It seems like every spacecraft headed away from Earth to some distant solar-system target takes a moment to look back and record its home planet for posterity. Most often there's a bit of science involved — the imaging instruments use Earth or the Moon as a calibration target. But what gets released to the news media is pure PR.

What's different about the image at right is that the NASA spacecraft responsible for it, Deep Impact, is nowhere near us. In fact, this photo shoot took place in late May when the spacecraft was 31 million miles (50 million km) away. That's only a skosh closer than the minimum separation of Mars and Earth during a really good year (as 2003 was).

You might recall that Deep Impact had its 15 minutes of fame when it slammed an artillery-size copper bullet into Comet Tempel 1. That was three years ago. So why is the spacecraft still taking snapshots of the inner solar system?

Deep Impact has been given a second life as a combination comet chaser (next up is Hartley 2 in 2010) and extrasolar-planet sleuth. This new mission has been dubbed EPOXI, for convoluted reasons. The spacecraft recently took a time-lapse video of the Moon transiting in front of a "first-quarter" Earth. Mission scientists think that such long-range photography may give them an edge when it comes to identifying Earthlike worlds around other suns.

Regardless, the result is very cool. Deep Impact took images every 15 minutes throughout a full Earth rotation, and the Moon steals the show during a 4½-hour dash across center stage.

Learn the how and why of it from NASA's press release, or just view/download the QuickTime video here. (A second version, utilizing an infrared channel that makes landmasses more obvious, is here.)

In some of the richest, most tumultuous parts of spiral galaxies — the dusty gas clouds veiling embryonic stars — molecules lurk that could serve as life’s Legos. These aromatic hydrocarbons are carbon-based compounds found nearly anywhere combustion occurs — from stellar nurseries to terrestrial barbeque pits. They appear abundantly in the interstellar dust of the Milky Way and nearby galaxies.

But Karl Gordon (Space Telescope Science Institute) and his colleagues claim that their Spitzer Space Telescope observations reveal something strange about the polycyclic aromatic hydrocarbons (PAHs) in the outer rim of M101, a big spiral galaxy just off the Big Dipper. Here, there aren’t any.

The dearth appears as the red patches in the infrared image here. Both the M101 study and observations of starburst galaxies led by one of the paper’s co-authors, Charles Engelbracht (University of Arizona), find a correlation between the PAH decline and an increase in the ionization of hot hydrogen regions. The astronomers think that the dead zones exist because harsh radiation from hot young stars destroys organic molecules. Radiation from stars in galaxies’ outer regions should be more damaging to organics, because these stars have a lower heavy-element content than stars closer to the galactic core, and high-energy radiation can more easily pass through their atmospheres. This radiation would also ionize the hydrogen regions.

At the same time, PAHs have a major role in star formation. Due to the way they absorb and re-emit radiation, PAHs help lower the temperature in a molecular cloud. Low temperatures are important because a cloud must be cold enough to clump up before stars can be born inside it: the clumps collapse under their own gravity to form the stars. If gas is hot, it exerts too much pressure to clump well. Since the PAH dust did not exist in the early universe, observing star formation in places like M101’s outer rim today allows astronomers to study how the first stars might have coalesced soon after the cosmos’s dawn.

The group’s paper appears in the July 20th Astrophysical Journal.
The best-resolution pictures are here.

When it comes to accurately weighing the supermassive black holes that lie at galaxies’ centers, there has been only one method: tracking the motions of stars and gas in the central region to calculate the mass of the unseen body that causes them to move that way. But for scientists trained to test and re-test results in as many ways as possible, having just one technique to measure something that’s not even directly detectable does not sit well.

But now there’s a new way to weigh. First suggested 10 years ago by Fabrizio Brighenti (University of Bologna, Italy) and William Mathews (UC Santa Cruz), the technique depends on the peak temperature of gas in the central region. The gas is compressed by the black hole’s gravitational influence and becomes hot enough to glow in X rays.

Using this temperature and the gas’s entropy (a quantity related to temperature, density, and a few other physical constants), Philip Humphrey (UC Irvine) and his colleagues calculate that the supermassive black hole at the center of the giant elliptical galaxy NGC 4649 has a mass 3.4 billion times that of the Sun. The value agrees well with previous estimates for the black hole’s mass as one of the largest in the local universe. It also outweighs Sagittarius A*, the Milky Way’s own central black hole, by more than 1,000 times.

The X-ray method works particularly well for galaxies like NGC 4649, which lies 50 million light-years away, because the material in their cores is not as "stirred up," explains Humphrey. In an active core infalling and ejected material interact more, and these disturbances make it difficult to distinguish the origin of temperature spikes, he continues.

In contrast, the gas in NGC 4649’s center appears to be in hydrostatic equilibrium — a state of balance between gravity pulling in and gas and radiation pressure pushing out. Astronomers refer to NGC 4649’s black hole as “quiescent,” or quiet, because it’s not swallowing the gas close by.

“There seems to be an observed correlation between the masses of central supermassive black holes and the galaxies they exist in,” Humphrey explains. But without verified mass measurements, scientists have only been able to conjecture what that connection is. Pinning down supermassive black holes’ masses will help astronomers to make more general statements about their occurrence and help test models of galaxy formation.

The study will appear in an upcoming issue of the Astrophysical Journal.

In recent years, NASA's overriding focus in Martian exploration has been to "follow the water" — especially the water of modern-day Mars.

Today it's all frozen. This past week the Phoenix lander has been clawing away at the rock-hard slab of ice just a few inches below its footpads. The smart money says that a thick layer of this "white gold" lies barely buried across much of the planet's polar regions.

But for decades spacecraft pictures have been telling us that ancient Mars was a far different place, and that liquid water freely coursed across its surface. How widespread were the flows, scientists wonder, and for how long? The key to knowing whether the Red Planet was ever hospitable to life is buried in those hard-to-know details.

One leap in our understanding came in 2005, when the European Space Agency's orbiter called Mars Express used its infrared spectrometer to discover extensive deposits of phyllosilicates (clay minerals) on the surface. The implication was clear: liquid water, and a lot of it, had saturated the ancient rocks and altered their chemistry.

Now a far-more detailed view of water-driven chemistry has been revealed by NASA's Mars Reconnaissance Orbiter and its powerful CRISM infrared spectrometer. In the July 17th issue of Nature, John Mustard (Brown University), Scott Murchie (Applied Physics Laboratory), and 34 collaborators describe just how ubiquitously water affected early Mars.

CRISM has identified thousands of clay deposits in the ancient southern highlands of Mars, thanks largely to the 20-fold improvement over Mars Express's OMEGA spectrometer in resolving details on the ground. And it's spotted new types of clay minerals rich in aluminum and chlorite and even hydrated silica (what we call opal here on Earth).

It's far from certain that all these rock-and-water minglings took place on the surface. Sometimes they did. For example, in June 2nd's Nature Geoscience, a research team led by Brown graduate student Bethany Ehlmann describes how clay minerals permeate two deltas laid down on the floor of a 30-mile-wide Martian crater called Jezero.

But conceivably, says Murchie, much (and maybe most) of the chemical alteration occurred deeper down. Liquid water could have percolating through subterranean cracks for hundreds of millions of years — even if the temperature topside remained near or below freezing.

There's more of this story to come, Murchie teases, as the CRISM team starts delving into the chronological arrangement of the deposits and identifying more minerals. As Ehlmann notes, "These clay minerals offer just a taste of the geologic setting" that will allow the team to reconstruct the planet's ancient environment.

The Astronomical League began its annual meeting today in Des Moines, Iowa. It's a combined meeting of the AL and the Association of Lunar and Planetary Observers (ALPO). J. Kelly Beatty left this morning to attend the conference where he will give a talk tomorrow.

Among the other events throughout the next two days will be an awards ceremony that will include presenting the League's Webmaster Award. The distinction is given to "acknowledge the club Webmaster who does an outstanding job of website design and administration." The winner was announced late last week.

So if you see Del Gordon, congratulate him on winning the 2008 AL Webmaster Award for his work with the Huachuca Astronomy Club in Arizona. Based on his submitted portrait, he seems like an easy-going guy.

Second place went to Chas Rimpo of the Howard Astronomical League in Maryland.

Third place went to Richard Richins of the Astronomical Society of Las Cruces in New Mexico.

Once again I was happy to be a judge in evaluating the 10 submissions. Other judges were Drew Carhart, the 2006 Webmaster Award winner from the Naperville Astronomical Association, and Matt Ganis, who works at IBM.

Our evaluations were tallied by Bob Schneider, of the Boise Astronomical Society, administrator of the Astronomical League Webmaster Award.

As a nascent science journalist, I don't often have a chance to write about people and places I know well. That's not true for today's blog item. A group of scientists from my home university, Villanova, and Eastern University (just down the road from 'Nova) has devised a way to combat one of the most frustrating problems facing astronomers today: information overload.

With the deluge of new ground-based instruments and space telescopes, professional astronomers have to worry just as much about analyzing the gobs of data they amass as they do about obtaining them. Some turn to amateurs for help, others to students desperate for experience (I know: they're my classmates). But some databases — like the Optical Gravitational Lensing Experiment (OGLE) survey, which among other things identifies variations in the light coming from stars — have too much information for even a sizeable group of people to sort out in a lifetime.

The team offers a solution with its Eclipsing Binary via Artificial Intelligence (EBAI) project, spearheaded by Andrej Prša (Villanova/University of Ljubljana, Slovenia) and Edward Guinan (Villanova). Compact enough to run on a laptop, the EBAI neural network analyzes dips in the light coming from binary systems as one component passes in front of the other. From these light curves it calculates five of the stars' physical parameters, including the ratio of their combined radii to their separation and the system's orbital inclination, which can significantly affect observations.

Now, if you're like me, your eyes glaze over when you hear "neural network." This Treky term is actually just an exciting name for a simple concept, Prša explains. A neural network has three levels: input, hidden, and output. The input layer represents the data you enter into the program — in this case, thousands of light curves taken from OGLE and the Catalog and Atlas of Eclipsing Binaries (CALEB).

The hidden layer is a sort of intermediate form or representation of the input data. Here the network consults its "knowledge" of light curves and the parameters describing the stars that create them — derived from the more than 33,000 synthesized light curves the team fed the network as "training." It then combines the light-curve data points in different ways until it finds parameters that work for the system and spits out the results. On a basic PC processor, EBAI can analyze 15,000 light curves in 10 seconds — and most of that time is spent in input and output, not computation.

The astronomers then feed these parameters into another modeling engine called PHOEBE, a community project based on a code written by Edward Devinney (Villanova) and Robert Wilson (University of Florida). With the EBAI results PHOEBE can give a good idea of the more than 50 parameters needed to fully describe eclipsing binaries, Prša says.

EBAI will work for any light curve, not just those of eclipsing binaries. "We picked eclipsing binaries because they are the toughest," Prša explains. "Other types of stars rarely exhibit such a variety in parameters." Eclipsing binaries are also important because they are astronomers' only direct means to calculate both the mass and diameter of a star, he continues. Today's stellar models are largely based on what scientists have learned from eclipsing binaries.

While other researchers have successfully applied automatic data processing to survey results, EBAI is unique in how — and how fast — it solves for output, Prša says. Its results are only statistically good, though. "Values for individual stars can be good, average, or bad," he admits. "But even if it messes up for 10% of them, it still gets 90% of them right." That 90% offers a solid starting point for PHOEBE's subsequent analysis, he says.

The EBAI and PHOEBE programs are available free to anyone who wants to download, change, and contribute to them. The team's first paper will appear in the Astrophysical Journal.

In case you hadn't noticed, there's been a lot of chaos in the outer solar system — not because distant objects are careening out of control, but rather because astronomers are struggling as they come to grips with how to characterize them.

Lost in the din over finding large, distant Eris and the subsequent debate over Pluto's planethood was the 2005 discovery of a Kuiper Belt object initially designated 2005 FY₉ and later numbered as 136472. Quite bright (magnitude 16.7) despite its distance, 2005 FY₉ turns out to be the third largest trans-Neptunian object (after Eris and Pluto); its diameter is roughly 950 miles (1,500 km). Curiously, 136472 is the only big Kuiper Belt object lacking a satellite.

These temporary designations can be so confusing. And then there's the nickname "Easterbunny," coined by the discoverers (Michael Brown, Chad Trujillo, and David Rabinowitz) because they'd found 2005 FY₉ on March 31st, near Easter.

Finally, we can all start using this iceball's final, permanent, official name. On July 14th, the U.S. Geological Survey announced that it'll be called Makemake (pronounced MAH-keh MAH-keh). The International Astronomical union followed up with a press release on July 17th.

Objects in the Kuiper Belt are named for creation deities, and the god Makemake is the creator of humanity and the god of fertility for inhabitants of the Pacific island Rapa Nui. Most of us know this place as Easter Island — and the link to Easterbunny isn't a total coincidence. Mike Brown describes how his team and the International Astronomical Union came to agreement on Makemake in his online blog.

Now only one Kuiper Belt "giant," 2003 EL₆₁, lacks a permanent name. It's big (1,200 miles long), has two moons, and has been numbered by the IAU (136108). But it might not get a name anytime soon — because there's disagreement about who discovered it.

Exoplanet research hit paydirt this week when a team of astronomers from Caltech, the University of California, Berkeley, and the Harvard-Smithsonian Center for Astrophysics announced its survey of the dense star-forming Orion Nebula. The researchers revealed that fewer than 10% of stars there have enough material in their protoplanetary disks to create Jupiter-size planets. The results agree — almost shockingly well — with other studies of how common gas giants are and raise the possibility that our solar system is an exception to the family rule. The paper will appear in the August 10th issue of the Astrophysical Journal.

A mere million years old, the Orion Nebula Cluster packs 1,000 stars into the same volume of space that one star inhabits in the solar neighborhood. The Sun itself probably formed in a cluster like it, eventually straying away from its siblings over billions of years. The solar system coalesced from the leftover gas and dust in the Sun's surrounding disk, so finding such disks around young stars — like those in the Orion Nebula — indicates planets could be developing. As a typical stellar nursery, the cluster should help us understand star and planet formation as it occurs across the Milky Way, says the paper's lead author Joshua Eisner (UC Berkeley) in a prepared statement.

Using the new Combined Array for Research in Millimeter Astronomy (CARMA), Eisner and his colleagues observed more than 250 stars in Orion's central region and found that less than 8% had dust disks thought massive enough to create a Jupiter. The radiation from hot, massive stars in the cluster probably clears out a lot of surrounding material and keeps high-mass disks from forming, explains co-author John Carpenter (Caltech). In all, about 10% of the observed stars emit radiation associated with warm dust disks.

The results agree almost perfectly with findings by Geoff Marcy (UC Berkeley) and his colleagues, who conclude that about 10% of nearby stars have Jupiter-mass planets orbiting within 5 astronomical units (Jupiter's distance from the Sun). "This is an extraordinary moment in astronomy," Marcy says. Gas giants must form out of disks sufficiently massive to give birth to them, and "here we see that the numbers of such disks and of such planets agree."

The results do not mean, however, that only 10% of stars have planets. In fact, more than 10% of stars in Orion are known to possess disks massive enough to create a Saturn, and the average disk that the team found easily has enough material to make Neptune. CARMA isn't sensitive enough to detect the disks that would form smaller planets like super-Earths, but as Eisner explains, "With the average disk mass that we find, there seems to be adequate material there for terrestrial planet formation around a large percentage of stars."

How large is a "large percentage"? At least 80%, according to Marcy. Other studies conclude that at least that many stars have adequate disk material to form Earth-mass rocky planets.

So while few stars may have Jupiters, 8 out of 10 may have Earths — and there are maybe 100 billion stars in the Milky Way. Makes you feel small, doesn't it?

As recently as the 1970s, dynamicists scoffed at the notion that some asteroids have moons, even though observers had reported seeing occasional double blink-outs when asteroids passed in front of stars.

They aren't scoffing any more. More than 100 bodies in the asteroid and Kuiper belts are now known to be binaries. In fact, by some estimates roughly 15% of small objects in the asteroid belt and in near-Earth orbits are paired. It's an embarrassment of riches: far too many exist to have been formed by bodies simply crashing into one another.

A new study, published by a trio of theorists in the July 10th issue of Nature, finds that some asteroids can spin so rapidly that they literally fly apart. The researchers invoked a process known as the YORP effect that causes small bodies to spin up or slow down when exposed to sunlight.

YORP stands for Yarkovsky, O'Keefe, Radzievskii, and Paddack — the four scientists who identified the effect. The phenomenon occurs when a body absorbs sunlight and reradiates it as heat. For a spherical object, this is a zero-sum exchange of the two forces. But if the asteroid is faceted, as most small ones are, then some heat gets radiated at an angle instead of radially, causing a tiny torque that can either speed up its rotation or slow it down.

The Nature authors focused on different types of "rubble-pile" asteroids, chunky collections held loosely together by gravity. In their computer simulations, rubbly asteroids behave much like a fluid. As their spin rates quicken to just a few hours, the asteroids first become elongated footballs as mass slides from their poles to their equators. Eventually chunks of matter fly off these whirling dervishes — sometimes reaccumulating onto the main bodies and sometimes creating moons close by.

These YORP-driven cleavings work so well, say the authors, that the process likely is the source of many, if not most, binary asteroids. The University of Maryland's press release about the new finding includes an animation showing how a rubble pile can spin itself to pieces.

The joy — and the frustration — of astronomy is that so much remains that we don’t know. Supernovae, for example, are phenomena that astronomers like to think they understand. But Mansi Kasliwal (Caltech) and her colleagues have observed something strange: a supernova called SN2007ax, which they claim is the faintest and reddest Type Ia supernova yet observed. Using optical, ultraviolet, and near-infrared observations, they conclude that supernovae like SN2007ax prove that we still do not fully grasp the scope of physical processes involved in exploding stars.

Usually when people hear “supernova” they think of the cataclysmic death throws of a massive star. But that’s not the only kind of supernova out there. Type Ia supernovae occur when a white dwarf (the core of a dead star that wasn’t massive enough to explode) sucks up enough material from a more massive companion to ignite runaway nuclear fusion in the white dwarf’s interior, destroying the dwarf.

Astronomers use Type Ia supernovae as “standard candles.” Generally, these supernovae brighten and fade at the same rate and reach peak brightness at similar luminosities, so astronomers measure the difference between their absolute and apparent brightnesses to determine their distances. Type Ia studies led to the discovery that the universe’s expansion rate is accelerating.

Not all Type Ia are standard, though. There’s a subclass of unusually faint supernovae that Kasliwal’s team claims “has been purposefully overlooked” by astronomers. These subluminous Type Ia supernovae present a mystery. They are redder, fade more quickly, and have different spectra than their dependable siblings. SN2007ax, for example, was 10 times fainter at its peak luminosity than a normal Type Ia.

Using observations from several instruments, including the two Keck telescopes on Mauna Kea, Kasliwal’s team also found that SN2007ax’s ejecta are expanding at a medium velocity and have a smaller amount of radioactive nickel than normal Type Ia events. The values are similar to those for other known subluminous supernovae.

“These subluminous supernovae are not good standard candles,” says Kasliwal. She adds, however, that since they’re so much fainter than normal Type Ia explosions, astronomers find fewer of them far away from us. As a small percentage of the total observations, these subluminous supernovae should not seriously threaten the results that use Type Ia as reference points — such as the universe’s accelerating expansion.

The most exciting thing about these events is what they imply for the explosion process, Kasliwal explains. SN2007ax may be “just the tip of the iceberg.” Although it probably marks a white dwarf’s death, slightly fainter Type Ia supernovae could arise from a completely different physical process — perhaps even a violent thermonuclear flash produced by helium accreting onto a rapidly revolving binary system of white dwarfs.

Neither this model nor other exotic theories fit all the observed characteristics for these rebellious supernovae, though. The point is that astronomers just don’t know what’s going on in every supernova. Until they do, our understanding of these enigmatic explosions will remain incomplete.

The team’s results will appear in the Astrophysical Journal Letters.

It seems like water is everywhere lately. Mars, Mercury, our office’s storage room … and now the Moon.

Back in March S&T's Kelly Beatty described how researchers found water in tiny green-glass granules created by ancient volcanoes on the Moon — results that banished a long-coveted perception of a perfectly dry satellite. The team’s conclusions, appearing in the July 10th issue of Nature, have caused a bit of a splash in lunar science with their possible challenges to accepted Earth-Moon evolution models.

Contemporary models of lunar formation tell of a cataclysmic birth: a Mars-size object collides with Earth and throws molten material into space, where some of the fragments coalesce into the Moon. Scientists think that the high temperatures involved in this process vaporized all volatiles (lighter elements and molecules like water).

Alberto Saal (Brown University) and his colleagues offer a different lunar vision. Given how much water the glassy beads contain, the magma’s original water content may have resembled that of Earth’s upper mantle, according to team member Erik Hauri (Carnegie Institution of Washington). If this magma came from the Moon’s interior, water must have been deep inside.

The findings don’t throw out the collision model entirely. While scientists often use the lack of lunar water to support the great impact theory, there have been few attempts to link Moon-forming simulations to predictions of where the volatiles end up, explains Robin Canup (Southwest Research Institute), who specializes in planetary and lunar formation models. If a part of the Moon formed quickly after the impact — or (as simulations predict) from parts of the colliding body that didn’t hit Earth directly and therefore weren’t heated enough for the lighter elements to evaporate — it might have retained its water, she suspects.

The water could also come from meteorites or asteroids that struck the Moon. Saal’s team suggests that these collisions must have happened within 200 million years of the event that launched the molten lunar building blocks into Earth orbit.

Canup adds that the these new results may only reflect water abundance in a fraction of the Moon’s interior. Still, she agrees that the findings are important enough to kick-start attempts to pin down just how much of its light elements the Moon lost.

Twenty seconds. That's all it took for technicians to apply an ultrathin reflective aluminum coating to the primary mirror for the Stratospheric Observatory for Infrared Astronomy, or SOFIA. And now the world's most advanced airborne observatory is one step closer to completion.

No ordinary observatory, SOFIA is a 747SP aircraft that's been highly modified so that the telescope can view through a large rectangular hole cut in its fuselage.

The hole's been cut, and the plane has already undergone flight tests. But now the primary mirror, 98 inches (2.5 meters) across, will be permanently installed in the plane's midsection, soon to be followed by its suite of scientific instruments. "First in-flight light" should occur about this time next year.

Aluminizing of the mirror took place last week at NASA's Ames Research Center in California, where SOFIA is being readied for its first scientific missions. Accompanying the mirror was its mounting cell, which consists largely of a special plastic reinforced with carbon fibers. Because this material absorbs water vapor from the atmosphere, the assembly sat in a giant vacuum chamber for nearly a week to drive out a pint's worth of accumulated moisture.

SOFIA has been in the works for more than 20 years, and like many cutting-edge projects it's had to overcome technical challenges. The project's darkest hour came in 2006, when NASA managers decided the effort was too far over budget and behind schedule to salvage. What saved SOFIA was heavy lobbying from its international partner, DLR (Germany's equivalent of NASA), along with high marks from an independent review board.

By flying high above the bulk of Earth's atmosphere, particularly its water vapor, SOFIA will observe goings-on in the universe that are visible predominantly at infrared wavelengths: watching the birth of stars and planetary systems, mapping interstellar dust clouds, and identifying complex molecules in space.

As the dimmest and coolest stellar wannabes in the galaxy, a class of brown-dwarf stars, called T dwarfs, cling to a thin shroud of mystique. They’re hard to spot, and when astronomers do find them their low luminosities make figuring out their masses and ages difficult.

The T in their name comes from the appended stellar types: OBAFGKMLT, where O stars are the hottest and most luminous, M stars are the coolest and least luminous, and L and T types are substellar bodies — failed stars that cannot sustain hydrogen fusion in their cores. I usually remember the list with “O Be A Fine Girl Kiss Me, Lady Titania.”

I was reading A Midsummer Night’s Dream at the time.

Astronomers’ knowledge of T dwarfs has just received a boost. A team led by Michael Liu of the University of Hawaii has determined various physical properties for a pair of orbiting T dwarfs, challenging current models for cool atmospheres and providing new fodder for theorists. The team combined existing data from the Hubble Space Telescope and their own ground-based adaptive-optics observations for the calculations in the paper that will appear in the Astrophysical Journal.

The system, 2MASS J15344984–2952274AB (hereinafter called 1534–29AB), is a field binary, which means that it doesn’t appear to be associated with other clusters or star-forming regions. With no neighbors to compare the dwarfs to, calculating its age is tricky. Precise mass and luminosity values, as well as theoretical models on how brown dwarfs cool, allowed Liu and his colleagues to pin down an age of less than 1 billion years. That’s really young: the Sun is 4½ billion years old, by comparison.

The dwarfs are both around 30 times Jupiter’s mass, well below the limit separating true stars from substellar dwarfs. But given these sizes, the dwarfs' temperatures are 100 kelvins lower than expected. Confident in their values, the team thinks this discrepancy indicates a small overestimate in current models. Others agree.

“It is indeed quite an excellent result, and in fact likely a harbinger of results to come as more of these close binary systems are found and tracked,” writes Adam Burgasser (MIT), who previously studied 1534–29AB.

The astronomers also determined a 15-year orbital period, almost four times the length originally calculated.

Both Liu and Burgasser think the information garnered from binary dwarfs will “provide real empirical tests of the brown-dwarf theory” and help astronomers tweak models of ultracool atmospheres. Looking at the dregs of stellar society may also help astronomers understand the stellar population as a whole.

In our quest to find planets around other stars, we often forget that we have alien worlds of our own, just a spacecraft-hop away. Look at the inner solar system: four terrestrial planets, formed by the same processes and at the same time, yet each is undeniably unique. And we don’t understand why.

That’s why NASA has headed back to Mercury.

The Messenger spacecraft (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging), launched in 2004, is designed to answer lingering questions and outright debates about the closest planet to the Sun. What’s with the weird-looking surface? Why does Mercury have a magnetic field? Is that water ice in those permanently-shadowed polar craters?

Answers are pouring in from the spacecraft’s first flyby on January 14th, including images of 20% of the surface that Mariner 10 (the only other craft to visit the planet) left largely unseen during its flybys in 1974 and 1975.

The Messenger results appear in 11 reports in the July 4th issue of Science. During the flyby, the first of three before it settles into orbit in 2011, the spacecraft confirmed that volcanism created many of the smooth plains seen across the planet. That's especially evident around the Caloris basin, a gigantic impact crater with a diameter over half that of the planet. High-resolution images also show eruptions from isolated volcanic vents.

One instrument, a spectrometer that records ultraviolet, visible, and near-infrared sunlight reflected off the planet’s surface, found significantly less iron in the surface than is present on the other terrestrial planets and the Moon. Ground-based astronomers had previously come to the same conclusion, though their observations weren't nearly as detailed. The dearth of iron is especially odd because Mercury’s iron core comprises 60% of its total mass — twice that of any other planet — and volcanic flows on Earth are usually iron rich.

Messenger’s images also verify that the tall scarps and “wrinkle ridges” Mariner 10 saw extend across a significant portion of the planet’s surface (if not all of it). And the explanation for these features? Mercury shrank. A lot.

As Messenger principal investigator Sean Solomon (Carnegie Institution of Washington) explains in a Science podcast, the planet contracted between 3 and 4 billion years ago when its inner core cooled and solidified. The total shrinkage wasn't much in relative terms &mdash just 0.05% to 0.1% — but that was enough to create overlap faults across the surface similar to those made by crashing tectonic plates on Earth. The lost heat that caused the contraction may have been converted into the energy required to maintain the magnetic field generated within the core.

The Mercurian Magnet

Mercury’s magnetic field perhaps surprises astronomers more than anything else about the planet. The field appears to result from an active source, as opposed to being frozen into the surface. As Solomon explained in a press teleconference earlier today, the energy to drive the field may come from turbulence in the planet's outer core caused by iron as it solidifies and sinks.

The highly-dynamic field interacts with the solar wind along the magnetosphere’s boundary, a miniature version of the envelope that protects Earth from the solar wind and cosmic rays. Solar particles still punch their way through Mercury’s “flimsy” magnetosphere, though, often impacting the surface, explained FIPS project leader Thomas Zurbuchen (University of Michigan) in the teleconference. These particles can change the surface’s color and eject ionized material into the planet’s thin atmosphere or directly kick ions from the atmosphere into space. The atmosphere is so thin that its atoms are more likely to collide with Mercury’s surface than each other.

Messeenger's Fast Imaging Plasma Spectrometer (FIPS) detected a slew of ions — including sodium, sulfur, calcium, and even water — in the atmosphere and magnetosphere. These ions surround the planet in a cloud and form a comet-like tail pointing away from the Sun.

“What is in some sense a Mercury plasma nebula is far richer in complexity and makeup than the Io plasma torus in the Jupiter system,” said Zurbuchen in a prepared statement.

Still, a lot of questions will have to wait until Messenger settles into orbit, after which it can study Mercury long term and confirm these preliminary results. The Sun will be more active in 2011, too, and mission scientists expect a spectacular shower of information as the faster and more turbulent solar wind interacts with Mercury’s magnetosphere, atmosphere, and surface.

“These are very exciting results for me,” said investigator William McClintock (University of Colorado, Boulder). “I can’t wait until orbital observations begin.”

For more information and images of the iron planet, visit the Messenger website.

Related Articles:

Mercury's Better Half
Reunion With Mercury
Catching the Messenger of the Gods

Solar scientists from Europe and the U.S. have had it good since 1995. That's when the Solar and Heliospheric Observatory (SOHO) began conitnuous monitoring of the Sun from space at all kinds of wavelengths. Results from its 12 instruments have revolutionized much of what's known about our star.

But little did the SOHO scientists realize that their solar sentry would become the most prolific comet discoverer in history. As of June 25th, SOHO's tally has reached 1,500. Who knew!?

It turns out that a profusion of small comets swarms near the Sun, undetectable from Earth, as part of what's called the Kreutz group — fragments from a large body that veered too near the Sun centuries ago and broke apart. Roughly 85% of SOHO's comets come from this one breakup.

As each fragment plunges inward, the Sun's energy causes its ice and dust to boil off into space, creating a flashy but short-lived display. But it's usually a one-time-only performance: passing just a million miles from the solar surface at perihelion, few of these errant icebergs survive.

So who discovers all these Sun-grazing comets? Amateur astronomers mostly. A dedicated worldwide group scans the SOHO images as they're radioed to Earth for UFOs (Unidentified Frying Objects). A veteran Kreutz-chaser, Rob Matson, once discovered five SOHO comets in one day.

You can get in on the action too — mission scientists have set up a special website to get you started. Happy hunting!