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.

Herschel’s Population of Trans-Neptunian Objects

ESA’s Herschel space observatory has observed 132 of the known 1400 cold worlds that inhabit a region of the Solar System beyond the orbit of Neptune, some 4.5–7.5 billion km from the Sun.

These ‘trans-Neptunian objects’, or TNOs, include worlds such as Pluto, Eris, Haumea and Makemake, and make up a vast population of such objects thought to occupy these far-flung reaches of the Solar System.

TNOs are particularly cold, at around –230ºC, but these low temperatures lend themselves to observations by Herschel, which observes at far-infrared to sub-millimeter wavelengths. Indeed, the space observatory observed the thermal emission from 132 such objects during its nearly four-year lifetime.

These measurements provided their sizes and albedos (the fraction of visible light reflected from the surface), properties that are not otherwise easily accessible. The graphic presented here shows a sample of the population of TNOs observed with Herschel, arranged to showcase these properties.

What is most striking is their diversity. They range from just below 50 km to almost 2400 km in diameter; Pluto and Eris are the largest. Two worlds have distinctly elongated shapes: Haumea (seen in white) and Varuna (brown). Some even host their own moons (not shown).

The albedo measurement implies a variety of surface compositions: low albedo (brown) is an indication of dark surface materials, such as organic material, while higher albedo (white) suggests pure ices.

TNOs are thought to be some of the most primitive remnants of the planet-forming era. Thus the results of the Herschel “TNOs are cool: A survey of the trans-Neptunian region” open key time program are being used to test different models of Solar System formation and evolution.

Image credit: ESA/Herschel/PACS/SPIRE; acknowledgements: M. Rengel and P. Lacerda (Max-Plack-Institute für Sonnensystemforschung, Germany), T. Müller (Max-Planck-Institut für extraterrestrische Physik) and the Herschel “TNOs are Cool” Team.
Description

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

The Earth and Moon, by Messenger

The pair of bright star-like features in the upper panel are not stars at all, but the Earth and Moon! MESSENGER was at a distance of 98 million kilometers (61 million miles) from Earth when this picture was taken. The computer-generated image in the lower left shows how the Earth appeared from Mercury at the time. Much of the Americas, all of Europe and Africa, the Middle East, and much of Asia were visible.

MESSENGER took this image as part of a campaign to search for natural satellites of Mercury. Mercury has no moons that we know of. If any exist, they must be small (less than a few kilometers), or we would have seen them by now. The strategy for the satellite search involves taking multiple images of locations at predetermined distances from Mercury, from 2.5 to 25 times the planet radius. Pictures of these points in space are captured at intervals ranging from seconds to nearly an hour, depending on their distances from Mercury. A moving satellite will appear at different positions in images of the same region of space taken at different times.

The Earth and Moon appear very large in this picture because they are overexposed. When looking for potentially dim satellites, long exposures are required to capture as much light as possible. Consequently, bright objects in the field of view become saturated and appear artificially large. In fact, the Earth and Moon are each less than a pixel in size, and no details on either can be seen. The "tails" pointing downward from the Earth and Moon are artifacts caused by the image saturation. These can be seen clearly in the zoomed image in the center lower panel.

This image was taken on the same day that images with Earth in the scene were acquired by the Cassini spacecraft, as part of a mosaic of the backlit Saturn system (http://saturn.jpl.nasa.gov/news/waveatsaturn).

One day later in the search for satellites of Mercury, MESSENGER again took similar images of the Earth and Moon. The date of those images, July 20, was the 44th anniversary of the Apollo 11 landing on the Moon. All six Apollo landing sites are illuminated in the MESSENGER images, although they cannot be resolved. Their locations are marked on the simulated image in the lower right panel.

The background for this image is the region between the constellations Sagittarius (the archer) and Scutum (the shield). The right ascension and declination of Earth are 18h 55m 44s and -18° 25' 31", respectively. Pluto is also in the field of view but far too dim to be seen.

Date Acquired: July 19, 2013, 11:54:41 UTC
Image Mission Elapsed Time (MET): 1016558881
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
WAC Filter: 2 (clear filter)
Field of View: The WAC has a 10.5° field of view

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Kerberos and Styx

From the USGS Astrogeology Science Center:

The IAU has approved the names Kerberos for Pluto's moon formerly known as P4, and Styx for Pluto's moon formerly known as P5. For detailed information about this naming, see the IAU press release. For more information about Kerberos and Styx and other moons and planets in our Solar System, see the Planet and Satellite Names and Discoverers page in the Gazetteer of Planetary Nomenclature.

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.

How Would the Solar System Look to Aliens?

Dust in the Kuiper Belt, the cold-storage zone that includes Pluto, creates a faint infrared disk potentially visible to alien astronomers looking for planets around the Sun. Neptune's gravitational imprint on the dust is detectable in new simulations of how this dust moves through the solar system. The simulations show how the distant view of the solar system might have changed over its history.

Video credit: NASA; text credit: NASA

Dwarf Planet Eris

This artist's impression shows the distant dwarf planet Eris. New observations have shown that Eris is smaller than previously thought and almost exactly the same size as Pluto. Eris is extremely reflective and its surface is probably covered in frost formed from the frozen remains of its atmosphere.

Illustration credit: ESO/L. Calçada

Note: For more information, see Faraway Eris is Pluto's Twin.

Hubble Discovers a Fourth Moon Around Pluto

Astronomers using the Hubble Space Telescope have discovered a fourth moon orbiting the icy dwarf planet Pluto. The tiny, new satellite – temporarily designated P4 -- popped up in a Hubble survey searching for rings around the dwarf planet.

The new moon is the smallest discovered around Pluto. It has an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Charon, Pluto's largest moon, is 648 miles (1,043 km) across, and the other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter (32 to 113 km).

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," said Mark Showalter of the SETI Institute in Mountain View, Calif., who led this observing program with Hubble.

The finding is a result of ongoing work to support NASA's New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our solar system. Hubble's mapping of Pluto's surface and discovery of its satellites have been invaluable to planning for New Horizons' close encounter.

"This is a fantastic discovery," said New Horizons’ principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby."

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet’s entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites observed around Pluto.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago. Scientists believe material blasted off Pluto's moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.

"This surprising observation is a powerful reminder of Hubble's ability as a general purpose astronomical observatory to make astounding, unintended discoveries," said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

P4 was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.

Photo credit: NASA; map credit: NASA, ESA, and A. Feild (STScI)

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

Four Exoplanets Seen

An interesting day in astronomy. The Minister of Space Exploration is pleased to say that two articles were recently published, announcing the discovery of four exoplanets around two stars. The announcement of exoplanets, in and of itself, isn't terribly newsworthy; after all, over 300 have been discovered so far. What make these four stand out is that all of them have been observed visually; that's newsworthy.

In the first article, published by Science, a star 128 light years away, designated HR 8799 and located in the constellation of Pegasus, was found to have three exoplanets orbiting it (labeled b, c and d in the photograph). The planets appear to be between five to ten times the mass of Jupiter, with the closest planet being the smallest and the furthest planet being the largest, somewhat like our solar system. The planets were imaged in infrared radiation and are still glowing with the original heat from their creation roughly 60 million years ago. (The Earth, by contrast, is 4.5 billion years old.)

The other exoplanet recently discovered orbits Fomalhaut, a star only 25 light years away from Earth, located in the constellation Piscis Australis, or the "Southern Fish." NASA reports:

In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of the region around Fomalhaut. (Note: A coronagraph is a device that can block the bright light of a central star to reveal faint objects around it.) It clearly showed a ring of protoplanetary debris approximately 21.5 billion miles across and having a sharp inner edge.

This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto.

Hubble astronomer Paul Kalas, of the University of California at Berkeley, and team members proposed in 2005 that the ring was being gravitationally modified or "shepherded" by a planet lying between the star and the ring's inner edge.

Now, Hubble has actually photographed a point source of light lying 1.8 billion miles inside the ring's inner edge.

...

Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star, and is therefore gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from our sun.

The planet is brighter than expected for an object of three Jupiter masses. One possibility is that it has a Saturn-like ring of ice and dust reflecting starlight. The ring might eventually coalesce to form moons. The ring's estimated size is comparable to the region around Jupiter and its four largest orbiting satellites.

Kalas and his team first used Hubble to photograph Fomalhaut in 2004, and made the unexpected discovery of its debris disk. At the time they noted a few bright sources in the image as planet candidates. A follow-up image in 2006 showed that one of the objects had changed position since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion.

To give an idea of how large an 872-year orbit is, the dwarf planet Pluto orbits the sun once every 248 years (about 28% of the time Fomalhaut b takes); the dwarf planet Eris orbits the sun once every 557 years (about 64% of Fomalhaut b's orbit).

Of course, none of these four exoplanets may actually qualify as being the first seen visually; the Minister reported on a possible exoplanet being seen in mid-September.

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One other article of note. In an article on the evolution of rocks and minerals, The Economist notes:

Understanding just how dramatically life shapes minerals will play an important role in the exploration of the universe, says Dr Hazen. Knowing which minerals form at different stages of a planet’s evolution, and which depend upon life to be present, are crucial to understanding the mineralogy of other planets and moons.

With NASA’s Messenger probe now going into orbit around Mercury, Dr Hazen predicts that it will find only 300 or so minerals on the planet. If there are 500-1,000 detected, then it will suggest that there is a lot more to Mercury than anyone originally thought. And if minerals that depend upon life for their formation show up, then researchers will be flummoxed. The same is true for Mars and other planets—including the exoplanets that have been known about but which have just been seen for the first time orbiting stars outside the Solar System. Dr Hazen argues that considering minerals in evolutionary terms is a powerful way to help identify how far a planet has developed geologically. Moreover it can tell you whether life was present at some point—and even whether it is present now.

Photo Credits: Of the HR 8799 system, The Economist; of the Fomalhaut system, NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (Univ. California, Berkeley), M. Clampin (NASA/Goddard), M. Fitzgerald (Lawrence Livermore NL), K. Stapelfeldt, J. Krist (NASA/JPL), courtesy of APOD

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.