The Whale and the Heart
Closest Approach for New Horizons
3 billion miles away, 4 and a half hours if you happen to travel at the speed of light, New Horizons' closest approach to the dwarf planet Pluto came and went in silence. The spacecraft, which must rotate its entire self to point at Earth and transmit to us, was busy carrying out the hundreds of individual maneuvers carefully pre-programmed to point all of its science instruments in the right direction to make their observations. So all was quiet in the Mission Operations Center, the spacecraft's control center, at 7:50 AM EDT. But it wasn't empty.
"We always talk about the spacecraft as being our child," said Julie Bowman, Mission Operations Manager for New Horizons, the lead engineer for in-flight operations. Though the spacecraft was entirely left to its own devices, "we wanted to be with it. It's a little bit like when you send your child off. We were nervous and proud at the same time."
Though the first data sent following closest approach won't reach Earth until 8:53 pm EDT -- the "phone home" signal carrying information about the health and status of the spacecraft -- there was still the stunning image of Pluto that had arrived on Earth on Monday, painstakingly processed by the science team overnight.
Pluto, as imaged by New Horizons on Monday. This is a heavily compressed image from the spacecraft's LORRI imager, with color data obtained earlier in the mission added after the fact. Credit: NASA/JHUAPL
The image is the closest look Earth-bound observers will get until data from the close approach itself arrives on Wednesday, and it shows a surprising diversity of surface features and terrain.
"This is clearly a world where both geology and surface climatology plays a role," said Alan Stern, Principal Investigator for the mission. "We see a history of impacts, a history of surface activity, some features that we might be able to identify as tectonic," meaning they were produced by processes taking place under the dwarf planet's surface.
Standing out prominently is the smooth-looking, heart-shaped feature in the lower right portion of planet. Cathy Olkin, Deputy Project Scientist, was asked if this region could be a giant impact crater or basin, similar to those visible on Earth's moon.
"That's an active thing we're debating," she said. Scrolling around the image of Pluto on an iPad, she seemed at once overflowing with ideas about what could be seen, but struggling to suppress the temptation to speculate. "It's all interesting."
"We know that the heart is enriched in methane [ice]," explained Bonnie Buratti, Co-Investigator with a focus on surface investigations. The region has actually been under observation since 1952, visible in ground-based observations as a region of unusual brightness in very low resolution imagery. Many decades of these observations have shown that the left side, or "left ventricle" as it's been referred to by the science team and media, has been getting darker over the years. Now that it can viewed up close, Buratti was willing to speculate that the ices on that side have been sublimating over time, causing the heart to be overtaken on the left by the darker material. In 100 years, she suspects, it will be smaller, but still present.
Olkin further speculated that the dark regions, forming an equatorial band that the science team has been calling "the whale," could be tholin or "tholin-like" material. Tholins are very complex carbon-based molecules that, in aggregate, form a thick, tarry substance that would explain the darker appearance. It's expected these could form via the interaction of methane and nitrogen in the atmosphere with ultraviolet photons and other high energy particles from the Sun, which cause those compounds to sink towards the surface, warming and undergoing chemical reactions in the process.
A prominent crater visible in the darker region, with a bright center, was dubbed "the whale's blowhole" by a member of the science team. Olkin speculated that the impact that created the crater could have penetrated the dark material, exposing lighter subsurface material. She also pointed out with great interest what could be "tendrils" or other linear features near the north pole (visible in the top quarter of the image).
Although images from close approach will be orders of magnitude better in resolution, there's a lot more than just imagery the science team will need to refine this speculation. For regions that exhibit possible topography, like the lumpy-looking regions to the right of the heart that Olkin described as "hummock-y," stereo images will be useful in taking a three dimensional look. Infrared spectroscopy will be vital in creating maps that show the actual composition of these regions that we can see in the picture, and confirming or refuting some of the speculation that's been mooted. Spectroscopy data is very large, and takes time to send down from the spacecraft, so it may be a while before a complete compositional map can be created.
Another vital set of data is color information. The picture that stole that spotlight on Tuesday morning was taken by the panchromatic (black-and-white) LORRI imager, which was then processed using much lower-resolution color data downlinked days earlier. Color data is taken by the MVIC instrument. Since New Horizons was designed to be a simple and resilient as possible for its long journey, it doesn't employ moving parts like the rotating filter wheels used by other spacecraft to build up color images. MVIC has four static imagers -- blue, red, methane, and near-infrared -- that each scan the surface, producing individual one-color images that must be reassembled on Earth.
"So it's not like we just have RGB and you combine them," Olkin said. The science team learned on-the-fly how to assemble an image that captures natural color using the data from these four imagers. During New Horizons' Jupiter gravity assist, the spacecraft photographed Jupiter's volcanic moon Io, with its distinctive yellow hue, and members of the science team developed the methodology for assembling natural color images by comparing their Io results to existing literature that they knew was correct, taken by previous missions like Galileo and Voyager. High-resolution, accurate color information will be important for defining regions of differing composition and terrain.
The team now anxiously awaits the "phone-home" signal, due Tuesday night, which will help the verify whether all of the expected observations actually occurred. From that point on, said Buratti, it will be "like opening a birthday present every day," as the data trickles in and is assembled to paint a larger, contextualized portrait of Pluto, Charon, and the outer moons, many times more complete than the mind-boggling image we've already seen.
"I think people will look back and say this is truly the golden age, the classical age of space exploration," said John Grunsfeld, NASA's Associate Administrator for the Science Mission Directorate. "There's very little terra incognita in our solar system." On Tuesday, an effort founded on the highest human aspirations, decades in the making, ebbed a bit more of that terra incognita away.