If one were to characterise NASA’s New Horizons mission, it would be fitting to call it the ultimate exercise in patience. Having spent almost a decade in the conceptual phase, the Pluto-bound spacecraft was eventually launched in January 2006 on a speedy trajectory toward the Solar System’s distant and unexplored region of small icy bodies beyond the orbit of Neptune, known as the Kuiper Belt. Having broken the record for the fastest man-made object to ever leave our planet—with an Earth-relative speed of 58,536 km/h—New Horizons nevertheless spent the better part of nine years quietly traversing the interplanetary void in order to reach its ultimate destination. Finally, following a journey of more than 4.5 billion km, the spacecraft emerged from its electronic hibernation one last time on Dec. 6 of last year, just in time to begin preparing for the final leg of its approach toward Pluto in mid-January, while marking the official start of the mission’s first Pluto encounter stage, called Approach Phase 1. Now, with less than 100 days remaining before New Horizons flies through the Pluto system on July 14, the mission has marked another milestone on its way to the distant dwarf planet by advancing to Approach Phase 2, during which Pluto science observations will start to swing into high gear.
One of the primary objectives of Approach Phase 1, which lasted from Jan. 15 to April 4, was the optical reconnaissance of the vicinity of the Pluto system, as part of a larger, four-stage optical navigation campaign that began during the spacecraft’s last Active Check Out period last year. The purpose of this optical navigation campaign is to give the mission’s science team a detailed characterisation as much as possible of the dust and particle environment around Pluto, allow them to search for new moons around the dwarf planet, as well as help them fine-tune New Horizons’ position and trajectory relative to both Earth and Pluto. All of these tasks are of the utmost importance for ensuring the overall success of the mission, without which New Horizons would face the risk of veering off course and lose Pluto by thousands of kilometers, or get knocked out by a collision with an unseen moonlet or any dust debris that could be orbiting the dwarf planet, as the spacecraft will be hurtling through the Pluto system at a breakneck speed of 14 kilometers per second (which is more than 50,000 km/h).
“A large part of what we’re doing is to receive images that are coming from the spacecraft and use those to help refine our course, to then compare that to what they should be on a perfect trajectory and from the differences make decisions about either how we fire engines or change the timing of the encounter,” explained Dr. Alan Stern, principal investigator for New Horizons at the Southwest Research Institute in Boulder, Colo., during a past mission podcast.
To that end, the mission’s teams have been practicing for years while conducting many hundreds of simulations in the process, so as to be better prepared to respond in any emergencies that could potentially jeopardise the mission in the critical few weeks before closest approach to Pluto. Furthermore, and as an added precautionary measure aimed at better safeguarding New Horizons from any unexpected hazards it might encounter along its path, the responsibility for the mission’s navigation has been spread between two independent ground teams, called the “P-Nav” and “I-Nav” teams respectively.
“We have two teams, because it’s best to have both belts and suspenders if you want to make sure your pants won’t fall down,” comments Stern jokingly. “By having two teams working independently, we can at least check the discrepancies between the two and have a much higher chance of ensuring that things are going to work out just fine.”
“When you only have one crack at it, it has to work right and it has to work right the first time,” adds Bill Owens, optical navigation lead engineer for the mission’s I-Nav, at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
All of this practice was put to good use on March 10, when New Horizons successfully executed a 93-second thrust burn, which helped to reduce its speed by 1.14 meters/second and shifted the spacecraft sideways by 3,442 kilometers toward the distant dwarf planet, as seen from Earth. Aided by the latest optical navigation images that New Horizons has beamed back so far, this latest course-correction manuever allowed ground controllers to better align the spacecraft’s trajectory with Pluto’s projected orbit in space, while also moving the scheduled time of closest approach in July backwards by 14 minutes and 30 seconds. “We’re flying by an object that is a huge distance from Earth and we’re trying to hit a box that is 100 by 150 km wide,” says Mark Holdridge, encounter mission manager, at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) in Laurel, Md., which oversees the entire New Horizons mission. “That had been the reason for [doing] maneuver planning and trajectory control needed to thread that needle and get to that small box that is way the heck out there.”
The coming of April brought the start of Approach Phase 2 which began in April 5 and will last until June 23. During this period New Horizons will undertake its third out of four optical navigation campaigns, which will take place between April 5 and May 15 with the help of the spacecraft’s onboard Long-Range Reconnaissance Imager, or LORRI, and the Multi-spectral Visual Imaging Camera, or MVIC, of the Ralph Visible and infrared imager/spectrometer instrument acting as a backup. Both of these instruments will provide scientists with more valuable data on the conditions of the dust environment around Pluto, as New Horizons is now on final approach to the small icy world and getting closer by 1.2 million kilometers with each passing day.
“We are going to be starting in just a few weeks taking long exposure images of the whole region around Pluto, so that we can see if there are any new moons that might be producing any debris that could be dangerous to the spacecraft, or if we actually see rings of debris themselves orbiting Pluto in regions that might be dangerous to us,” explained John Spencer, member of the mission’s science team at JHU/APL during a recent Google Hangout that was conducted by Universe Today. “If we see anything like that, we’ll be pouncing on it, we’ll be analysing what orbits those moons could be having, or what kind of rings we might get from those moons and make sure that the spacecraft is going to be safe as it goes through the system.”
During Approach Phase, New Horizons is also devoted to science measurements of the background high-energy particle population around Pluto, which are coming from the Sun. Two of the spacecraft’s onboard spectrometers, the Solar Wind Around Pluto, or SWAP, and the Pluto Energetic Particle Spectrometer Science Investigation, or PEPSSI, have been continuously operating even during the mission’s long hibernation periods, gathering valuable data on the behavior of the charged particles of the solar wind in the vicinity of the Pluto system, where plasma intensity is 1,000 times less than that around the Earth’s magnetosphere. Initial results have indicated that due to a less-intense-than-expected solar activity that has been observed during the last decade, the area where the solar wind starts to interact with Pluto’s atmosphere might be much bigger than originally thought. “This means that New Horizons may cross the interaction boundary between the solar wind and Pluto’s atmosphere up to dozens of Pluto radii [and several hours] before its closest approach on July 14, creating a scientific bonanza for studies of the composition and escape rate of Pluto’s atmosphere!” says Fran Bagenal, co-Investigator for the New Horizons mission, at the University of Colorado.
Even though New Horizons is currently at a distance of approximately 115 million km from Pluto (which is just over two-thirds of the Earth-Sun distance), with the small world occupying no more than 3.5 pixels in the spacecraft’s cameras, the beginning of Approach Phase 2 will also mark the start of the first long-range studies of the brightness and color variations on Pluto’s frozen surface. Even at its current distance from the dwarf planet, these observations by New Horizons will only get better with each passing day, finally providing scientists with their first insights on the topography of the mysterious icy world. “Currently the best images [from New Horizons] show Pluto as barely resolved, as only a dot in the distance,” comments Stern. “But by the time that Approach Phase 2 is over in June, you’ll be seeing images of Pluto [that will be] like nothing ever [taken] from any Earth-based system. So, we’re really looking forward now to transition from an astronomer’s kind of target, to a planetary science target and that’s gonna be happening in the near future.”
Meanwhile, the general public can participate until April 24 in a campaign that is undertaken jointly by NASA and the International Astronomical Union, called “Our Pluto,” in order to submit names for the geologic features on the surface of the icy world that will be revealed by New Horizons for the first time for everyone to see, during its closest approach in July. The latter—provided that everything goes according to plan—will mark the triumphant culmination of humanity’s first reconnaissance of the entire Solar System, which started 50 years ago. “This really tells you that this is the last great fly by of an unknown region in the Solar System,” says Jim Green, Director of NASA’s Planetary Science Division at the agency’s Headquarters in Washington DC. “It’s really so to speak, the end of the initial exploratory phase of understanding our place in the Solar System.”
Even though the phrase “history-making” has been overused to the point that it has become trite and clichéd, as a characterisation it nevertheless befits New Horizons’ upcoming fly by of Pluto perfectly. For in just over three months from now, everyone alive today will have the fortune of witnessing the revelation of a previously unknown planetary world before their eyes for the first time.
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