NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

This image mosaic synthesizes some of the best views that NASA's Dawn spacecraft obtained during more than a year in orbit around the asteroid Vesta. A towering mountain, rising more than twice the height of Mount Everest, sticks out from the south pole at the bottom of the image. A chain of three craters known as the "Snowman" can be seen at top left.

NASA's Dawn mission is saying Hasta la Vesta with a series of parting shots showing the asteroid Vesta, unveiled as the probe hightails it for the dwarf planet Ceres, the next stop on its eight-year, 3-billion-mile (5-billion-kilometer) itinerary.

The newly released pictures were taken as the Dawn probe wound down more than a year's worth of observations while in orbit around Vesta, an acorn-shaped world in the main asteroid belt. Dawn's $466 million mission was launched in 2007 and is aimed at studying the composition of Vesta as well as Ceres, two huge asteroids that are thought to preserve a record of the solar system's earliest days.

Dawn's data showed that Vesta has a chemically complex surface and an iron core. Based on its scars, the protoplanet appears to have suffered a mighty cosmic impact not just once, but twice in the past couple of billion years.

If it weren't for Jupiter's disruptive gravitational influence, Vesta might well have grown to become a major planet. "We can now say with certainty that Vesta resembles a small planet more closely than a typical asteroid," UCLA's Christopher Russell, the mission's principal investigator, said last month.

Dawn officially left Vesta's orbit during the night of Sept. 4-5, and its ion propulsion drive is gently pushing the probe toward a rendezvous with Ceres in 2015. Russell and his colleagues said today's images represent the last routine daily delivery from the mission during the three-year cruise, although other images may be highlighted as fresh findings are made.

"Dawn has peeled back the veil on some of the mysteries surrounding Vesta, but we're still working hard on more analysis," Russell said in today's news release. "So while Vesta is now out of sight, it will not be out of mind."

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

An elevation map from NASA's Dawn probe shows the topography of the northern and southern hemispheres of Vesta, updated with readings gathered during Dawn's last look back. Colors represent distance relative to Vesta's center, with lows in violet and highs in red.

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

This image from NASA's Dawn mission, released Sept. 10, shows a shadowy view of the asteroid Vesta's northern hemisphere, using pictures obtained during Dawn's last look back. The mosaic is composed of five images obtained by Dawn's framing camera on Aug. 26, while the probe was at an altitude of 4,000 miles (6,000 kilometers).

Researchers expect to see a markedly different world when Dawn gets to Ceres. Unlike 330-mile-wide (530-kilometer-wide) Vesta, 583-mile-wide (940-kilometer-wide) Ceres is so massive that its gravity has crushed the world into a basically spherical shape — which is why the International Astronomical Union classifies it as a dwarf planet. Ceres has a differentiated crust, icy mantle and core, and may have a higher water content than Earth.

"Almost everything we see at Ceres will be a surprise, and totally different from Vesta," Russell said last week.

Months after Dawn's arrival at Ceres, NASA's New Horizons probe will fly by my favorite dwarf planet, Pluto, and its brood of moons. How will Ceres and Pluto compare? Will we see polar frost caps on Ceres? Ice volcanoes on Pluto? Stay tuned for 2015, the year of the dwarf planets.

More about Vesta and the dwarfs:

• Vesta is a protoplanet, scientists say
• Flash interactive: The new solar system
• Take a tour of five dwarf planets
• Pluto debate is about more than just one little world

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA / JPL-Caltech

It turns out that Eris, shown in this artist's conception, may be Pluto's denser twin.

It's been almost a year since astronomers suggested that Eris, the icy world whose discovery prompted Pluto's controversial reclassification in 2006, wasn't as big as they originally thought. Now the official word has leaked out unofficially: Pluto just might be the largest dwarf planet after all — although Eris is still seen as more massive.

The latest measurements were reported last week in Nantes, France, at a joint meeting of the American Astronomical Society's Division of Planetary Sciences and the European Planetary Science Congress. But as the Planetary Society's Emily Lakdawalla explains, it took a while for the report to become public, due to worries about the journal Nature's rules on embargoes and confidentiality.

Here are the statistics: Based on measurements made last November during the dwarf planet's occultation of a faraway star, Eris' diameter is estimated at 2,326 kilometers (1,445 miles). A similar set of measurements, published in 2009. estimated that Pluto was at least 2,338 kilometers (1,453 miles). When you include the margin of error, Pluto is essentially Eris' equal in size.

"It could be smaller, it could be larger; basically, it is a twin," Lakdawalla quoted Paris Observatory astronomer Bruno Sicardy, the lead researcher for the Eris measurements, as saying at the conference.

Lakdawalla held back from reporting what Sicardy said because she was asked to. The research paper about the measurements is under consideration for publication in Nature, and Sicardy said the journal's editors told him he could discuss the results only if he instructed his audience not to report them publicly. The implication was that Sicardy's paper would be tossed out if his team's findings appeared in the press.

The audience was all abuzz about the findings, of course, but Lakdawalla said she wouldn't "break anything until somebody else breaks it."

She did, however, refer to the zipped-lip situation in a Twitter message to Embargo Watch's Ivan Oransky. Long story short, Oransky checked with Nature and was told that "researchers with papers in submission at a Nature journal can certainly present at a scientific meeting but shouldn't court the press." Oransky blogs about the back-and-forth today on Embargo Watch, but the bottom line is that Sicardy needn't have feared having his paper rejected, as long as he confined his public remarks to the presentation.

If Nature sticks to the reported publication plan, the paper will be published on Oct. 26. Today, a lot of the details came out not only on Lakdawalla's blog, but also on Scientific American's Observations blog — which is interesting, because Scientific American is part of the Nature Publishing Group. (SciAm's John Matson helpfully included a link to Sicardy's conference report.)

So what else do Sicardy and his colleagues say? Although Pluto and Eris are roughly the same size, Eris is more massive, which implies it's "mainly composed of rocky material, with a relatively thin ice mantle," the astronomers say. They suggest that Eris once had a thicker layer of ice, most of which was "blasted away" as the result of a catastrophic cosmic collision.

Sicardy and his colleagues also note that when you factor in Eris' distance, its observed brightness and its relatively small size, the dwarf planet stands out as one of the brightest bodies in the solar system, after the Saturnian moons Tethys and Enceladus. They suggest that the dwarf planet is so bright because it has a surface layer of nitrogen or methane frost, due to the freezing-out of its atmosphere.

A similar freeze-out might well happen on Pluto as it heads out to the farthest point of its orbit around the sun. Eris, meanwhile, is coming closer to the sun — and at some point the nitrogen or methane might thaw back into the atmosphere.

The two worlds seem destined to stand in the planetary pantheon as separated twins — in possession of moons, seasons, their own distinctive geologies and potentially some kind of cryovolcanic activity. Should they really be regarded as non-planets, or is it better to see them as a different class of planets? I argue for the latter in my book, "The Case for Pluto," but I'd love to hear what you think. Please feel free to add your comments below.

More about dwarfs and other planets:

• Pluto debate is about more than one little world
• Interactive: The new solar system
• Eris looks a lot like Pluto
• Eight decades of Pluto
• The Pluto files on msnbc.com

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle.

A. Schaller / STScI

New observations suggest that the dwarf planet Eris, shown in this artist's conception, isn't as big as scientists thought it was.

Everyone knows that the solar system is no longer seen as a nine-planet set. When you count the dwarfs, there could be scores or hundreds of planets out there -- as I write in my book, "The Case for Pluto." But a case could be made that Pluto is once again the ninth-largest planet orbiting the sun, based on observations reported over the weekend.

Pluto was ousted from the No. 9 spot five years ago, after the discovery of another dwarf planet on the solar system's icy frontier, known today as Eris. It was Eris' apparent status as an object slightly bigger than Pluto that brought the controversy over the definition of a planet to a head. If Pluto was an honest-to-goodness planet, shouldn't Eris be one as well?

In 2006, the International Astronomical Union approved a definition that established a new class of objects, called dwarf planets, which were big enough to be basically round but not gravitationally dominant enough to "clear out the neighborhood of their orbit." What's more, the IAU ruled that dwarf planets were not really planets.

My book delves into the questions raised about that definition, particularly in light of what we've been learning about planetary systems since then. Now there's a new question: Is Eris bigger than Pluto after all? Based on observations of Eris' occultation of a faraway star in the constellation Cetus, the answer could well be no.

Sky & Telescope's Kelly Beatty reports that the latest observations suggest Eris is actually slightly smaller than Pluto. He quotes the Paris Observatory's Bruno Sicardy as saying Eris is "almost certainly" no wider than 1,454 miles (2,340 kilometers), compared with Pluto's estimated width of 1,456.5 miles, plus or minus 6.5 miles (2,344 kilometers, plus or minus 10 kilometers).

"If the early results hold up, this time it's the dwarf planet Eris' turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt," Beatty writes. The Kuiper Belt is the broad zone of icy objects that lie beyond Neptune's orbit.

Gathering the data for the measurements was a grand astronomical feat: Three teams of scientists watched the distant star disappear when Eris crossed in front of it. By analyzing how long the star was covered over, as seen from three vantage points in Chile, the astronomers could calculate how wide Eris' round disk was. Previous estimates were based on indirect data, such as Eris' brightness.

Further observations will be required to reduce the uncertainties surrounding the two worlds' widths. The current estimates are so close that Caltech astronomer Mike Brown, whose team discovered Eris and two other dwarf planets, can justifiably say Eris and Pluto are "more or less the same size." And when you rank the two by mass rather than size, Eris clearly comes out on top. That implies that Eris' interior is denser and thus rockier than Pluto's.

"How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?" Brown writes. "As of today I have absolutely no idea. ... Something is going on in the outer solar system, and I don’t know what."

Whether Pluto is bigger or smaller than Eris really doesn't affect its status as a dwarf planet. But it does illustrate that small celestial objects can deliver some big scientific surprises -- and that it's a huge mistake to write off the little guys of the solar system.

Correction for 6:50 a.m. ET Nov. 8: I originally wrote that Pluto might be the ninth-widest object in the solar system, neglecting to take into account that some moons are wider than Pluto (and in fact wider than Mercury). Thanks to Stevesliva for pointing that out.

More on dwarf planets:

• Pluto debate is about more than one little world
• Interactive: Guide to the new solar system
• Pluto maps raise new questions
• 80 years of Pluto

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also check out "The Case for Pluto," Alan's book about the controversial dwarf planet and the search for new worlds.