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!

The bright star Regulus, shining in Leo 77 light-years away, has long befuddled astronomers. It's not the brightness that puzzles them — though Regulus does emit more ultraviolet and X-rays than models predict. The problem is its spin.

Like many stars of spectral type B (i.e. hot, massive, and young), Regulus rotates much faster than it should, especially considering that it's already about halfway through its (short) hydrogen-fusing lifetime. In fact Regulus spins only 15% below the speed at which it would start to fly apart. This rapid rotation causes Regulus to bulge out around the middle. The result is a star that's not spherical, but oblate.

Until recently scientists could only speculate on what causes Regulus's zealous spinning. Astronomers know that a star can speed up if it is a member of an interacting binary system — a close binary whose stars transfer material. In these systems, one star may blow off a heavy stellar wind that blankets its companion in extra mass. In other cases, one star overflows the edge of its gravitational well and spills a thin stream of gas down onto the other. In each the gas hits the star like water from a hose hitting the edge of a ball. The water makes the ball spin; in the case of the receiving star, the stream causes the star to "spin up."

Astronomers already know Regulus to be a multiple system. The main star, Regulus A, has a distant companion that is itself a binary, consisting of two small, dim stars about Regulus A's age. These shine only 1/170 and 1/44,000 times as brightly as the primary. They lie too far away from A to trade mass with it, though, so they cannot explain the spin.

There's also the dilemma that many other type-B stars, such as Vega and Achernar, spin about as quickly but lack any close companion to explain the mystery. Achernar, for example, is even more stretched by its spin than Regulus A: it is 56% wider at the equator that at the poles, whereas Regulus A is only 32% wider. Achernar does have a companion, but it's your run-of-the-mill star, writes Doug Gies, one of the paper's authors, so "such a star is probably not a former mass donor."

But mass transfer may be the answer for Regulus after all. A team of astronomers from the United States, Chile, and Germany has released definitive evidence that Regulus A has a before-undetected close binary companion with at least 1/3 the Sun's mass. The companion has a period of just over 40 days and orbits Regulus A at Mercury's distance from the Sun.

The astronomers, combining data from 10 instruments and more than 20 years of observations, found the companion by studying minute wobbles in Regulus A's spectral lines caused by the companion's gravitational pull. The companion itself is too faint for astronomers to see, and its gravitational effect on the primary star is so small that it remained hidden in radial velocity measurements dating as far back as the late 1800s.

The astronomers offer two possibilities for what the companion is. It could be a low-mass main-sequence star, a cool red dwarf still burning hydrogen in its core. Or it could be a white dwarf, the hot core of a once-massive star that sloughed off its outer layers into space — or was unraveled by a cannibalistic Regulus A — to be left as a superheated skeleton.

The team favors the white-dwarf explanation. Since the Regulus system is relatively young (between 50 and 150 million years old, depending on whom you ask), the white dwarf would not have cooled off yet and could be much hotter than the primary star. This higher temperature would make it blaze at ultraviolet wavelengths, explaining the mysterious emissions purportedly coming from Regulus A. Mass transfer onto Regulus A from the star the white dwarf used to be would also explain the primary's fast spin.

Disagreement exists, though, as to whether the white dwarf would be hot enough to account for the X-rays. On the other hand, a small main-sequence star of that age would probably have a very active atmosphere emitting strongly in X-rays.

The binary model may not explain other quickly-rotating B stars and Regulus could be a fluke. But Gies notes that astronomers have found signs of hot subdwarf companions that went unnoticed for decades around other well-studied stars, so "the lack of evidence may not be surprising."

Despite the lingering questions, the news of Regulus A's companion falls on welcoming ears. "There have always been hints that Regulus was a spectroscopic binary," writes Edward Guinan of Villanova University, "but this study nails it down conclusively."

Out-of-this-world greenhouses might be feasible. That's the word from the Phoenix lander scientists, who announced via a teleconference on Thursday that the craft has found the surface of Mars to be surprisingly Earthlike, containing a macédoine of inorganic minerals that life would need to survive.

"We have basically found what appears to be the requirements, the nutrients to support life, whether past, present, or future," said Sam Kounaves (Tufts University), whose miniaturized wet-chemistry lab has radioed back the results of its first analysis. "The sort of soil there is the type of soil you'd probably have in your backyard."

The experiment, part of the Microscopy, Electrochemistry and Conductivity Analyzer (MECA), mixed dirt with water brought from Earth and took readings from sensors sensitive to the presence of inorganic salts containing chlorine, magnesium, sodium, and potassium. Still to come is a test for sulfur-bearing salts.

A key finding is that the sample's pH level — how acidic or alkaline it is — lies between 8 and 9, meaning it's slightly alkaline (neutral is 7) but a far cry from the extremes some researchers had predicted. While acid-loving plants like strawberries might have a hard time growing in Martian soil, Kounaves quipped, it'd be perfect for "asparagus, green beans, and turnips."

"There's nothing about it that would preclude life," said Kounaves. "In fact, it seems very friendly." He likened it to soils found in the dry valleys of Antarctica.

A second experiment, the Thermal and Evolved Gas Analyzer (TEGA), heated a sample to 1,000°C (1,800°F) to study the gases released. Lead investigator William Boynton (University of Arizona) reports that the extraterrestrial bake-off revealed carbon dioxide and water vapor that must have been chemically bound to minerals, confirming that dirt at the landing site had interacted with water in the past.

"We don't know whether that interaction occurred in this particular area in the northern polar region, or whether it might have happened elsewhere and blown up to this area as dust," said Boynton.

Phoenix has yet to find any trace of organic molecules, those containing carbon and nitrogen, though the lander's tests are still unfinished. Analyses of water ice unearthed by the lander should provide the most direct test for organic compounds, and Boynton says his detector should be able to detect abundances down to one part per million or less.

Michael Hecht (Jet Propulsion Laboratory), lead scientist for MECA, says that with these new results the lander's teams could actually start a list of the fertilizers necessary to have a successful greenhouse on the Red Planet.

Who knows? Maybe someday the veggies at the supermarket will bear stickers that say "Product of Mars."


Related Articles:
"Holy Cow!" - Phoenix Spots Ice
The Two Faces of Mars

Only within the last decade have stargazers come to appreciate what residents of north-central Pennsylvania have long known: the night skies over Cherry Springs State Park are among the darkest anywhere in the eastern U.S.

Two sizable annual gatherings — the Cherry Springs Star Party (hosted by the Astronomical Society of Harrisburg, PA) and the Black Forest Star Party (Central Pennsylvania Observers) — help the area attract thousands of amateur astronomers annually.

So it's fitting that Cherry Springs was recently declared an International Dark-Sky Park, only the second site to earn this designation from the International Dark-Sky Association. The IDA announced its decision earlier this month at its 20th-anniversary meeting in Tucson, Arizona.

The Pennsylvania Department of Conservation and Natural Resources, which manages the park, had already become a leader in night-sky protection and appreciation. But to meet the IDA's stringent guidelines for a dark-sky sanctuary, additional steps were taken to ensure that those pristine black skies would stay that way.

For example, park staff retrofitted all outdoor lighting to be fully shielded and to utilize low-wattage compact-fluorescent bulbs. Some areas of the park were even cordoned off as "no-light zones" to protect nocturnal animals and to maximize the view above for amateur astronomers.

"This designation is continued validation that this region has something special to offer to our visitors," noted DCNR Secretary Michael DiBerardinis in a prepared statement. "We are proud of what we have protected, and hope our visitors will enjoy the remoteness of the Pennsylvania Wilds and Cherry Springs State Park for many years to come."

One modest threat to the park's darkness comes from a proposed cluster of several dozen wind turbines to be built nearby. Tower lighting to warn aircraft in the vicinity could degrade the view very slightly, depending on what kind of lighting system gets installed. Results from a study to assess and minimize the lights' impact were made public in late May, and you can access them here.

In late 2006 Gary Honis and Stan Stubbe of the Pennsylvania Outdoor Lighting Council prepared their own assessment, titled "Protecting the Night Sky Resource of Cherry Springs Dark Sky Park". Honis worries that the tower lights will become a problem for wide-field astrophotography.

When NASA's Viking orbiters completed the first comprehensive map of Mars in the late 1970s, scientists back on Earth puzzled over why the Red Planet appeared two-faced. Its southern hemisphere, dominated by heavily cratered highlands, stood in marked contrast to the flat, low-lying plains found in the north.

In 1984, veteran researcher Don Wilhelms and up-and-comer Steven Squyres proposed that Mars had been struck a mighty blow early in its history — an impact so powerful that it blasted away much of the northern crust and left the planet with its Janus-like complexion.

Other scientists weren't so sure. For one thing, the resulting "crater," marked by the highland-lowland boundary that girds the globe, had an elliptical (not circular) outline. And the fit was particularly bad around the continent-size complex of towering volcanoes collectively called Tharsis. As an alternative, some researchers speculated that the young planet's interior had churned in an asymmetrical way that drove an enormous blob of hot magma upward on one side but not the other.

Yet Wilhelms and Squyres might have been right all along, as three papers in this week's Nature bolster the giant-impact scenario.

One tackles the poor fit of the highland-lowland boundary around Tharsis, which masks about 30% of it. A research trio led by Jeffrey Andrews-Hanna (MIT) used computer modeling, together with assumptions about the state of early Mars's rigid exterior (its lithosphere), to "remove" Tharsis. What remained was an elliptical boundary measuring about 6,600 by 5,300 miles (10,650 by 8,520 km) centered at 66°N, 151°W. (Andrews-Hanna offered a sneak peak of his team's result last March at a meeting of planetary scientists in Texas.)

A crater that size — larger than Asia and Europe combined — would rank as the largest impact in the solar system. But why wasn't it circular?

Margarita Marinova (Caltech) and two colleagues address that problem in the second Nature paper. Using three-dimensional modeling, they find that when impacts get this big, the spherical shape of Mars itself would have affected the outcome. And there's a "sweet spot" in the range of likely impact energies and collision angles that creates huge elliptical craters matching the Martian situation.

Marinova's team is betting that the impactor was 1,000 to 1,700 miles across and struck Mars a glancing blow (30° to 60°: from horizontal) at no more than 6 miles per second. Maverick planetoids this big must have been fairly common in the early solar system, and statistically they'd most likely strike other objects in this just range of angles.

Although not powerful enough to shatter the planet, the titanic blast would would have stripped away one hemisphere's worth of crust and smothered the other one with debris. The underlying mantle, superheated by the impact energy and exposed to space, would have flash-melted and later resolidified into new crust.

In fact, as Francis Nimmo (University of California, Santa Cruz) and three colleagues argue in the third paper, this Martian makeover should have resulted in a northern crust with a distinctly different composition than that in the southern highlands. It'd be a close match, they suggest, to a class of Martian meteorites called shergottites.

Interestingly, Nimmo's team concludes that the geophysical conditions for this mega-impact would have been just right about 100 million years after the planets formed — and, perhaps coincidentally, just about the time something the size of Mars slammed into Earth and created the Moon.

You can't access these articles unless you have a subscription to Nature, but abstracts are available here, here, and here.

It's not every day that a famous historical event, scrutinized by generations of classical scholars, can be re-dated by two astronomers and their college honors class. But that's exactly what Donald W. Olson and Russell Doescher of Texas State University did, with the help of students Kellie Beicker and Amanda Gregory. They report their findings in the August 2008 Sky & Telescope, which has just hit the newsstands.

Tipped off by Don in advance, I was fortunate to be able to join the team's research trip to the southern coast of England last summer. The white cliffs of Dover, subject of a memorable song from World War II (listen), were also the setting for a much earlier clash of civilizations. Along this very shore, Julius Caesar first landed with two legions of Roman soldiers in 55 BC.

Caesar, in his first-hand account of the invasion, carefully noted the phase of the Moon, the approach of the equinox, and above all the unexpected ocean tides his fleet encountered. So it's a simple matter for any astronomer to determine the precise date of the invasion, right?

Wrong! No lesser astronomers than Edmond Halley and George B. Airy carefully studied the astronomical aspects of 55 BC in hopes of letting historians know the exact date and location where Caesar and his legions came ashore. But Airy and Halley disagreed with each other. And what's more, they both got it partly wrong, as Olson's Texas State team found out on their research trip.

Some years back, Don realized that the summer of 2007 offered a unique chance to settle this tricky problem once and for all. In 55 BC the full Moon came about three days before lunar perigee and about 3.5 weeks before the equinox, just as in 2007, so the key tidal factors would be virtually identical. On less than a dozen dates in the last 2,061 years has this match been so good. August 2007 offered the perfect chance to find out just where and when Caesar came ashore in 55 BC.

What Needed to Be Determined

There were two top uncertainties to answer about the ocean currents when the Roman fleet arrived off the white cliffs of Dover:

 (1) Which way was the current flowing on the traditionally accepted invasion date on the afternoon of August 26 or 27, 55 BC?

 (2) Which way was the current flowing on an invasion date four days earlier, one that the Texas State researchers had already started to focus on?

To address the first question, our group went to the coastal town of Deal, the area historians have long believed to be the Roman fleet's eventual landing spot because it's roughly seven miles north of the stretch of white cliffs Caesar says he first encountered. That beach is indeed "open and flat," just as Caesar described. I noticed it wasn't sandy at all, being thickly paved with golf-ball-size pebbles its entire length, and wouldn't have been an easy place for Roman warriors to scramble ashore as they dodged a hail of spears and arrows from Britain's hostile Celtic tribes.

On the date in 2007 that corresponded closely to August 26 or 27, 55 BC, we walked out to the end of the Deal pier, which sticks out hundreds of feet into the English Channel. There Don tossed an apple into the ocean at roughly the same time of afternoon Caesar described the movement of the fleet. Sure enough, the apple drifted southwest toward Dover. No way could an invasion fleet, arriving in oar-powered triremes and other ancient warships, have come up from Dover on that particular afternoon.

To address the second question (the current's direction on the revised invasion date, August 22 or 23), we chartered a sightseeing boat that normally takes tourists around the Dover inner harbor. The skipper agreed to take us well beyond the breakwater, into the open Channel and northward along the white cliffs. To view my not-quite-ready-for-YouTube video clip as we pulled away from the Dover dock, click here (QuickTime player required).

Once we were out in the open sea, the skipper turned off the boat's engine. Kellie and Mandy began noting GPS readings and times, crucial data for determining the current's rate and direction by the drift of our boat. And yes — we were drifting northeast toward Deal. So on that afternoon, with lunar conditions so nearly matching those of 55 BC, the Roman fleet would have had no trouble making its way along the coast toward Deal.

I don't know about the others in our group, but I was starting to feel a little queasy as our small boat bobbed around in the choppy seas. I was glad when we got back ashore at Dover. The Roman fleet, its mission only just begun, had no such option.

For more more about the Texas State researchers' findings, check their press release as well as the August Sky & Telescope.

So You Never Knew All This About Caesar?

Time was when all high-school students translated Caesar's Commentary on the Gallic War in second-year Latin class. You know, the famous narrative that begins, "Gallia est omnis divisa in partes tres...." The other night I dug out my old textbook. (And Mr. Dalton, if you're out there, I saved a great caricature of you that was surreptitiously drawn by a fellow classmate of mine back in 1961!)

Flipping those old pages to Caesar's Book IV, I saw that I'd underlined the words, "Eadem nocte accidit, ut esset luna plena...," meaning "That night, it happened that there was a full Moon...." Caesar was from the Mediterranean, where there is very little tide, and he didn't know that true ocean tides have nearly their maximum range whenever the Moon is full. As a result, his invasion fleet faced unexpected challenges as they looked for a suitable landing beach on the British shore.

Picking up on subtle astronomical clues like these has been a hallmark of the many projects undertaken by Don Olson and his Texas State researchers for past articles in Sky & Telescope — whether in a well-known painting, the original marathon, or a famous photograph by Ansel Adams.

The Roman army's historic landing on the coast of Britain in 55 BC involved perhaps 100 ships and 10,000 men. But this was a rather limited incursion, by Caesar's own standards. Buoyed by the sensation his exploits caused back home, Caesar returned to Britain the following spring (54 BC) with an invasion fleet perhaps 10 times larger. It was like a D-day in reverse.

Caesar's crossing of the Channel, nearly 21 centuries ago, would alter the course of British history forever.

Whether you just took your first picture of the night sky or have years of experience, there was something for everyone in Hoffman Estates, Illinois, during the MidWest Astro-Imaging Conference June 20-21.

Sponsored by Al Degutis of AstroPhoto Insight Magazine, the event offered a broad selection of presenters specializing in everything visible in the sky from the Moon to the faintest reaches of space.

Notable presenters included NASA/Goddard Space Flight Center staff scientist Jerry Bonnell, who co-authors the Astronomy Picture of the Day, offering tips for submitters on making your work stand out.

Imaging artists Ken Crawford, Neil Flemming, R. Jay GaBany, Warren Keller, and Bob Pilz all gave presentations detailing their processing work flows using Adobe Photoshop (as well as Warren's tuxedo-clad "Russian Filmmaker" alter-ego).It was especially nice to see lunar imaging specialist Pilz demonstrate his tricks for capturing top-rate lunar photos - a pursuit often neglected at similar events.

A great gem numerous imagers mentioned with frequency was a plug-in filter for Adobe Photoshop called Focus Magic. Designed for forensic applications, This filter performs a type of deconvolution useful for any type of image, and works quite well.

Speaking of Photoshop, representatives of software and cameras were on hand to demonstrate the latest in digital technology. Adobe, Canon and other imaging-product vendors including Diffraction Limited, DC-3 Dreams, MLUnsold Digital Imaging were represented in the vendor area. Both Quantum Scientific Imaging and Fishcamp Engineering showed off their versatile cameras.

Workshop sessions were held in smaller classrooms, allowing those interested to experiment and follow along with various software packages such as ACP, AIP4WIN, MaxIm DL, and ImagesPlus.

We call our Sun an “average star.” It’s average because its size and brightness fall smack dab in the middle compared to stars throughout the universe. Some stars make the Sun look like an ant standing next to Abe Lincoln; others, an elephant overshadowing a ladybug.

Stars differ in mass much less than they do in diameter or brightness, but they still cover a wide scale. The lightest stars have about 1/12 the Sun’s mass (any lower and they can’t sustain nuclear fusion), while the most massive top scores more.

So how heavy can a star actually be? For the universe’s present era, astronomers assume an upper limit of about 100 solar masses. This number comes from the need to preserve something called hydrostatic equilibrium. The term means that the two main forces working on a star have to balance out. One is gravity, which pulls a star together. The other is — no joke — the pressure of the star’s own heat and light.

As energy streams out of the nuclear-fusing core, the photons push outward on the star’s layers. This pushing is called radiation pressure. For supermassive stars (which create a lot of heat and light), the radiation pressure on the star’s outermost layers overcomes the inward pull of gravity, blowing layers away as a stellar wind. This critical ratio between mass and radiation intensity is the Eddington limit, named after the astronomer who first figured it out in the 1920s.

But astronomers have a hard time observing this upper mass limit in the universe. The problem is that high-mass stars are rare. They don’t form often, and when they do they burn themselves up much faster than smaller stars. The most massive stars may only live for a few million years (the Sun, on the other hand, will burn hydrogen for about 12 billion years). Even with today’s equipment astronomers still have yet to identify any stars weighing in at 100 solar masses.

That’s not to say the theory’s wrong. Astronomers have found a handful of stars in the 85-solar-mass range. But the data have everyone wondering: are there any 100-solar-mass stars?

A team at the University of Montreal now answers a tentative yes. Looking deep into NGC 3603, one of the youngest and most massive star clusters in the Milky Way, the scientists studied five extremely luminous stars. One is an only child, and the others are in two binary pairs. For one of the binaries, the team used shifts in the two stars’ spectral lines caused by the stars’ speeding around each other to calculate a rough mass for each companion. They came up with 85 to 145 solar masses for one, 75 to 105 solar masses for the other.

Those are large uncertainties, and the team knows it. They plan to go back for higher quality data. Even with the possible error, though, their paper still claims the 85-to-145 star is “the most massive star ever directly weighed.” If so, it will be a feather in theorists’ caps.

Most of us think of Alaska or Iceland when we imagine aurorae. Maybe you’ve seen them at lower latitudes, like in Minnesota or New York, or (rarely) even Arizona. But aurorae light up other places, too. I don’t mean places like Egypt; I mean places like Jupiter and Saturn.

On these planets (like on Earth) glowing rings encircle both poles. But Saturn’s main auroral oval has puzzled astronomers for a long time. Unlike for aurorae on Earth and Jupiter, researchers can’t determine which of three processes creates it.

We’re not ready to decide that debate yet. But scientists from the UK and the United States describe in the June 18th Nature something even more interesting: a second auroral oval on Saturn that clearly does not work the same way as Earth’s.

Aurorae arise when charged particles stream in along a planet’s magnetic-field lines and crash into its upper atmosphere. On Earth, these particles come from the solar wind. They enter Earth’s magnetosphere through “open” field lines at the poles that connect to the field embedded in the solar wind itself, providing a direct path from the Sun to the top of our atmosphere.

On Jupiter, however, the charged particles come from the volcanic moon Io. Thin, ionized gas from Io is caught by Jupiter’s magnetic field, which rotates rapidly with the planet. The ions can’t keep up with Jupiter’s fast rotation at the equator, though; they stop co-rotating with the planet and slide along the magnetic field lines to Jupiter’s polar regions. The aurorae that result appear at latitudes specifically tied to this “co-rotation breakdown.”

On Saturn, the newly discovered second auroral oval glows at Saturn’s co-rotation breakdown latitude, too.

The new oval is only a quarter as bright as the main oval, though. Researchers have not yet completed ultraviolet studies to corroborate the infrared data that led to the discovery.

Scientists also don’t know yet where the ions feeding into Saturn’s magnetic field come from. “Until relatively recently, it was thought that sputtering off the surface of the icy moons and rings would be the dominant source for Saturn’s plasma,” writes principal investigator Tom Stallard (University of Leicester). He notes that the moon Enceladus and its ice-geyser plume likely provide Saturn’s magnetosphere with about one tenth the material that Io injects into Jupiter’s.

Nevertheless, given their results the team concludes that modeling Saturn’s aurorae as a “hybrid” of Earth and Jupiter’s lights is now unreasonable.

On October 6, 1990, scientists in the United States and Europe sat waiting. Five years beforehand, they had watched the Space Shuttle Challenger explode in a horrifying blaze above Cape Canaveral, taking with it not only the shuttle's crew but also the craft that was due to take the team's joint project to the stars later that year. Now, on board Discovery, their explorer was finally aloft. They watched elated as the satellite separated successfully from the shuttle and set off for Jupiter, where it would use the giant planet's gravity to sling itself back to the inner solar system — and toward the Sun.

So began a mission that would span almost 18 years. In that time the Ulysses spacecraft has fundamentally changed the way astronomers see our corner of the galaxy. At the end of June, failing generator power, inevitable after so many years' service in space, will leave Ulysses a frozen solar satellite, orbiting our star's poles on a comet-like path.

Ulysses's primary mission was to study the Sun and its heliosphere, the giant magnetic bubble carved out in space's near-vacuum by the charged particles that make up the solar wind. One of Ulysses's many discoveries was the existence of a second, faster solar wind speeding from the Sun's poles. Scientists originally only knew of the slower wind moving directly past us through the ecliptic plane (the solar system's "equator," in which the Earth and other planets orbit the Sun). The fast wind races through space at roughly 500 miles (800 km) per second, twice the slow wind's speed.

"We found that this is really the normal solar wind," said Ulysses project scientist and mission manager Richard Marsden in an ESA television broadcast. "It fills much of the heliosphere over much of the solar cycle."

Astronomers still debate the slow wind's origin, though they're reasonably certain that the fast wind's particles stream directly out of holes in the Sun's upper atmosphere. The fast wind also better reflects the photosphere's composition than the slow wind does. (The photosphere is the Sun's "surface," the yellow layer we actually see. The Sun's "surface temperature" is then the photosphere's temperature — roughly 6,000 kelvins or 11,000°F.)

The solar wind varies in conjunction with the Sun's 11-year sunspot cycle, which ends when the star's magnetic poles flip. Mission data showed that the solar magnetic field resembles that of a bar magnet and that the solar-activity cycle corresponds to that bar magnet turning like the hands on a clock face, said Marsden. Activity minima happen when the magnet aligns with the Sun's rotational axis (12 and 6 o'clock), maxima when it lies perpendicular to it (3 and 9 o'clock).

Ulysses also proved the complexity of the solar magnetic field's size and shape, which allows energized particles from high and low latitudes to switch places and possibly escape to threaten astronauts and satellites.

"Over its long life, Ulysses redefined our knowledge of the heliosphere and went on to answer questions about our solar neighborhood we did not know to ask," said project scientist Ed Smith (NASA/Jet Propulsion Laboratory) in a prepared statement.

But Ulysses's life wasn't all glory. Several delays plagued its development from its official inception in 1977 to its 1990 Discovery launch. Soon after reaching space, an extended boom set the satellite wobbling. The motion, which prevented precise measurements, threatened to ruin the mission. The team eventually determined that the nodding arose from flexing caused by the Sun heating the boom. The spacecraft has since successfully compensated with its thrusters.

Ulysses's longevity ultimately depended on its plutonium-fueled power source, called a radioisotope thermoelectric generator (RTG). The generator powers the craft with the heat produced as the plutonium decays, but as this process also depletes the plutonium supply the amount of energy available to power Ulysses decreases over time. The energy loss wouldn't have been a problem if Ulysses had stuck to its five-year plan. Pushing it to complete multiple pole passes instead of one means keeping the satellite's 10 instruments on a precarious schedule of on-and-off use. While the instruments conserve power when off, they also stop producing enough heat to keep the spacecraft's fuel from freezing in the pipelines.

Then, on January 15th of this year, the main transmitter failed to turn back on. The team knew it had reached the end.

"It's like saying goodbye obviously to an old friend," said Marsden.

Ulysses will leave behind its own epic — written in the 1,500 papers using its data and the revelations it brought to astronomy. It was the first to detect interstellar dust, finding 30 times the amount expected and proving the heliosphere was not impenetrable as scientists thought. And its collection of helium atoms from deep space helped to prove that the universe does not have enough matter to stop its own expansion, nurturing theories on dark matter and the shape of space-time.

"When the last bits of data finally arrive, it will surely be tough to say goodbye to Ulysses," said mission operations manager Nigel Angold (ESA). "But any sadness I might feel will pale in comparison to the pride of working on such a magnificent mission."

After a few snags with its Delta II vehicle — which delayed its launch from June 3rd, to the 5th to the 7th, and finally to the 11th — NASA's newest space observatory is finally in orbit.

The Gamma-ray Large Area Space Telescope, or GLAST, rode a pillar of smoke and fire as it rocketed skyward from Florida at 12:05 p.m. EDT on the 11th. Now it's safely orbiting Earth at an altitude of 350 miles (560 km). The mission's engineers and scientists will give it a thorough checkout over the next two months, and the first observations from its instruments should come in about three weeks.

GLAST is designed to study the most energetic processes in the universe, such as the jets of superheated matter ejected from black holes and powerful explosions known as gamma-ray bursts. You can read about the instruments and their objectives here.

For the moment, however, the science can wait. Instead, check out the terrific sets of images and videos from the launch.

The Hubble Heritage Project released this image on Tuesday of part of the Coma Cluster of galaxies in the constellation Coma Berenices. The cluster lies more than 300 million light-years away and appears near the Milky Way's north galactic pole, well above our galaxy's plane of gas and dust. It's one of the densest galaxy collections astronomers know of.

The image covers a section about a third of the way out from the cluster's center. It contains one prominent spiral galaxy (where star formation is ongoing in the bluish regions) and several elliptical and S0-class galaxies filled with old stars (golden tan). Background galaxies far behind the cluster also peek in, and some of them are greatly redshifted.

Edwin Hubble first separated galaxies into three main classes — spirals, ellipticals, and irregulars — in 1926 by how they appeared on photographs. Ellipticals look like spheres with varying degrees of squashed-ness. They are made up of only older, yellower stars because they have little of the gas and dust needed to create new ones. Spirals, like the Milky Way, have bright arms filled with dust and gas clouds and brilliant young stars that wind around a yellower (usually) central bulge. Hubble split this category into ordinary spirals and barred spirals, depending on whether the center was round or an elongated bar. Irregulars also have young stars but don't look like much of anything.

Ten years later, Hubble added the S0 class as a link between ellipticals and spirals. Like ellipticals, S0 galaxies consist of older stars, but unlike the featureless gold "fuzzballs," the S0s show vague structures such as bars, rings, or disks.

Astronomers now realize that the "Hubble type" does not correspond to a galaxy's age, but only whether star formation stopped. Nevertheless, Hubble's galaxy classifications and his "tuning fork diagram" portraying them have stuck.

What the HST image doesn't show is the superheated gas filling the cluster. Although the gas is invisible at optical wavelengths, it blazes in X-rays. Scientists think jets from the centers of some of the galaxies may keep the gas toasty, since it glows at a far higher temperature than expected for the cluster's age. The hot gas may also tear cold gas from the cluster's galaxies, depriving them of the material necessary to form new stars and leaving them with faded features.

For the full-resolution image and more information, see the European Space Agency's press release. The image is about 9 arcminutes wide (1.7 million light-years at the cluster's distance). Further image details are also on the Hubble Heritage site.

Big space objects aren't the only ones who throw their weight around. At least that's what scientists are finding in Saturn's outer rings, where new observations from NASA's Cassini spacecraft pin responsibility for twists in the planet's F ring on tiny moons. The results appear in the June 5th Nature.

"Saturn's F ring is perhaps the most unusual and dynamic ring in the solar system," says Carl Murray, the team's leader and a professor at Queen Mary, University of London, in a press release. "It has multiple structures with features changing on a variety of timescales from hours to years." The rope-like F ring is the third outermost of Saturn's rings and lies roughly 87,000 miles (140,000 km) from the planet's center.

The team found two different types of features: "jets" and "fans." The jets are extensions of ring material either toward or away from the planet that look like smears through a straight chalk line. They likely arise from collisions between material in the ring's central concentration and surrounding satellites. Fans, on the other hand, are a series of furrows that converge in the ring that scientists believe result from the gravitational effects of embedded satellites.

Numerical simulations for two features indicate diameters of 9 and 44 miles (14 to 70 km) for the guilty satellites, although multiple smaller satellites may offer a better solution for producing fans than one moon the size of Rhode Island. Other fans also suggest smaller culprits. Two objects seen by Cassini, called S/2004 S 6 and F07090QB, fit the team's data.

Earlier studies determined that the moon Prometheus plays a prominent role as well. Murray's team believes that Prometheus, which orbits just inside the F ring, directly clears channels in the ring and also off-balances embedded objects that then gravitationally affect the surrounding material.

The scientists think that the processes responsible for the jets and fans parallel how planets coalesce by accreting material from a dust plane. They hope to use the F ring observations to better understand planet formation.