of far superior quality than those of the previous ones. From here on, New Horizons’ image resolution will by far exceed that of the images of Pluto that have been taken with the Hubble space telescope, which up till recently were the best ones ever we had of the distant planet, allowing scientists to unveil ever more fascinating details about this fascinating world that has remained a complete mystery ever since it was discovered almost a century ago. “By late June the image resolution will be four times better than the images made May 8-12, and by the time of closest approach, we expect to obtain images with more than 5,000 times the current resolution,” says Hal Weaver, project scientist for New Horizons at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD.
The time has finally arrived, when everyone alive today will have a front-row seat to humanity’s first ever close up exploration of Pluto and its moons. The seven weeks that separate us from this historic event, will be a thrilling time indeed.
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