New Horizons flew past Pluto and its moons earlier today, but the spacecraft stayed out of contact with Earth while finishing its historic observations.

Update: At 8:53 p.m. EDT, right on time, New Horizons "phoned home" with a strong radio link and a list of engineering status readings. From every indication, the spacecraft executed its preprogrammed observations exactly as planned, and its hardware and other systems remain healthy.

The late astronomer Carl Sagan was fond of saying that ours is the only generation that will explore the solar system for the first time. In that spirit — and adopting a generously broad definition for "generation" — it's worth noting that two milestones of planetary exploration occurred on this date.

Mariner 4 image of Mars
One of the images taken by mariner 4 in July 1965 shows craters in the Memnonia Fossae region of Mars. The large crater cut off through the center at the right of the frame is 156-km-wide Dejnev

The first was Mariner 4's flyby of Mars, passing just 6,120 miles (9,846 km) from the Red Planet's surface. (Historical footnote: the moment when the two were closest actually occurred on July 15, 1965, at 1:00:57 Universal Time.)

With a maximum transmission rate of just 3313 bits per second, it took a long time for the results from its camera, magnetometer, dust detector, and other instruments to reach Earth. But it was worth the wait. The 21 black-and-white images (and part of a 22nd) transmitted to Earth revealed a bleak, cratered landscape far different than what most researchers had imagined — an ultimately false impression that persisted until later missions proved otherwise.

Today we witnessed another important interplanetary "first." NASA's New Horizons spacecraft zipped past Pluto and its five moons, coming closest to the celebrated dwarf planet at 11:49:57 Universal Time (± about 30 seconds) and at a distance of roughly 7,750 miles (12,500 km). This morning principal investigator and head cheerleader Alan Stern (Southwest Research Institute) urged a room packed with hundreds of onlookers to chant the countdown's final seconds — notably, starting at "9."

New Horizons' countdown crowd
A large, enthusiastic crowd counts down to New Horizons' moment of closest approach to Pluto at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Click on the image to see S&T's video of the countdown.
NASA / Bill Ingalls

We know that New Horizons zipped past Pluto at 8.6 miles (13.8 km) per second — the laws of motion assure that — but what we don't know, yet, is whether the spacecraft completed its complicated sequence of measurements. In fact, it's been incommunicado since 11:17 p.m. EDT last night.

Word of the flyby's outcome will come later, at roughly 9:00 p.m. EDT, when the spacecraft "phones home" with about 20 minutes' worth of engineering status reports and some very carefully selected science results.

Two Worlds, Two Histories

Pluto's heart, July 13th
NASA’s New Horizons spacecraft took this image of Pluto with its Long Range Reconnaissance Imager (LORRI) on July 13, 2015. The color image has been combined with lower-resolution color information from the Ralph instrument that was acquired earlier. This This is the last and most detailed image sent to Earth before the spacecraft’s closest approach on July 14th.

Still, the handful of close-up images and other data that have already reached the ground are astounding. Here at the Johns Hopkins University's Applied Physics Laboratory, which serves as the control center for New Horizons, the day has been filled with brief encounters with weary but very happy mission personnel.

Asked about her feelings during the 22-hour-long break in radio contact at a press briefing this morning, mission operations manager Alice Bowman explained that she's like a worried parent trusting her child to do what we trained it to do. "I'm nervous and proud at the same time."

Today the team released dramatic new close-ups of Pluto and Charon, a true odd couple of worlds.

First out was a eye-popping image of Pluto, taken yesterday with the LORRI (Long Range Reconnaissance Imager) when the spacecraft was 476,000 miles (768,000 km) from its target. Showing details down to 3 miles across, it reveals dramatic details in and around a bright equatorial feature first seen weeks ago that's nicknamed "The Heart." Roughly 1,000 miles across, it appears rather smooth in its western (left) portion but more irregular toward the eastern end. (However, Jeff Moore, who heads the missions geology/geophysics team, cautions that compression of the image data might be artificially smoothing out small-scale details.)

This "heart" is flanked by broad dark regions of unknown origin or composition. They're probably not rocky outcrops, even though Pluto's density argues that the interior should consist mostly of rocky material. More likely it's some kind of carbon-rich veneer. The scientific rationale is that, given enough time to be irradiated by the Sun's ultraviolet light and space radiation, the frozen methane (CH4) known to exist on Pluto will gradually change to more complex (and dark) hydrocarbons.

Pluto and Charon in false color
False-color views of Pluto (left) and Charon include images taken through three visible and near-infrared filters taken on July 13, 2015, by New Horizons. Note the distinct halves in the heart-shaped region on Pluto and the reddish hue in Charon's polar cap.
NASA / JHU-APL / Southwest Research Institute

Then, at mid-afternoon briefing, deputy project scientist Cathy Olkin (Southwest Research Institute) revealed a new false-color image of Pluto and Charon that accentuates the regional differences and offers the first hints about their composition. For example, the "heart" looks pretty consistently toned across its breadth, but the false-color stretch shows a distinct left-right boundary with a peachy western lobe and a bluish cast dominating the mottled eastern side. There's also a tinge of yellow-orange in the northern polar cap (near the top).

The muted gray of Charon's surface reveals, when seen in false color, that its dark polar cap is reddish. The likely mechanism, explains Wil Grundy (Lowell Observatory), hinges on the fact that the night side of Charon probably acts as a "cold trap" for methane molecules escaping from Pluto's atmosphere. Over time space radiation has converted these trapped molecules into the same kinds of reddish high-mass organic compounds seen on Pluto.

Charon revealed
Chasms, craters, and a dark north polar region are revealed in this image of Charon taken by New Horizons on July 11, 2015.
NASA / JHU-APL /  Southwest Research Institute

Finally, John Spencer (Southwest Research Institute) showed a richly detailed LORRI image of Charon. The surface looks beat up, and the abundant craters (or, for now, vague circular features) suggest that the surface is ancient.

One large crater has created a splash of bright (icy?) material — complete with rays — across the otherwise dark surface. This suggests to team geologists that Charon's exterior has gradually darkened over time as molecules of water ice sublimate away and leave behind bits of carbon-rich solid matter that had been mixed in. (The same process has darkened the surface of other icy moons, such as Callisto.)

The New Horizons Waiting Game

Stern and his science teams keep reassuring us that the best is yet to come. For example, they're awaiting an image of Pluto, due tomorrow, showing its surface with 10 times better resolution that we've seen so far.

But tonight the great images to come will be less important than that keenly anticipated burst of telemetry to confirm that New Horizons has done all its handlers have asked of it and is ready to send its gigabytes of findings back to Earth — even as Pluto and Charon start to recede into the interplanetary space behind it.


Image of Peter Wilson

Peter Wilson

July 14, 2015 at 9:04 pm

The smooth area looks like a giant human footprint, as if someone stepped on Pluto. It also appears to be of recent origin...

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