With the clock swiftly ticking down to humanity’s eagerly anticipated first close-up reconnaissance of Pluto and its moons on July 14, courtesy of NASA’s New Horizons spacecraft, mission controllers on the ground are continuously monitoring the robotic explorer’s fast-track course through interplanetary space and adjusting it when necessary, in order to ensure a successful flyby. To that end, New Horizons successfully executed a critical, small, 45-second thruster burn on June 14, thus helping to keep the spacecraft where it should be along its path as it speeds through the Pluto system at a breakneck speed of 13.78 km per second.
Currently located more than 4.5 billion km away, deep within the outer reaches of the Solar System, New Horizons’ one-of-a-kind close passage through the Pluto system is make-or-break. For this reason, a large part of the spacecraft’s activities, ever since it was awakened from its electronic hibernation for the final time last December, comprised of a series of optical navigation campaigns that were pre-scheduled by the mission’s ground team to take place at various times throughout the entire approach phase to the Pluto system. In addition to the constant monitoring of its radio communication with Earth as New Horizons traversed the expanses of interplanetary space on its way to Pluto, these optical navigation campaigns were key for ground controllers to determine the exact position of the distant planet against the fixed background stars. This, in turn, allowed them to maintain New Horizons’ proper course toward Pluto and to schedule for any appropriate correction maneuvers when necessary, without which the spacecraft would face the risk of veering off course and lose Pluto by hundreds or even thousands of kilometers.
This extensive preparation by the mission’s ground teams was put to good use on June 14, when New Horizons successfully executed a 45-second burn, which allowed it to change its velocity ever so slightly, by just 52 centimeters per second. Even though that sounds like a miniscule change, it is nevertheless a significant one in orbital mechanics, without which New Horizons would miss its pre-scheduled arrival point at Pluto by 755 kilometers, while also arriving there a whole 84 seconds earlier than expected. With the help of NASA’s Deep Space Network antennas, which are used to communicate with the spacecraft, ground controllers were able to receive telemetry confirming that the burn had gone according to plan on 6:23 a.m. EDT, more than 5 hours after it had been executed on 12:05 a.m.
This course correction is only the second one to be undertaken during New Horizons approach phase to Pluto, after a previous 93-second one on March 10, and had reduced the spacecraft’s speed by 1.14 meters/second, while shifting it sideways by 3,442 kilometers toward the distant dwarf planet as seen from Earth. A third opportunity for a trajectory correction maneuver is also marked just a week from now, on June 24, to be followed by a fourth one on June 30 and a final one on July 4, just 10 days before New Horizons’ closest approach to Pluto. Whether these future trajectory correction maneuvers will take place, however, will depend on the results of the mission’s ground teams’ continuing study of the optical navigation images that are being returned by New Horizons’ onboard Long-Range Reconnaissance Imager, or LORRI, and the assessment in particular of the debris hazard to the spacecraft from any free-floating dust particles that may be present around Pluto.
“We have a whole series of observations of which we are currently in the middle of as we approach [to Pluto], searching the system for moonlets or debris rings,” told me Dr.John Spencer, a science team member for New Horizons from the Southwest Research Institute in Boulder, Colo., during a recent NASA media teleconference. “[Starting on early June], we will have the chance to divert the spacecraft if we see anything scary and then we will have a series of chances going [as well]. The last time we could divert the spacecraft to a different trajectory based on what we might see on the approach images, would be [approximately] 14 days before closest approach, more or less on July the 1st. So, we’re taking images up until that time, so that we’ll have the best possible information on which to make that decision.”
The chances for such a debris hazard are fortunately getting ever smaller as time goes by, as evidenced by the analysis of hundreds of 10-second exposures that were taken by LORRI between May 29-30 and June 5, which failed to reveal the presence of any threatening dust debris rings or moonlets that could be four times dimmer than Pluto’s faintest known moon, Styx. Even though the analysis of New Horizons’ optical navigations images will continue, the mission’s science team is fairly confident that the spacecraft will not face any such hazards in its course all the way through the Pluto system and beyond. “There’s a lot of chaos in the [Pluto] system, but there are parts of the system where things are pretty well-behaved and predictable,” says Spencer. “The big moon Charon in particular, is sweeping around Pluto in a very predictable and regular orbit every six days and it is very effective in clearing stuff out of anywhere near its own orbit in a matter of days. If you had a big cloud of debris somehow introduced into that region, it would be cleared out in days or weeks, and that is the part of the system that we’re flying through. We’re flying interior to the orbits of [Pluto’s] outer moons but right in the orbit of Charon. And Charon will do a lot of the [cleaning] work for us, despite whatever craziness is happening in the outer part of the system.”
With four weeks remaining prior to New Horizons’ historic flyby of Pluto, the mission’s next major milestone is set to occur on June 23, a date which will mark the start of Approach Phase 3, by which time New Horizons will be approximately 20 million kilometers away from Pluto. By then, the spacecraft’s onboard cameras will be returning images of Pluto and Charon of such clarity, which will allow scientists to study the atmospheric structures of this fascinating double dwarf planet system, which will allow for the creation of detailed global maps for both of these mysterious celestial objects.
Meanwhile, the resolution of New Horizons’ images continue to increase steadily with each passing day, until the time of closest approach on July 14, when the spacecraft will be able to take images of Pluto’s surface with a resolution of 0.1 km per pixel, which will finally reveal the mystifying distant planet’s topography in great detail for the first time in history.
“Every day we break a new distance record to Pluto, and every day our data get better,” says Dr. Alan Stern, Principal Investigator for the New Horizons mission at the Southwest Research Institute, Boulder, Colo. “Nothing like this kind of frontier, outer solar system exploration has happened since Voyager 2 was at Neptune way back in 1989. It’s exciting – come and watch as New Horizons turns points of light into a newly explored planetary system and its moons!”
Easily ranked as one of the biggest and most popular space exploration events of the whole decade, New Horizons’ upcoming close flyby of the Pluto system is bound to attract the attention of billions of people around the world.
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