We’re nearly there.
Eighty-five years after it was first identified by U.S. astronomer Clyde Tombaugh and subsequently named by English schoolgirl Venetia Burney, 39 years since its surface was first spectroscopically imaged, 37 years since its large binary companion, Charon, was discovered, almost a full decade since humanity despatched its first robotic visitor, and since it was ignominiously demoted in status, the dwarf world Pluto is almost ready to surrender some of her closely guarded secrets from the ragged edge of the Solar System. In recent weeks, she has steadily emerged from the gloom, with images from the Long-Range Reconnaissance Imager (LORRI) and Ralph telescope aboard NASA’s New Horizons spacecraft steadily resolving a curious reddish-brown world, of hearts and whales, bright and dark regions, and it is clear that whatever it finds will generate many more questions than answers and continue to whet scientists’ appetites long into the future. Earlier this week, Dr. Bobby Williams— Executive Vice President of Space Navigation and Flight Dynamics at KinetX Aerospace, the primary navigation team for the mission—took time to explain to AmericaSpace the key challenges faced as New Horizons was guided across the Solar System to the farthest target ever reached by a human-made machine.
When New Horizons roared into a blue Florida sky on 18 January 2006, it was met with excitement and frustration in equal measure: excitement because, after so many fruitless attempts to send a spacecraft to Pluto—ranging from the ill-fated Pluto Fast Flyby (PFF) to the Pluto Kuiper Express (PKE), which breathed their last in ferocious NASA budget cuts in the 1990s and at the turn of the millennium—a mission to explore the last of the nine “traditional” planets in the Solar System was underway, tempered with frustration that it would require such a long period of time in order to reach its quarry. On 21-24 September 2006, just eight months out, New Horizons acquired its first fuzzy image of Pluto during a tracking test of LORRI. The tiny world was approximately 2.6 billion miles (4.2 billion km) away at that stage. Pluto was, to be fair, little more than a faint point of light in a dense field of stars, but the image was remarkable in that it proved that the spacecraft could track long-range targets.
By now, New Horizons was well en-route toward its Jupiter Gravity Assist (JGA) rendezvous with the Solar System’s largest planet in February 2007. In the weeks after launch, it had executed a trio of Trajectory Correction Maneuvers (TCMs), followed by another in September 2006. During this period, in-flight tests were conducted on the spacecraft’s Alice ultraviolet imaging spectrometer and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), as well as LORRI, and New Horizons passed the orbit of Mars in April 2006. Two months later, it successfully imaged Asteroid 132524 APL—named in honor of the Johns Hopkins University (JHU) Applied Physics Laboratory (APL) in Baltimore, Md., which runs the New Horizons mission—and offered an opportunity for the Ralph telescope to demonstrate ability to track rapidly moving objects.
Passing within 1.4 million miles (2.3 million km) of Jupiter on 28 February 2007, the spacecraft picked up an additional gravitational impulse, which accelerated it towards the outer Solar System by a further 9,000 mph (14,000 km/h). This effectively shortened its travel time to Pluto by three years, thereby making a July 2015 rendezvous achievable. During the Jovian passage, New Horizons imaged the planet’s large moon Callisto at infrared wavelengths and acquired a number of monochromatic photographs of the giant planet itself. It examined the Little Red Spot from close range, for the first time, and observed 11 eruptions from the volcanically active moon, Io.
After concluding its operations in Jupiter’s environs in June 2007, the spacecraft was placed into the first of four hibernation cycles, with redundant systems shut down, and the computer periodically transmitting signals back to Earth—“green” if all was well, “red” if assistance from ground controllers was necessary—as New Horizons voyaged outwards. It crossed Saturn’s orbit in June 2008 and that of Uranus in March 2011.
However, it was the discovery of two tiny moons of Pluto, subsequently named Kerberos and Styx, in July 2011 and July 2012 respectively, which first raised the possibility that the spacecraft might run into unseen moons, debris, or ring material, which could prove catastrophic at a flyby velocity of 30,000 mph (50,000 km/h). “The object of our scientific affection may actually be a bit of a black widow,” said Principal Investigator (PI) Dr. Alan Stern, speaking in October 2012, whilst New Horizons Project Scientist Dr. Hal Weaver added that “a collision with a single pebble, or even a millimeter-sized grain, could cripple or destroy” the spacecraft.
In response to this risk, an 18-month study was completed in mid-2013 and led to the development of up to five Safe Haven by Other Trajectories (SHBOT) profiles. Each of these profiles, if circumstances required them to be adopted, presented different options in terms of the quality of scientific data-return. “In the interest of limiting the amount of work required to develop instrument sequences for five different trajectories, NASA limited our final choice to three trajectories,” explained Dr. Bobby Williams of KinetX Aerospace. “There was a further-out flyby, way outside the 7,750 miles (12,500 km) of our current approach. It went out far enough that it degraded the science across the board. We had a ‘deep inertia’ BOT that went inside our current trajectory. That one also degraded science, because the instruments on-board the spacecraft don’t have scan rates compatible with such a fast approach. And that left the remaining SHBOT. We had the ‘hot’ and the ‘cold’, and this was the ‘medium’. It was only slightly removed from our current trajectory, the nominal trajectory, and it had the minimal loss of science. It required a slightly different sequence of instruments.” In precis, Dr. Williams stressed that, had any of the SHBOT profiles been required, there would have been would have been some loss of science.
“Over the last month, at different periods, we took a lot of images, looking for unknown objects in orbit around Pluto: dust, rings, small rocks, things like that,” he continued. “We call those objects ‘unknown hazards’, or ‘U-hazards’, and we didn’t find any. That means any such objects, if they exist, are fairly small, pebble-sized things.” A final U-hazard meeting took place on 2-3 July. “If we had seen something then, it would have had to have been very small,” said Dr. Williams. “Had something small turned up, we had one last mitigation for impacting dust. We would have turned the high-gain antenna into the ‘ram’ direction—what they call ‘looking into the velocity’—so that the antenna would shield the cabling and the instruments behind it on the spacecraft.” Fortuitously, none of these steps proved necessary and New Horizons remained on its nominal trajectory through the Pluto-Charon system.
“We can’t downlink the data very fast,” Dr. Williams explained, pointing to the immense distance between Earth and Pluto. “We were running at 500 bits/sec this weekend. Now, we can go up to 1,200 bits/sec. That’s essentially like the old dial-up modems, used on the telephone, years ago. It’s not very fast.” Not only that, but with a single large image requiring about an hour to download, simply sending that image would interrupt the science-gathering efforts of the other instruments. “The spacecraft can’t spend the time to spin around and downlink the images to us. We gather the ‘core load’ in a solid block from 7-12 July. All the science goes on a recorder, including the images. That recorder is then used to play back the data after the flyby.”
Early on 14 July, New Horizons is expected to pass within 7,750 miles (12,500 km) of Pluto, allowing the spacecraft’s instruments to map the dwarf planet’s surface and also large parts of Charon. “The distance is just inside Charon’s orbital radius from Pluto,” explained Dr. Williams, when asked about the reasons for this particular flyby distance. “Charon, of course, is not in the way when we go through. It’s on the other side of Pluto. One of the reasons we chose it was to pick a cleared-out area. We knew Charon would have moved any particles out of the way. In orbiting, it tends to perturb small objects, like rocks, that could be in orbit. The other reason is that we’re going at a very fast speed relative to Pluto. The closer we go in, the faster you’d have to move the cameras to keep from smearing the image. Even on a short exposure, it’d just be a blur going by.
“We have very high-resolution cameras flying at a distance that’s kind of the ‘sweet spot’ for not getting smear in the images. Some of our images are what they call ‘push-brooms’. It’s an instrument that takes multi-spectral scans with each line. Each sensitive line in the imagers is a single line of the spectrum band, and by just scanning across the object as you move the spacecraft slowly from a distance, you get a full spectral image of the surface. From that spectrum, you can determine the elemental composition of the things we’re looking at. Whether it’s iron, or water, or methane. And that’s how we can tell what Pluto is made of and what it’s covered with, which is very, very important to the scientific goals of the mission.
“From 7,750 miles (12,500 km), we have imagers where one pixel would subtend 670 square feet (62 square meters) on the surface,” he continued. “That means you can see large boulders. Anything the size of a football field would be easy to see. That’s the kind of resolution we’re going to get. We don’t know now if there are mountains, hills or valleys on Pluto. We know there are color variations. The Hubble Space Telescope has detected a dark, orange-brown-looking color and some bright, reflective-looking areas, which probably indicates some kind of snow covering; probably methane snow. If there are large geographic features, like canyons, we’ll see it. We’ll see if there’s a large, icy-covered pole—which is expected from early images—though it could change as we get more resolution. We’ll certainly see big features in geology. If there is any visible atmosphere or atmospheric processes, we should be able to see that on a large scale.”
When Voyager 2 was preparing for its encounters with Uranus and Neptune in the early 1980s, it faced a significant challenge, since it was traveling to regions of the Solar System far darker than anything previously encountered at Jupiter or Saturn. High noon on Uranus was not as bright as dusk on Earth and more than one member of Voyager 2’s imaging team likened his role to photographing a pile of charcoal briquettes at the foot of a Christmas tree, dimly lit by a single-watt bulb. Of course, the imaging technology which New Horizons brings to the table is considerably more advanced than the 1970s-specification Voyager hardware. “For the optical data, we do have much better imagers now than we had back in the day for Voyager for Neptune and Uranus,” said Dr. Williams. “The cameras then were much more crude than what we have now. We have gain-controls and we have exposure time that we can change. They’re run on a Charge-Coupled Device (CCD). As early as July 2014, we were using images of Pluto for our optical navigation. We could even pick up very, very faint images before that.”
As for the dark, gray-hued Charon, significant new insights are expected to be gained. Having said this, no real-time commands will be issued during Closest Approach. “The round-trip light-time from Earth to the spacecraft is almost nine hours now,” said Dr. Williams. “If we sent a command, we wouldn’t even know for nine hours if it had even been received, so we don’t do real-time commands during the ‘core load’, which is during close approach. The spacecraft is running on a pre-determined instrument sequence to maximize the imaging on the closest approach and departure.” Current planning calls for solar and Earth occultations of both Pluto and Charon, in order to acquire spectral measurements and determine the presence of atmospheric constituents or rings.
Monitoring the spacecraft across its long journey toward a primary destination which is farther from Earth than anything previously attempted, has proven a monumental challenge and has stretched the worldwide tracking assets of the Deep Space Network (DSN)—whose primary 230-foot-diameter (70-meter) stations are based in Goldstone, Calif., near Madrid in Spain, and outside Canberra, Australia—close to their limits. “We have had to revive a very old capability they had built for Voyager: three-way ranging,” Dr. Williams told AmericaSpace. “Voyager still exists, and still is being tracked for telemetry, but we haven’t done radiometric tracking of it for a couple of decades at this point. Likewise the Pioneers. With three-way ranging, we do an uplink from one ground station, let’s say Goldstone in California. It uplinks to the spacecraft. The signal hits the spacecraft and is transponded, or repeated, back towards the Earth. But because of the Earth’s rotation, the transmitting station has already set. It’s moved away from the spacecraft, so the signal is received by another station, say Canberra, Australia.
“The other technology that we personally created is optical navigation software,” he continued. “Optical navigation is not new, but we’ve improved on it. We have optical navigation software that has many different techniques for finding the center of both unresolved and resolved bodies. Unresolved bodies are less than a pixel. One point of light is spread out in what is called a ‘point-spread function’, so that it covers more than one pixel. From the way that it covers surrounding pixels that you can determine where the center is—where the peak of the signal is coming from—to a precision better than a pixel. We can usually find a center down to one-tenth or two-tenths of a pixel. That’s a very precise measurement. It lets us find star centers and unresolved bodies, like Pluto, from very far away. It’s not unique, because NASA’s Jet Propulsion Laboratory (JPL), essentially invented and developed optical navigation for deep space applications over many years. But we believe we’ve got a fairly modern instantiation of it. We’ve made recent developments in both the center-finding and modeling. We have at least five ways to find the center of unresolved bodies and five ways to find the center of resolved bodies.
“For resolved bodies, bodies that are many pixels across, you have to do something different, because the light can be effected by the albedo – the markings and features of the surface. You may be looking at a sphere that is not wholly lit, or the body may not be a sphere. It may be a rocky body, like an asteroid, that looks like an potato. We developed software to find the center on those bodies, too. Even though we didn’t use it on Pluto, we will use it on future missions we’re working on.”
And after Pluto, what then? New Horizons was always intended to be redirected, if possible, to explore one or more Kuiper Belt Objects (KBOs), with several candidates already under consideration. “We’re planning a following mission,” said Dr. Williams. “We have to write a proposal to NASA and get approved, because it would require additional funding. We’ve detected three objects beyond Pluto that are reachable, theoretically, by the fuel that we should have left on-board after the flyby. We know how much deflection of the orbit we could accomplish, based on that fuel. That gives us a space in the sky, beyond Pluto, that the Hubble Space Telescope (HST) searched. They found three objects within only a few weeks of effort, though it was very intense effort.”
These three “Potential Targets,” whose identification was reported last October by the Space Telescope Science Institute (STScI), are provisionally designated “PT1,” “PT2,” and “PT3” by the New Horizons team, and are all located between 43 Astronomical Units (3.9 billion miles or 6.4 billion km) and 44 AU (4.1 billion miles or 6.6 billion km) from the Sun, making a rendezvous achievable at some stage between late 2018 and mid-2019.
“We already have a maneuver planned,” continued Dr. Williams. “It’s in late October. We’d start a maneuver after we download a large portion of data; not all of it, because it takes about 14 months to get all the data. After we get the bulk of the data—all the ‘Group 1 Science’, as they call it—we’ll perform a maneuver that will take us to this other object. The other object is about three years beyond Pluto. We’ll write a proposal to do that maneuver, approach that object and do the same instrument scans as we did on Pluto on that very newly discovered object. A year and a half ago, no one knew the object existed. It’s very newly discovered. It would be exciting to flyby that!”
In spite of receiving electricity from pellets of plutonium-238 dioxide in a General Purpose Heat Source (GPHS) Radioisotope Thermoelectric Generator (RTG)—which should provide sufficient power to keep the spacecraft and instruments running through 2030-2035—the key concern is fuel consumption. “We guard our fuel usage and try to optimize every burn to get the most out of every gram of fuel,” Dr. Williams told AmericaSpace. “We are carrying hydrazine and that’s what we’d use to deflect the trajectory to the new KBO. It also requires some maneuvering and attitude-control to point the instruments at the object.”
During the trek out to the KBO, there are plans to execute a series of “distant flybys” of one or more other objects, passing at a range of about 1 AU, approximately the 93 million miles (150 million km) which separates Earth from the Sun. “We can’t reach them, but we can get a closer image than even Hubble can get, because we are so far out with a very capable camera,” Dr. Williams explained. “It really would be the next step, crossing that frontier into the Kuiper Belt, discovering and exploring all we can, as long as the fuel and the power hold out. That’s our goal.”
The author would like to express his sincere thanks to Dr. Bobby Williams, Executive Vice President of Space Navigation and Flight Dynamics at KinetX Aerospace, for his time in responding to AmericaSpace’s questions, and also to Ms. Linda Capcara, Principal and Co-founder at TechTHiNQ, for arranging the correspondence.
Stay with AmericaSpace for regular updates and LIVE COVERAGE of New Horizons’ approach and flyby of the Pluto system.
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