By the time it reached the Pluto system, the spacecraft had traveled farther away and for a longer time period more than nine years than any previous deep space spacecraft ever launched. Besides its suite of scientific instruments, New Horizons carries a cylindrical radioisotope thermoelectric generator a spare from the Cassini mission that provided about watts of power at launch decaying to watts by the time of the Pluto encounter.
A second firing of the Star 48B solid rocket accelerated the spacecraft to a velocity of about 36, miles per hour 58, kilometers per hour , the highest launch velocity attained by a human-made object relative to Earth. The spacecraft was now set on a trajectory to the outer reaches of the solar system. Controllers implemented course corrections Jan. A month later, on April 7, , New Horizons passed the orbit of Mars. During the flyby, New Horizons carried out a detailed set of observations over a period of four months in early Although observing the moons from distances much farther than Galileo, New Horizons was still able to return impressive pictures of Io including eruptions on its surface , Europa and Ganymede.
During hibernation, most major systems of New Horizons were deactivated and revived only about two months every year. The second, third, and forth hibernation cycles were Dec. The discovery of new Pluto moons Kerberos and Styx during the mission added to concerns that there might be debris or dust around Pluto. Mission planners devised two possible contingency plans in case debris increased as the spacecraft approached Pluto, either using its antenna facing the incoming particles as a shield or flying closer to Pluto where there might be less debris.
On Dec. At that time, it took four hours and 25 minutes for a signal to reach Earth from the spacecraft. The spacecraft began its approach phase toward Pluto on Jan. Two days later, with about four months remaining before its close encounter, New Horizons finally became closer to Pluto than Earth is to the Sun. Pictures of Pluto began to reveal distinct features by April 29, , with detail increasing week by week into the approach.
A final second engine burn June 29, , accelerated New Horizons toward its target by about 11 inches per second 27 centimeters per second and fine-tuned its trajectory. There was concern July 4, , when New Horizons entered safe mode due to a timing flaw in the spacecraft command sequence. Fortunately, the spacecraft returned to normal science operations by July 7.
Three days later, data from New Horizons was used to conclusively answer one of the most basic mysteries about Pluto: its size. Mission scientists concluded that Pluto is about 1, miles 2, kilometers in diameter, slightly larger than prior estimates.
Its moon Charon was confirmed to be about miles 1, kilometers in diameter. The download of the entire set of data collected during the encounter with Pluto and Charon — about 6. Such a lengthy period was necessary because the spacecraft was roughly 4. The atmospheric haze and lower than predicted atmospheric escape rate forced scientists to fundamentally revise earlier models of the system.
Pluto, in fact, displays evidence of vast changes in atmospheric pressure and possibly had running or standing liquid volatiles on its surface in the past.
There are hints that Pluto could have an internal water-ice ocean today. On Charon, images showed an enormous equatorial extension tectonic belt, suggesting a long-past water-ice ocean. In the fall of , after its Pluto encounter, mission planners began to redirect New Horizons for a Jan. Four course corrections were implemented in the fall while a fifth was carried out Feb. The goal of the encounter was to study the surface geology of the object, measure surface temperature, map the surface, search for signs of activity, measure its mass, and detect any satellites or rings.
On April 3, , the spacecraft was halfway from Pluto to its new target. During that time, the flight computer broadcast a weekly beacon-status tone back to Earth, and another data stream once a month on spacecraft health and safety data. On the anniversary of its Pluto-Charon flyby, July 14, , the New Horizons team unveiled new detailed maps of both planetary bodies.
On Jan. There was much discussion of Sputnik planum, now called Sputnik planitia more on that shortly , the big bright spot on Pluto that many people call Pluto's "left ventricle" but which would actually be the right ventricle if you were Pluto. Leslie Young talked about how Pluto's climate modelers didn't expect a giant nitrogen-rich basin on Pluto, and that's sent everyone back to the drawing board, understanding how the massive reservoir of volatile materials releases and takes in gas over time.
Bill McKinnon showed topographic maps of Sputnik planitia that explained why the name has changed: they now know that Sputnik is actually a deep basin, a massive ice sheet whose top lies 2. That makes it not a plain but a basin, and thus, it's a planitia. In fact, it's most likely an impact basin, an elliptical one by kilometers in extent, which would have formed from the glancing impact of a kilometer body.
James Tuttle Keane explored the history of that impact a bit more, presenting a hypothesis that the impact and subsequent condensation of a thick layer of nitrogen ice within the basin made it a "positive mass anomaly," like the lunar mascons or the Tharsis plateau on Mars.
If you add extra mass to the side of a rotating globe, the globe wants to put that mass on the equator, and the entire outer shell of a world can reorient. It's especially easy to do that if the world has a subsurface liquid layer, as Pluto certainly once had and might still have. Sputnik planitia is now near Pluto's equator, suggesting that -- like Tharsis -- it's a mass anomaly that has reoriented Pluto's lithosphere.
That would have telltale effects in the patterns of tectonic fissures on Pluto's surface. Oliver White noted that the color of the basin shifts from north to south and that there is more evidence of sublimation pits in the north.
Moreover, he pointed out that the northern boundary between the darker and brighter material is coincident with the location of Pluto's northern arctic circle, which is currently bathing in continuous summer sunlight. There may be a net sublimation of nitrogen from the northern part of the heart, leaving behind a darker lag and sublimation pits, and net deposition in the south, giving it a fresher, frostier surface.
John Spencer mentioned Kelsi Singer's work mapping craters on Pluto and Charon, noting that both Pluto and Charon have heavily cratered surfaces that are equally heavily cratered, and both show a relative lack of small craters. Because both worlds are missing small craters in similar proportions, it's probably not due to resurfacing; it's probably a quality of the population of things that hit them, i.
Spencer also showed a really cool topographic map of Charon showing that it has broad topographic troughs that aren't obvious in visual images. This is work being done by Ross Beyer and Paul Schenk that has yet to be published. Finally, Spencer showed a processed version of a post-flyby Charon image that I don't recall seeing before, in which you can barely see Charon by Plutolight. Spencer said that there was the suggestion of a dark south pole on Charon mirroring its dark north pole , "perhaps.
Early analysis of the stellar and radio occultations they performed with Charon to try to detect its atmosphere resulted in a non-detection. Now the team can quantify that: if Charon does have any atmosphere, it is less dense than our Moon's atmosphere. To put it another way, physically speaking, there have to be some molecules and ions floating around above any solid surface in a vacuum, but there are way fewer of those molecules above a given area of Charon's surface than there even are above the Moon's surface.
Charon's surface is dominated by water ice, but the spectroscopists have located outcrops of ammonium or ammonium hydrates. The geologist in me keeps balking at typing "outcrops of ammonium," but at Pluto's cold temperatures, these materials do function as rocks do on Earth, so "outcrops" they are. Stern also mentioned the intriguing presence of sublimation pits in Charon's surface -- intriguing because nothing would be sublimating there today.
Anne Verbiscer talked about the phase curves of Pluto and Charon. A phase curve is a study of how strongly a surface reflects light depending on the angle at which light travels from the light source, from the surface, to your eye or your camera. Many surfaces in the solar system display a strong "opposition surge," in which the surface appears to suddenly brighten when you view it with the light source directly behind you.
From Earth, with Hubble, we can only see Pluto and Charon at phase angles ranging up to 2 degrees, because we're so much closer to the Sun that we almost always see them nearly at opposition.
New Horizons got a much wider range of phase angles though not lowest phase , and Verbiscer showed phase curves, with Charon having a much stronger opposition surge than Pluto. Charon, she said, has a highly porous surface of opaque water ice particles, and Hydra and Nix are similar. Pluto has a more compact surface, with transparent grains of methane and nitrogen.
Overall, Pluto's global scattering properties are similar to those of Neptune's moon Triton. Looking a bit more closely at the way that Pluto's surface reflects light, Bonnie Buratti showed the results of some work she's been doing on quantitative albedo mapping. She reminded the audience that most of the variations in intensity within a spacecraft image are not intrinsic, but are rather due to changes in viewing geometry.
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