Neptune

During August 16 and 17, 1989, the Voyager 2 narrow-angle camera was used to photograph Neptune almost continuously, recording approximately two and one-half rotations of the planet. These images represent the most complete set of full disk Neptune images that the spacecraft will acquire. This picture from the sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. The large dark oval near the western limb (the left edge) is at a latitude of 22 degrees south and circuits Neptune every 18.3 hours. The bright clouds immediately to the south and east of this oval are seen to substantially change their appearances in periods as short as four hours. The second dark spot, at 54 degrees south latitude near the terminator (lower right edge), circuits Neptune every 16.1 hours. This image has been processed to enhance the visibility of small features, at some sacrifice of color fidelity.

Image credit: NASA/JPL

Note: For more information, see NASA Pluto-Bound Spacecraft Crosses Neptune's Orbit, 25 Years After Neptune: Reflections on Voyager, and New Horizons Crosses the Orbit of Neptune.

Countdown to Pluto

Are we there yet?

One of the fastest spacecraft ever built -- NASA's New Horizons -- is hurtling through the void at nearly one million miles per day. Launched in 2006, it has been in flight longer than some missions last, and it is nearing its destination: Pluto.

“The encounter begins next January,” says Alan Stern, of the Southwest Research Institute and the mission’s principal investigator. “We’re less than a year away.”

Closest approach is scheduled for July 2015 when New Horizons flies only 10,000 km from Pluto, but the spacecraft will be busy long before that date. The first step, in January 2015, is an intensive campaign of photography by the Long Range Reconnaissance Imager or “LORRI.” This will help mission controllers pinpoint Pluto's location, which is uncertain by a few thousand kilometers.

"LORRI will photograph the planet against known background star fields," explains Stern. "We’ll use the images to refine Pluto’s distance from the spacecraft, and then fire the engines to make any necessary corrections.”

At first, Pluto and its large moon Charon will be little more than distant pinpricks—“a couple of fat pixels,” says Stern--but soon they will swell into full-fledged worlds.

By late April 2015, the approaching spacecraft will be taking pictures of Pluto that surpass the best images from Hubble. By closest approach in July 2015, a whole new world will open up to the spacecraft’s cameras. If New Horizons flew over Earth at the same altitude, it could see individual buildings and their shapes.

Stern is looking forward to one of the most exciting moments of the Space Age.

“Humankind hasn't had an experience like this--an encounter with a new planet--in a long time,” he says. “Everything we see on Pluto will be a revelation.”

He likens New Horizons to Mariner 4, which flew past Mars in July 1965. At the time, many people on Earth, even some scientists, thought the Red Planet was a relatively gentle world, with water and vegetation friendly to life. Instead, Mariner 4 revealed a desiccated wasteland of haunting beauty. New Horizons’ flyby of Pluto will occur almost exactly 50 years after Mariner 4’s flyby of Mars—and it could shock observers just as much.

Other than a few indistinct markings seen from afar by Hubble, Pluto’s landscape is totally unexplored. Although some astronomers call Pluto a “dwarf” planet, Stern says there’s nothing small about it. “If you drove a car around the equator of Pluto, the odometer would rack up almost 5,000 miles—as far as from Manhattan to Moscow.” Such a traveler might encounter icy geysers, craters, clouds, mountain ranges, rilles and valleys, alongside alien landforms no one has ever imagined.

“There is a real possibility that New Horizons will discover new moons and rings as well,” says Stern.

Yes, Pluto could have rings. Already, Pluto has five known moons: Charon, Styx, Nix, Kerberos, and Hydra. Numerical simulations show that meteoroids striking those satellites could send debris into orbit, forming a ring system that waxes and wanes over time in response to changes in bombardment.

“We’re flying into the unknown,” says Stern, “and there is no telling what we might find.”

Video credit: NASA

Pluto and Its Five Moons

This image, taken by the NASA/ESA Hubble Space Telescope, shows five moons orbiting the distant, icy dwarf planet Pluto.

The green circle marks the newly discovered moon, designated S/2012 (134340) 1, or P5, as photographed by Hubble's Wide Field Camera 3 on 7 July 2012.

Other observations that collectively show the moon's orbital motion were taken on 26, 27 and 29 June and on July 9.

The moon is estimated to be 10 to 25 kilometers across. It is in a 95,000 kilometer diameter circular orbit around Pluto that is assumed to be aligned in the same plane as the other satellites in the system.

The darker stripe in the center of the image is because the picture is constructed from a long exposure designed to capture the comparatively faint satellites of Nix, Hydra, P4 and S/2012 (134340) 1, and a shorter exposure to capture Pluto and Charon, which are much brighter.

Photo credit: NASA, ESA, and M. Showalter (SETI Institute)

Note: For more information, see Hubble Discovers New Pluto Moon; also, Fifth Moon Discovered Around Pluto. And ScienceCasts reminds us that the New Horizons spacecraft will be flying past Pluto in 2015. (This video is a little dated already in that the narrator announces the then-recent discovery of a fourth moon.

The Mysterious Molasses Markings of Pluto

A team of researchers led by Marc Buie of the Southwest Research Institute recently released the best Hubble images [of Pluto] to date [see above].

The data reveal an icy molasses-colored world with a surprising amount of activity. Buie compared Hubble images taken in 1994 vs. 2003 and discovered that Pluto's northern hemisphere has brightened while the southern hemisphere has dimmed. Ground-based observations suggest that Pluto's atmosphere doubled in mass during approximately the same time period. And no one is certain what's causing the molasses-colored splotches on Pluto's surface.

...

Pluto can get so cold, researchers believe, that its atmosphere can actually freeze and fall to the ground. If Earth's atmosphere did that, it would make a layer 30 feet thick, but Pluto has less to work with. When it’s on the ground, Pluto's entire blanket of air is no more than a frosty film of nitrogen and methane.

"Until the mid-1980s, Pluto's northern hemisphere was tilted away from the sun for over 100 years, accumulating a substantial amount of frost," says Buie. "Now the northern hemisphere is coming into sunlight and appears, as shown in the Hubble images, to have been growing brighter."

The atmosphere might also be changing in response to Pluto's highly eccentric orbit. During the late 1980s, Pluto approached as close to the sun as it ever gets (about 2 1/2 billion miles) and gradually started warming. Now the temperature on Pluto is up to a balmy -385 degrees Fahrenheit! Surface frosts exposed to such "warmth" may be subliming — that is, changing back into a gas.

"Pluto, right now, has the best atmosphere it's had in our lifetime," says [Mike] Brown.

And about that molasses…

Researchers think these dark areas may be primordial organic matter.

"We know there's methane on Pluto," says Brown. "Here's what we think happens: Sunlight hits the methane and breaks it apart into its chemical components -- hydrocarbons. Over millions of years this process makes a dark reddish-brown oil or tar like substance that sticks to the ground. These darker areas spread larger as they absorb more sunlight and cause additional frost to sublimate."

"Now, Pluto is headed away from the sun again," says Brown. "It will gradually get colder and colder and its atmosphere will refreeze to its surface. In fact, that should have already started happening, but apparently it has not. It's a mystery."

NASA's New Horizons probe is en route to investigate. The spacecraft left Earth in January 2006 and has been racing toward Pluto for an encounter in July 2015, hopefully before the atmosphere refreezes.

"New Horizons will map the entire sunlit portion of Pluto," says Buie. "And as it swings closer, it will get very detailed images, maybe as good as 50-100 meter resolution."

"This will allow us to explore some of the interesting areas we've pinpointed," he continues. "For example, the recent Hubble images reveal a very bright spot – brighter than anything else on Pluto – near the equator. And just to the left of that bright spot is some of the darkest terrain on Pluto's surface. We want to examine the area where these bright and dark areas are touching and figure out what's causing the differences. This is a good target because it includes every kind of terrain Pluto has to offer."

Photo credit: NASA/ESA/SWRI

Jupiter and Io, by New Horizons

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This is a montage of New Horizons images of Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby [pdf] in early 2007. The Jupiter image is an infrared color composite taken by the spacecraft’s near-infrared imaging spectrometer, the Linear Etalon Imaging Spectral Array (LEISA) at 1:40 UT on Feb. 28, 2007. The infrared wavelengths used (red: 1.59 µm, green: 1.94 µm, blue: 1.85 µm) highlight variations in the altitude of the Jovian cloud tops, with blue denoting high-altitude clouds and hazes, and red indicating deeper clouds. The prominent bluish-white oval is the Great Red Spot. The observation was made at a solar phase angle of 75 degrees but has been projected onto a crescent to remove distortion caused by Jupiter’s rotation during the scan. The Io image, taken at 00:25 UT on March 1st 2007, is an approximately true-color composite taken by the panchromatic Long-Range Reconnaissance Imager (LORRI), with color information provided by the 0.5 µm (“blue”) and 0.9 µm (“methane”) channels of the Multispectral Visible Imaging Camera (MVIC). The image shows a major eruption in progress on Io’s night side, at the northern volcano Tvashtar. Incandescent lava glows red beneath a 330-kilometer high volcanic plume, whose uppermost portions are illuminated by sunlight. The plume appears blue due to scattering of light by small particles in the plume.

Are We There Yet?

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This image demonstrates the first detection of Pluto using the high-resolution mode on the New Horizons Long-Range Reconnaissance Imager (LORRI). The mode provides a clear separation between Pluto and numerous nearby background stars. When the image was taken on October 6, 2007, Pluto was located in the constellation Serpens, in a region of the sky dense with background stars.

Typically, LORRI’s exposure time in hi-res mode is limited to approximately 0.1 seconds, but by using a special pointing mode that allowed an increase in the exposure time to 0.967 seconds, scientists were able to spot Pluto, which is approximately 15,000 times fainter than human eyes can detect.

New Horizons was still too far from Pluto (3.6 billion kilometers, or 2.2 billion miles) for LORRI to resolve any details on Pluto’s surface – that won’t happen until summer 2014, approximately one year before closest approach. For now the entire Pluto system remains a bright dot to the spacecraft’s telescopic camera, though LORRI is expected to start resolving Charon from Pluto – seeing them as separate objects – in summer 2010.