Long, long ago, when I was a college student, my profs taught me about stellar black holes, the remains of collapsed stars so dense and massive (3 to 15 Suns's worth) that not even light could escape from them. Then along came evidence for "supermassive black holes," hoarding millions or billions of times the Sun's mass, and by the 1990s it became clear that these behemoths must lurk at the center of most galaxies.

Several years ago astronomers began to speculate about the existence of a third class of supercompact object that, heft-wise, lies somewhere in between. The evidence for these intermediate-mass black holes was largely circumstantial: an ultra-luminous stream of X rays coming from inside a galaxy — but not from its center. The candidates included sources in the famous globular cluster Omega Centauri and in Messier 81 (Bode's Galaxy).

But no observer could make an ironclad case for an IMBH. Skeptics argued that these beacons could simply be stellar-mass black holes that are beaming X-ray jets at Earth, thus appearing more massive than they actually are. Or they could be massive white-dwarf stars gobbling matter from companions at a furious rate, belching copious X rays in the process. Or they could even be unrelated X-ray sources juxtaposed either directly in front or behind the candidate galaxy.

Now observers have come forward with two candidates that represent the strongest cases yet for midsize black holes.

The first, reported in Nature for July 2nd, involves the edge-on spiral galaxy ESO 243-49 in the southern constellation Phoenix. Observers led by Sean Farrell (now at the University of Leicester) first identified it in a 2004 X-ray survey compiled by XMM-Newton, a European space observatory launched in 1999. When the researchers made a follow-up observation last November, they found that their target had dimmed and its X-ray spectrum had changed.

Dubbed HLX-1 (for Hyper-Luminous X-rays), the source is offset from the galaxy's center by about 8 arcseconds. Farrell and his colleagues believe they've ruled out alternate explanations like X-ray beaming, a voracious white dwarf, or a binary neutron star. They can't completely dismiss a more distant X-ray beacon coincidentally aligned with the galaxy — there's a 1-in-11 chance of that.

But if it really belongs to ESO 243-49, then Farrell's team estimates that it's a black hole with (based on its X-ray brightness) at least 500 times the Sun's mass.

A second strong candidate has been identified by Rodrigo Ibata (University of Strasbourg) and others, who're making their claim in a paper submitted to Astrophysical Journal Letters. This time the host is a globular cluster, Messier 54, which is embedded in the Sagittarius dwarf galaxy.

Ibata's team used the Very Large Telescope in Chile to measure the orbital velocities of stars whirling tightly around the cluster's densely packed core. The high speeds observed argue that they're circling something quite massive. But what is it? Ibata and his colleagues admit that one explanation is a central crush of stars in peculiar orbits. But if it's an IMBH, then its mass must be about 9,400 Suns. Observations with the Chandra X-ray Observatory, which found a bright source at the same location, appear to back up this claim.

This estimated mass is strikingly similar to that of the candidate object at the center of Omega Centauri, a globular that astronomers suspect is what remains of a galaxy stripped of everything but its core. The same fate has been suggested for the Sagittarius dwarf galaxy. So might M54 be the core of a once-larger system, the decimated loser of an intergalactic fight?

NASA's newest lunar orbiter arrived on the scene just a few days ago, but it's wasting little time building its portfolio of stunning Moon shots. Today members of the Lunar Reconnaissance Orbiter Camera (LROC) team released the instrument's first views of the crater-scarred terrain below.

Although designed to calibrate LROC's two cameras — one for wide views and the other for ultra-detailed telephoto work — the "first light" views along the day-night terminator demonstrate dramatically what's in store as the spacecraft prepares for mapping the entire lunar surface.

The view here, a region east of Hell E crater in the lunar highlands south of Mare Nubium, reveals details down to about 10 feet (3 meters) across. The deep shadowing suggests a craggy and inhospitable surface, explains LROC team leader Mark Robinson in a press release. But in reality, he notes, "the area is similar to the region where the Apollo 16 astronauts safely explored in 1972."

Click here for additional details about the craft's first images of the lunar surface.

Besides LROC, ground controllers have already activated two other instruments: the Lunar Exploration Neutron Detector, or LEND, designed to identify regions enriched in hydrogen (a tracer for deposits of water ice); and the Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The remaining four will be switched on next week.

At any given astro-gathering, I'll sometimes try to stump the attendees with this question: "How many spacecraft have visited Jupiter?"

Most people get the easy ones: Pioneers 10 and 11 (1973-74), Voyagers 1 and 2 (1979), and Galileo (1995). A few recall that Cassini (2000) and New Horizons (2007) have zipped by en route to their final destinations.

But only True Space Cadets can cite the 1992 flyby of Ulysses, a craft designed jointly by the European Space Agency and NASA to study the Sun.

Ulysses needed to fly closely past Jupiter so that the planet's gravity could radically alter the craft's orbit — yanking it up and out of the ecliptic plane so that it could loop back over and under the Sun's poles. This helios incognita is unobservable from Earth, and space physicists first started dreaming about a solar polar mission in the late 1950s. Only with the advent of planetary gravity assists in the 1970s did such a mission become realistically attainable.

Once in its highly inclined, highly elongated 6.2-year orbit, Ulysses was ready to explore the Sun as never before. In all, it made three swings over the solar poles: in 1994-95, when the Sun was near the minimum of its 11-year activity cycle; again in 2000-01, during the most recent solar maximum; and a third pass from late 2006 through early 2008. (Ulysses never really got close to the Sun; its perihelion is 1.4 astronomical units.)

I was musing about all this yesterday while watching a webcast of Ulysses' final hour of contact with its home planet. The craft is nearly out of hydrazine fuel for its stabilizing thrusters, and there's not enough money to continue the mission for another year. With the Sun mired in a prolonged activity slump and Ulysses once again headed away from it, NASA and ESA agreed to pull the plug once and for all.

The webcast was full of nostalgic reminiscences by the graying team members. Most expressed sadness at the mission's termination — yet they all admired the spacecraft's plucky durability. A major communication glitch in early 2008 appeared to doom the mission, but clever engineers found a fix. Given the craft's delicate state, the final shutdown was supposed to occur a year ago, but the funding and hydrazine managed to last for one more year.

Here are the final few entries in a blog posted yesterday by the operations team at the Jet Propulsion Laboratory:

UTC Timestamp: 30-Jun-2009 19:59
Telemetry is back in lock at 64 bits per second engineering format. The command counter confirms reception of the final command.

UTC Timestamp: 30-Jun-2009 20:10
Telemetry loses lock as the spacecraft switches to use the low gain antennas.

UTC Timestamp: 30-Jun-2009 20:15
Ground station can not find the carrier. Transmitter is off on the spacecraft. Goodbye Ulysses.

About the size of a large golf cart, Ulysses operated for 18 years, 8 months, 24 days — and in doing so it managed to eclipse (by just 20 days) the previous endurance record for an ESA spacecraft, that being the International Ultraviolet Explorer. (Those space lifetimes, though remarkable, are still well short of the Voyagers' longevity, currently at just under 32 years.)

By studying the Sun all those years, Ulysses has paid big scientific dividends. Most notable was the realization that the solar wind, the tangle of hot, ionized gas and magnetic-field lines rushing outward from the Sun's lower atmosphere, has a distinct character over the poles. There the flow tends to be less turbulent and about twice as fast — some 500 miles (800 km) per second — than what's coming from equatorial latitudes.

Moreover, fast flows arise from relatively cool areas in the solar corona and slow ones from hot areas, counter to expectations. As U.S. project scientist Edward J. Smith (Jet Propulsion Laboratory) explained yesterday, this means that the solar wind must somehow be magnetically driven and not simply pushed out into space because the corona is hot. "The solar wind is closely related to the magnetic flux of the Sun," Smith emphasized, "which means the whole concept of its origin needs review."

Not bad for a spacecraft that has no camera! Click here for a nice summary of the mission and here to read more about Ulysses' scientific legacy.

So will we ever hear from Ulysses again? Probably not. Its instruments and radio transmitter have been switched off, and the systems that keep its main antenna pointed toward Earth have been disabled. Once it gets farther away, the remaining hydrazine will probably freeze, rupturing its fuel lines. However, engineers did activate two omnidirectional receivers, which would allow them to someday resume contact with the soon-to-be-tumbling craft.

Never say, "Never."

If the universe obeyed Newton's laws of gravity, there would be about a 60% chance that Mercury would head toward the Sun or Venus during the Sun’s lifetime. But according to a new study, corrections to Newton's laws using Einstein's theory of gravity (general relativity) lower these chances to about 1%. That’s good news, because if Mercury had a near miss with Venus or the Sun, it could wreak havoc on Earth.

Jupiter, the solar system's most massive planet, affects Mercury's trajectory. Drastic consequences for Mercury are possible, if its orbit elongates into a highly eccentric ellipse, allowing it to reach beyond Venus.

If that happens, Mercury would likely smash into Venus or the Sun, according to a new study by Jacques Laskar and Mickael Gastineau (Paris Observatory, France), published in the June 11th issue of Nature.

For all of Jupiter’s mass, sending Mercury farther out than Venus requires an "alignment of planets" of sorts — a near-perfect geometry that physicists call a resonance — allowing a small effect to build up over time.

In the case of Jupiter and Mercury, the accidental matching is provided by the speed at which their elliptical orbits move around the Sun — the precession of their perihelia — which happens to be nearly synchronized. But the warping of space-time near the Sun predicted by general relativity introduces a slight mismatch, by speeding up Mercury’s precession. So the resonance is less likely to happen.

We can thank Einstein once more (and Laskar too) for informing us that Mercury has only a 1% chance of going out of whack. Laskar's estimate following Newtonian gravity was 60%. "[The Newtonian calculation] had to be wrong," says Jack Lissauer (NASA/Ames Research Center), arguing that, with such high probabilities, Mercury would already have hit Venus or the Sun during the past few billion years.

"The interesting thing is that it can still happen quite a long time after planets have formed," says John Chambers (Carnegie Institution of Washington). "We’re sort of changing the idea that planetary systems are formed, and then stay the same."

Mercury's demise brings little consequences for Earth, unless Venus tidally disrupts Mercury, sending fragments our way. "There would be a lot of these chunks, and they could hit Earth," says Lissauer.

If Mercury goes haywire but misses Venus and the Sun on its new, highly eccentric trajectory, Mercury can perturb the orbits of Mars, Venus, or Earth; the more massive outer planets would remain unaffected.

It's difficult to know whether this will happen, because the solar system is chaotic. "That the system is chaotic doesn't mean anything can happen," Laskar explains, but relates to the "butterfly effect." All the planets and smaller bodies gravitationally influence each other, meaning that any imprecision on today’s trajectory is multiplied by ten every 10 million years.

Because of this chaos, astronomers will never be able to predict the positions of planets beyond a few hundred million years, and nobody can guarantee that Earth will even orbit the Sun so far in the future. "We can never overcome this limitation," says Laskar, even if the inaccuracy started out less than a trillionth of a trillionth of an inch.

"If I jump in the air, Earth moves more than that," Laskar says. So instead of aiming for the unknown exact trajectory of Mercury, Laskar and Gastineau went for a random sampling of what might happen. Modifying Mercury's current position in steps of 0.38 millimeters, they created 2,501 trajectories. These tiny variations could be thought of as due to Laskar jumping up, an asteroid or star passing by, the influences of tides, or the finite size of the planets. We can't tell the 2,501 trajectories apart today, but as they diverge in the future, they provide a sample of the innumerable possible outcomes.

The result is expressed in terms of probabilities, yielding the 1% for Mercury going haywire, and only a small fraction of this percent for it going haywire but missing Venus or the Sun. If it misses, Mercury can then send Mars or Venus hurling toward Earth, with dramatic consequences. Even without a direct hit, Earth's orbit can be enormously modified, Laskar says, "In all these cases, we have very strong climate change."

Mercury may cause chaotic events in the future, says Renu Malhotra (University of Arizona), but it might even owe its current eccentricity, inclination, and high density to some previous collision. Maybe this isn't a coincidence after all…

See a movie on Jacques Laskar's website. For an expert's opinion, read the News & Views in Nature by Greg Laughlin (University of California, Santa Cruz).

So far, so good... After a leisurely 4½-day coast through space, NASA's two newest recon robots reached the Moon on June 23th and then bid each other adieu.

The Lunar Reconnaissance Orbiter fired a braking rocket at 6:27 a.m. EDT (10:27 Universal Time) and slipped into a looping orbit that will carry it over the lunar poles. Four additional burns are needed to tighten the circuit into a "commissioning orbit" 20 by 135 miles (20 by 216 km) high, and in two months that will shrink further to final circular orbit just 30 miles (50 km) above the surface. From that vantage seven instruments will map and survey the desolate landscape below for at least one year.

Meanwhile, a second probe remains attached to the liquid-fueled Centaur rocket that helped launch the mission. Aided by the Moon's gravity during its brief swingby, the Lunar Crater Observation and Sensing Satellite, or LCROSS, swung into a much wider polar loop and, technically, is still orbiting Earth every 37 days.

Three loops from now, on October 9th, mission controllers will guide the emptied Centaur, followed 4 minutes later by LCROSS, into a permanently shadowed crater near the south-polar feature Cabeus. Pummeling the lunar surface at 1½ miles (2½ km) per second should raise giant clouds of debris, perhaps entraining some of the water that theorists believe lies frozen in the lunar shadows.

Mission managers plan to select the exact target about a month ahead of time. For now, the impacts are scheduled for within a half hour of 11:30 Universal Time (7:30 a.m. EDT, 4:30 a.m. PDT).

That timing is crucial, as the rising plumes need to be in view of the arsenal of observatories atop Mauna Kea and the western U.S., the Hubble Space Telescope, and a Swedish astronomical satellite named Odin.

(Why Odin, you might ask? Its two instruments are well equipped to detect faint wisps of water vapor and other volatile compounds.)

It's hard to gauge just how bright the impact clouds will be, but some calculations suggest 4th magnitude isn't out of the question — and that gives well-equipped amateurs a shot at seeing or recording the splashy event.Interested? For now you can learn more from the LCROSS team tasked with coordinating the observations.

SkyandTelescope.com plans to provide full details about where, when, and how to watch. You can also read more about this exciting mission in Sky & Telescope's June 2009 issue.

Astronomers have had good reasons for quite a while to think that shock waves in supernova remnants create cosmic rays. These are charged particles that have been accelerated to very near light-speed; some of them have been boosted to energies far higher than the best particle accelerators can do on Earth.

But no one knows how much of a supernova’s energy is actually converted into cosmic rays: is it enough to account for all the cosmic rays we see?

Yes, according to a new study published today in Science Express (Thursday, June 25th). Expanding supernova remnants could put half or more of their energy into cosmic-ray acceleration, accounting for all the "intermediate-energy" particle Earth receives. No one knew the process was so efficient.

High and very-high energies

Cosmic rays are the highest-energy particles in the universe. At the very highest-end, a single proton can pack as much kinetic energy as a well-served tennis ball (10²⁰ electron-volts). These probably have extragalactic sources related to active galactic nuclei. Much more abundant are particles with millions of times less energy, but still much more than the best accelerators on Earth can impart. These are the ones in the new study, and they likely come from within our galaxy.

“Historically, supernovae have been the prime suspect,” says co-author Jacco Vink (Astronomical Institute Utrecht, Netherlands). As Walter Baade and Fritz Zwicky suggested in 1934, only supernovae have enough energy to spare.

Enrico Fermi even described a plausible mechanism in 1949. When a star explodes, says Vink, “the supernova ejecta material goes at a speed much larger than the speed of sound” (pressure waves) in the surrounding interstellar medium. So it forms a shock wave, in which particle density and magnetic field strengths vary wildly. Such a magnetic field can repeatedly send charged particles back and forth behind the shock front, where they can gain momentum -at the expense of the shock wave's speed- until they reach extremely high energies.

The Missing Energy

Three supernovae explode in the Milky Way on average per century. On paper, that's more than enough energy to accelerate all the cosmic rays we see. But is the process efficient enough actually to do it? “The only missing link now was: can they convert this energy into cosmic rays?” says Vink. “The answer seems to be: Yes!”

“At least half the energy of the shock goes into cosmic rays,” says lead author Eveline Helder (Astronomical Institute Utrecht, Netherlands). That, anyway, is the case for the part of the ejecta ring that Helder and collaborators studied in the young supernova remnant RCW 86, located 8,200 light-years away. Chinese (and even maybe Roman) records from 185 AD mention a corresponding nine-month-lived star.

The researchers could not see the formation of cosmic rays directly. They compared the energy brought into the interstellar medium by the shock wave with the energy currently left over as heat, and found a mismatch.

The scientists deduced the energy of the shock wave by comparing X-ray images three years apart, taken by NASA's Chandra X-Ray Observatory. The motion of the shock wave between the images amounted to approximately 6,000 km per second, or about 2 percent of the speed of light.

Behind the shock, the scientists found the thin gas to be much less hot than expected: 30 million degrees centigrade instead of half a billion. To measure the temperature of the gas, the scientists used ESO's Very Large Telescope to observe visible light emitted by hydrogen atoms. The width of the hydrogen emission lines, created by the Doppler shift due to the random thermal motion of atoms, told the temperature.

The numbers showed that half the initial energy from the shock wave was missing. Lacking any other explanation, the researchers had to assume it was spent accelerating cosmic rays

This is more than a simple confirmation of previous results. “It’s a much more sophisticated set of measurements,” says Luke Drury (IAS Dublin, Ireland). “This one I think will generate much more interest from the community.”

The new result confirms that the cosmic-ray production process is remarkably efficient. “Everyone was a bit nervous about saying that, because it seemed so outlandish,” says Drury.

This research may also help in understanding similar acceleration processes occurring in gamma-ray bursts and radio jets, says Don Ellison (North Carolina State): “Supernova remnants offer the best laboratory to study the mechanism.”

Mercury may have its volcanic plains, Mars its buried glaciers, and the Kuiper Belt its dwarf planets — but arguably there's no more exciting place in the solar system right now than the Saturnian moon Enceladus. It's drawn the attention of planetary dynamicists, geophysicists, volcanologists, glaciologists, oceanographers, astrobiologists, and plasma physicists — not bad for a modest moon that's barely 300 miles (490 km) across.

What's gotten everyone buzzing is the realization that Enceladus is singlehandedly creating Saturn's broad E ring, thanks to the Cassini orbiter's discovery that eruptions near its south pole are spewing water-charged plumes hundreds of miles into space. Cassini observations show that the plumes are erupting from a quartet of 80-mile-long rifts (dubbed "tiger stripes") that are much warmer than their surroundings.

The heat to power all this activity — an astounding 15 gigawatts, based on the latest Cassini estimates — must be due to a tidal interaction with Saturn that's causing the icy crust to flex rhythmically during each orbit. This causes frictional heat to build up inside the icy moon. On paper, the heat must have created a subsurface global ocean at some depth, the "lubricant" that decouples the flexing crust from the moon's deeper interior.

In 2007 Jennifer Meyer and Jack Wisdom (MIT) discovered that Enceladus moves in and out of orbital resonances with the planet's other satellites. Right now, they say, a coupling with neighboring Dione has pumped up the orbital eccentricity of Enceladus. It's barely perceptible, just 0.005, yet that's enough to trigger the moon's tidal yin-yang with Saturn.

A few weeks ago, at a meeting of geophysicists in Toronto, a trio of dynamicists led by James Roberts (Applied Physics Laboratory) explained there almost has to be a layer of liquid water down there somewhere — it's the only physically plausible way to channel so much heat out of the interior. But they also emphasized that Enceladus can't possibly remain warm-hearted indefinitely — and consequently, given the current orbital arrangement, a subterranean water ocean is "likely to freeze on a geologically short timescale."

So if a hidden ocean exists, what's it like? Initially, the Cassini team envisioned a body of water so close to the icy surface that rivulets can escape upward along fractures and rush into space after explosively decompressing into vapor. Not everyone agrees with this "Cold Faithful" geyser model. Others have speculated that an ocean might not be needed at all. Instead, frozen water-based compounds called clathrates, which lock up volatile molecules like methane, carbon dioxide, or nitrogen, are decomposing into the jets of gas and ice.

Fortunately, Cassini has been getting "up close and personal" with Enceladus of late. The craft brushed past the moon four times last year — including one daring dash directly through the towering plumes. Four more close flybys are planned within the next year, and another dozen could occur if NASA approves a proposed 7-year extension to the mission.

Two new results, published today in Nature, offer glimpses into the jets' true origin. In one, Frank Postberg (Max Planck Institute for Nuclear Physics) and others reveal that about 6% of the tiny particles in Saturn's E ring are quite salty, containing up to 1½% sodium chloride (NaCl). This makes sense if the ocean inside Enceladus has been in contact with deep-seated rocks for millions of years, long enough for sodium and other elements to leach into the water. Postberg's team concludes that the moon's ocean might be as salty as terrestrial seawater (6 to 20 grams of NaCl per kilogram).

But an extensive effort to detect sodium in the plumes spectroscopically from Earth has come up empty. Even a trace should have been evident — atomic sodium has very strong emissions in yellow light. Nicholas Schneider (University of Colorado), who headed the observing team, concludes that the lack of sodium in the plume vapor rules out the near-surface geyser model, because in such an explosive getaway both the gas and particles should mimic each other's composition.

Instead, Schneider believes the saltless spectra imply that the gas is slowly evaporating in deep caverns below the surface — just like water escaping as vapor from the top of Earth's oceans. The gas collects in pressurized pockets until it can jet to space through fissures.

“This idea of slow evaporation from a deep cavernous ocean is not the dramatic idea that we imagined before," Schneider notes in a press release. He adds that other explanations are possible, including warm clathrate-rich ice decomposing and escaping to space — or liquid water created from the friction and heat when tidal flexure causes slabs of ice to rub against each other.

“These are all hypotheses, but we can’t verify any one with the results so far,” said Schneider. “We have to take them all with, well, a grain of salt.”

Cute turn of phrase, Nick.

But seriously, folks, Enceladus in exciting because its putative ocean is rapidly emerging as one of the places in the solar system most conducive to the development of life. As Cassini investigator John Spencer (Southwest Research Institute) notes in his accompanying Nature perspective, this little hotbed of activity appears to have all three of the basic ingredients: a source of heat, a suitable chemical mix, and liquid water.

For the past couple of years, our Sun has been at the minimum of its 11-year activity cycle. Its face has been virtually spotless for months on end, and there've been no dire alerts of titanic solar storms about to slam into Earth.

The problem is that this "quiet Sun" has continued far too long &mdash two years ago, a special task force predicted that the transition from the just-ended Cycle 23 to the upcoming Cycle 24 would come around March 2008. It didn't. (To be fair, there was sharp disagreement within the group at that time.)

Much fanfare accompanied the appearance of a tiny high-latitude sunspot in early 2008, supposedly heralding Cycle 24's arrival. Yet for months and months afterward the Sun's face remained spotless.

Knowing when the upturn in solar activity begins and, more importantly, how strong it'll get at maximum has grown in importance over the years. When the Sun gets agitated, it buffets our planet with huge "storms" of high-speed plasma (ionized gas), punctuated by threatening flares of relativistic protons.

But despite centuries-long records of sunspot counts and 50 years of mapping the Sun's magnetic fields, scientists still don't understand what makes one cycle strong and another weak. The same task force that was bullish on Cycle 24 two years ago now believes the forthcoming activity could be the weakest in a century

Predictions for the coming solar cycle are featured in the August issue of Sky & Telescope, now on press. In the meantime, this week two groups of researchers offered hope that we're finally understanding the Sun's complex workings a little better.

The first comes from Rachel Howe and Frank Hill of the National Solar Observatory, who now believe that sunspots are linked to slow, eastward-moving "jet streams" about 4,000 miles (7,000 km) below the Sun's photosphere (its visible surface). They've analyzed 15 years of observations made using helioseismology, which monitors rhythmic pulsations at the surface created by pressure waves bouncing around the solar interior.

"Think of the Sun as a musical instrument," Hill explains. "A piano has 88 keys, but the Sun has five million 'notes' or modes of oscillation."

Howe and Hill find that a pair of deep-seated currents migrate from the solar poles toward the equator during each cycle, and that the migration has been unusually sluggish of late. They took three years to shift 10° equatorward, and only now have they reached a solar latitude of 22°, the point at which activity perks up and sunspots start to appear. They can't yet tell whether the flow somehow causes sunspots, only that the two phenomena appear to be related.

"Had this analysis been available two years ago," notes solar physicist Dean Pernell (NASA-Goddard Space Flight Center), "we'd have seen the delayed onset of Cycle 24 coming."

The new findings were presented this week at a meeting of the American Astronomical Society's Solar Physics Division, which had a special session on Cycle 24. Howe and Hill plan to publish their work in Astrophysical Journal Letters. Click here to learn more about this work — and to watch animations of the currents' movement.

The second announcement concerns sunspots themselves and the arrangement of the intense magnetic tangles within them. Writing in June 18th's Science Express, Matthias Rempel (High Altitude Observatory) and three colleagues used a supercomputer grinding out 76 trillion calculations per second to create the first comprehensive, three-dimensional model of these mysterious dark patches' inner workings.

The simulations reveal in detail how superheated gas streams along magnetic filaments from a spot's dark, central umbra to the lighter penumbra surrounding it. Solar physicists first recognized this outward flow about 100 years ago. But, as Rempel's team notes, "The onset of these flows is closely related to the magnetic field inclination" and that outflows occur whenever the magnetic field is inclined more than 45° from vertical.

The hope is that a better understanding of sunspots will allow scientists to predict their behavior more accurately and, in particular, to identify the ones most likely to trigger dangerous solar flares.

Finally, let me note that all of these solar scientists are building on the work of John A. Eddy, who died June 10th at the age of 78.

Eddy gained fame in 1976 as the solar sleuth who confirmed that from 1645 to 1715 the Sun remained virtually spotless. During this "Maunder Minimum," as he dubbed it, solar activity essentially ceased, and it remains perplexing for modern-day theorists. Coincidentally, this period closely matches one of the coldest climatic periods on record, the middle of the "Little Ice Age" that plunged Europe into a series of unbearable winters.

More recently, Eddy has explored solar-terrestrial relationships, and before his death he finished a book (to be published by NASA) titled The Sun, the Earth and Near-Earth Space: A Guide to the Sun-Earth System.

A nice obituary appears in Wednesday's New York Times, but you'll learn much more about him from the wonderful 1999 interview for the American Institute of Physics' oral-history project.

Blogger Anthony Watts has started an online petition to name the next prolonged sunspot absence the "Eddy Minimum." It would be a fine way to recognize a fine scientist — please join me in signing it.

Today an Atlas rocket boosted NASA's Lunar Reconnaissance Orbiter and its companion LCROSS impactor skyward at 9:32 Universal Time (5:32 p.m. Eastern Daylight Time). The launch from Cape Canaveral, Florida, had been delayed briefly by neighboring thunderstorms, but project managers eventually gave the "Go!" for liftoff.

The successful sendoff marks the space agency's first step in its much-anticipated return to lunar exploration — a program that might carry astronauts to the Moon's surface in as little as 10 years.

Yet, remarkably, the LRO-LCROSS combo is only the second NASA-sponsored lunar mission since the Apollo landings ended more than 36 years ago. (The only other flight was Lunar Prospector, in 1998–99.)

For the moment the two spacecraft are safely in a parking orbit around Earth. But soon the powerful Centaur final stage will power both out of Earth's gravitational grip and toward the Moon. Then LRO will separate. Five days from now both will swing close past the Moon; LRO will take up a looping polar orbit that will gradually tighten in the weeks ahead. Meanwhile, the Centaur and still-attached LCROSS, an acronym for Lunar Crater Observation and Sensing Satellite, will take up a far larger orbit with a period of about 37 days.

LRO's mission is to use its seven instruments to map and survey the Moon with unprecedented detail — a tall order considering the stiff competition being provided by India's Chandrayaan 1 orbiter and, until recently, by Japan's Kaguya and China's Chang'e 1.

That's great science, but what space junkies like me are anticipating most is when the Centaur and LCROSS plow into a yet-to-be-specified crater at one of the lunar poles sometime between October 7th and 11th. The hope is that these kamikaze impacts — closely watched by telescopes big and small on Earth, the Hubble Space Telescope, and LRO itself — will heave hundreds of tons of lunar dust into telescopic view. If that happens, the ensuing observations should settle a longstanding debate about whether frozen water lies hidden in the permanently shadowed recesses of the lunar poles.

In four more months, I hope to learn the answer!

Protoplanetary disks around three young stars in Ophiuchus have large central holes, astronomers have found, which were presumably cleared by still-growing Jupiter-mass planets. But there’s a problem: the stars are too young. How would planets have formed in just a couple million years?

This is not the first observation suggesting central gaps in the disks of gas and dust around newborn stars (S&T: November 2008, page 32). In the present case, infrared spectra from NASA’s Spitzer Space Telescope had already provided hints, says lead author Sean Andrews (Harvard-Smithsonian Center for Astrophysics). “One of the disks, SR 21, we knew had a hole from the Spitzer observations.” But the evidence was indirect; Spitzer’s 0.85-meter aperture could not resolve the hole, only detect the absence of short-wavelength infrared radiation that would be emitted by warm dust closer to the star.

In their new paper, Andrews and collaborators provide an actual image of a hole. “This is hot stuff! Clearly we are on the verge of catching planet formation in action,” comments Alan Boss (Carnegie Institution, Washington), who did not take part in the study.

Andrews and collaborators used the Sub-Millimeter Array interferometer on Mauna Kea, Hawaii, to form images with a resolution of 0.3 arcseconds — enough to show holes with a radius of 40 astronomical units at the distance where the stars are, about 400 light-years away in Ophiuchus. This is the nearest active star-forming region to Earth.

The researchers studied nine disks and found that three harbor large central cavities. But these numbers do not imply that every third disk will form planets, because the sample was biased. “We’ve picked the most massive disks to look at,” explains co-author David Wilner.

The disks are larger than our solar system, and the central holes are at least the size of Neptune’s orbit. “You need something with Jupiter’s mass to clear that,” Andrews explains.

In fact the mass clearing out the hole could be one giant planet, or several, or a swarm of rocks and rubble, or perhaps an undetected binary companion to the star. “They still haven’t shown that there isn’t a brown dwarf,” says Carol Grady (NASA).

Finding a fully-formed planet in a disk a few million years old would be big news. Theorists expect the slow process of collisions and accretion to take about 10 million years for dust and rocks to collect into planets and clear away a protoplanetary disk. On the other hand, gravitational instabilities in the disk might act more quickly, forming a stellar-like companion such as a brown dwarf or a lower-mass object mimicking a conventional giant planet. The line between Jupiter-like planets and brown dwarfs is difficult to draw, and scientists would like to know which mechanism drives the formation of each or both.

Interestingly, of the nine disks, the three with holes seem to be the oldest — possibly older than a few million years.

Jack Lissauer (NASA) is impatient to see the analysis techniques of this paper applied to the higher-resolution data that will come from the upcoming ESO Atacama Large Millimeter/submillimeter Array (ALMA), now being built on a high plain in the Andes. “The fact that they can do this with the SMA is exciting, because ALMA is coming up soon.” ALMA, along with other instruments, should enable detailed observations of the structure of the disks and maybe even reveal what lurks in the central clearing.

On November 7, 2008, 14-year-old Caroline Moore of Warwick, New York, discovered a supernova in the galaxy UGC 12682, making her the youngest person ever to find an exploding star. She made the discovery in an image taken by a 16-inch Meade LX200 telescope in Arizona that is part of the Puckett Observatory World Supernova Search, led by amateur astronomer Tim Puckett.

The peculiar object, designated Supernova 2008ha, has turned out to be the weakest supernova on record. It was about a thousand times more powerful than a nova (a nuclear explosion on the surface of an old, compact star called a white dwarf), but a thousand times weaker than a typical supernova (the cataclysmic explosion of an entire star).

"Supernova 2008ha was a really wimpy explosion," says Alex Filippenko (University of California, Berkeley), a leading expert on exploding stars.

The supernova appeared relatively faint for the host galaxy's 70-million-light-year distance, leading astronomers to initially think that it was either a supernova dimmed by a lot of interstellar dust or a “supernova impostor,” an eruption on the surface of a massive star that appears similar to a supernova.

On November 18th, Ryan Foley (Harvard-Smithsonian Center for Astrophysics) and his team obtained the first spectrum of SN 2008ha. It was odd, and did not immediately yield a classification. Eventually, Foley found it was very similar to another peculiar supernova, SN 2002cx, but it had a much lower expansion velocity. SN 2002cx itself had a very lower expansion velocity, meaning that SN 2008ha ejected material with much less energy than most supernovae. It showed no significant dimming due to interstellar dust, leading Foley to conclude that it was an extremely weak supernova.

"You can imagine many ways for a star to explode that might resemble SN 2008ha," says Robert Kirshner (also at the Harvard-Smithsonian Center for Astrophysics). "It could have been a massive star suddenly collapsing to form a black hole, with very little energy leaking out. But it looks a lot like its brighter cousins [Type Ia supernovae], which we think are nuclear explosion of white dwarfs. Maybe this one was an explosion of that general type, just much, much weaker."

Foley finds the spectral analysis particularly interesting. In most supernovae, the high velocity of the ejecta smears out many spectral features into one big feature, making it difficult to determine chemical composition. "With SN 2008ha, the velocity was low enough that individual features could be separated, showing us exactly what the supernova was made of,” says Foley.

Explosions of this kind may not have been seen before because of their inherent dimness. A new generation of telescopes and instruments is beginning to search greater distances than ever before, effectively monitoring millions of galaxies. But as Moore’s discovery shows, an attentive human eye is far from obsolete.

Valerie Daum is an intern at Sky and Telescope

Low on fuel and its highly successful mission over, Japan's Kaguya orbiter struck the Moon on June 10th. And astronomers in Australia successfully captured the intentional crash landing.

The impact occurred within a couple seconds of 18:25:10 Universal Time, which meant the just-past-full Moon was visible in predawn darkness from Japan, Australia, and the far western Pacific. Unfortunately, it was already daylight for the mammoth telescopes atop Mauna Kea in Hawaii.

The target point occurred at 80.4°E, and 65.5°S, a location on the near side right along the darkened southeast limb. Those coordinates correspond to the inner wall of a small crater near the much larger crater Gill.

Poor weather hampered many of the observatories positioned to witness the crash. The only known success comes from the Anglo-Australian Observatory in southeastern Australia, where Jeremy Bailey (University of New South Wales) and Steve Lee (AAO) used the 3.9-meter Anglo-Australian Telescope to capture a series of images at the near-infrared wavelength of 2.3 microns.

"The observed flash was within a few seconds of the final predicted impact time [18:25:07 UT]," Bailey told me. "The biggest uncertainty for us was the weather which has been poor at Siding Spring for the previous few nights. However, the clouds cleared enough for us to get the observation." The two observers posted their images soon after acquiring them.

"It was seen on the monitor so we knew straight away we had it," Lee adds. "It was one of the few times in observing where there is some sense of immediacy, followed by instant gratification when the event was seen."

Amateurs attempted to witness the crash but came up empty-handed. From Hong Kong, the astrophotographer Wah writes, “I was using Meade 8-inch SCT and ToUCam SC3 to capture video. I could not see any flash during the period.” In Canberra, Australia, David Herald used video on a small scope but found no evidence of a flash in his recording.

NASA scientists are keeping close tabs on all these observing reports, because Kaguya's flashy finale serves as a dry run for the forthcoming LCROSS mission. If the craft's launch takes place as planned on June 17th, ground controllers will direct a 2½-ton Centaur rocket, closely followed by LCROSS itself, into a permanently shadowed polar region between October 7th and 11th. Mission managers hope the crashes will create a towering splash of water-rich dust and vapor observable from Earth.

However, Kaguya was no lightweight — its mass was nearly 3 tons — and further analysis of the AAT images may show that only very large telescopes will have a chance to capture LCROSS's demise.

I'm not big on space-exploration nostalgia, though I'm surprised that the world's space buffs didn't make a bigger deal of the 50th anniversary of Sputnik 1's launch last October. (For Sky Publishing's take on that watershed event, click here.)

In any case, about a month from now NASA, the Smithsonian Institution, and other organizations will make much ado concerning the 40th anniversary of the Apollo 11 lunar landing in 1969.

There's good reason to pull out all the stops now, rather than to wait another 10 years. Apollo 11's crew of Neil Armstrong, Buzz Aldrin, and Michael Collins are all in their late 70s. The project's key managers are even older, if not already deceased. When it comes to Apollo reminiscences, time is of the essence.

So yesterday I found myself in the futuristic Kresge Auditorium on the MIT campus to relive NASA's finest and most audacious accomplishment. The three-day Giant Leaps symposium had managed to draw a stellar list of attendees — including publicity-shy Armstrong, not-so-shy Aldrin, and mission director Chris Kraft. As co-organizer Maria Zuber explained, "We decided to have it this early to make sure we could get the people here we wanted."

Fittingly, Frank Sinatra's "Fly Me To the Moon" drifted down from the speakers overhead as the crowd settled in.

MIT didn't have a large role in the Apollo effort, but it did have a crucial one: Charles Stark "Doc" Draper and his staff at the school's Instrumentation Laboratory built the 1-cubic-foot, 70-pound Guidance and Navigation Computer that reliably guided those capsules to and from the Moon.

(Two footnotes of note: (1) MIT received the first contract issued by NASA for the Apollo program; and (2) the GNC's precursor was the computer that Draper's team built for a self-navigating spacecraft designed to make a round trip to Mars in the early 1960s. NASA managers weren't interested in Draper's Mars probe, but they sure were wowed by its computer.)

Even with 40 yeas of hindsight, it's amazing that Apollo worked at all &mdash let alone succeeded so spectacularly. "When President Kennedy announced his decision to go to the Moon in May 1961," Kraft recalled, "I thought he'd lost his mind." The effort to fulfill Kennedy's commitment was full of gutsy calls. For example, Kraft reminded the audience that, during its final qualification flight in 1968 (Apollo 6), the mammoth Saturn V launch rocket experienced serious problems with all three of its stages — yet the very next launch carried Apollo 8's crew to the Moon and back.

More than one speaker asked aloud whether the Apollo program could be undertaken today. Harrison "Jack" Schmitt said that he and fellow Moonwalker Armstrong had often reflected on the program's keys to success, then he ticked off the list they'd come up with:

• a sufficient base of technology
• a reservoir of young engineers and skilled workers
• a pervasive environment of national unease
• a catalytic event (Yuri Gagarin's historic flight)
• an articulate and persuasive president
• adequate funding
• a culture of organizational liberty with tough, competent managers

A pair of panels at Giant Leaps tried their best to frame future advances in aeronautics and space exploration. And NASA is planning a return to the Moon, with its chronically underfunded Constellation program.

But I wouldn't get too excited about Constellation's prospects just yet. On June 1st, NASA's acting administrator tasked a blue-ribbon committee to review the agency's plans for human spaceflight. Headed by Norman Augustine, who chaired a similar task force in 1990, the panel's recommendations are due within 120 days. So stay tuned!

Over the years I've seen dozens of partial lunar eclipses. They're exciting, they're pretty — but scientifically useful? "No way," I used to say.

Then I read an article in June 11th's Nature by an observing team led by Enric Pallé (Instituto de Astrofísica de Canarias). He and four colleagues did something that, apparently, hasn't been attempted in more than 80 years: they recorded the lunar spectrum during last August's deep partial eclipse.

So why do that? Because, as they explain, the dim light reflected off the lunar surface within the umbra is sunlight that has passed through Earth's atmosphere. It's literally the spectrum of Earth transiting the Sun, and that could prove critical as astronomers attempt to seek habitable Earthlike worlds elsewhere in space.

Of the 353 exoplanets tallied to date, 59 of them were discovered because they transit their host stars. In a few of these cases, astronomers have been able to divine hints about composition from the altered spectrum of starlight passing through the planet's atmosphere.

By recording Earth's visible and near-infrared spectrum as mirrored in the Moon, Pallé and his team hit the jackpot. Not only did they detect the absorption bands of biologically important gases (water vapor, oxygen, carbon dioxide, and others), but these spectral fingerprints proved more obvious that expected and, critically, stronger they appear in sunlight reflected directly off Earth's atmosphere.

Most interesting to me was the detection of both oxygen and methane. These two gases can't coexist without reacting, so methane's presence must be continually replenished somehow. On Earth, the principal sources of atmospheric methane are decaying organic matter and flatulent livestock. What manner of strange organisms might be exhaling methane on some alien Earth?

So the next time you gaze upon the lunar disk swallowed by Earth's shadow (which won't occur until December 2010), just remember that it's not just a pretty sky show — there's real science in that dim, ruddy light.

The American Astronomical Society's 214th meeting in Pasadena, California, kicked off in earnest on Monday.

Daniel Perley, Joshua Bloom (both at the University of California, Berkeley), and their colleagues may have solved one of the biggest mysteries surrounding powerful stellar explosions known as gamma-ray bursts (GRBs): the nature of “dark” bursts. The astronomers used the Keck and Palomar observatories to follow up 29 GRBs detected by NASA's Swift satellite. Fourteen of these were classified as dark GRBs, which do not have a strong visible-light afterglow. Some previous theories have held that the visible component is not observable because many of the bursts are so far away that their visible light is redshifted by the expansion of the universe into the infrared part of the spectrum.

Though there was one high-redshift GRB in the sample, the vast majority of the bursts were clearly embedded in visible galaxies. Since the light from these galaxies was still in the visible spectrum, there had to be another explanation for the bursts' weak visible component. The culprit appears to be dust around the burst, which absorbs visible light but not gamma rays. The dust could be local to the region where the stars that make GRBs were born.

Few high-redshift GRBs would indicate that there were not as many bursts early in the universe’s history as some theories predict. Bursts, emanating from massive stars, are associated with star formation, and so a lower rate of very early bursts would also suggest a slower rate of star formation in the early universe.

In other results announced at the conference, astronomers unveiled two methods for refining the Hubble constant, the rate at which the universe is currently expanding.

James Braatz (National Radio Astronomy Observatory) used a network of radio telescopes to obtain a geometric measure of the distance to water-bearing clouds in the distant galaxy UGC 3789: 160 million light-years, plus or minus 15%. Based on simple geometry, Braatz says this method is more accurate and direct than other ways of measuring distances to faraway galaxies. His reading of the Hubble constant, 71 kilometers per second per megaparsec, agrees nicely with previous measurements, such as those made by the Hubble Space Telescope.

Jonathan Bird (Ohio State University) described how ultra-long-period (near 100-day) Cepheid variable stars could be used to extend the reach of galaxy measurements. Astronomers can use shorter-period Cepheids to estimate distances of galaxies out to about 80 million light-years. But the longer-period Cepheids could extend this method to greater distances.

Astronomers recognized the importance of shorter-period Cepheids in the early 20th century because their intrinsic brightness relates to how long it takes for one to wax and wane over time. This relationship has not held steady with long-period Cepheids, but Bird is hopeful that with optical corrections for atmospheric anomalies, a relationship for long-period Cepeids can be worked out. Unlike Braatz's maser method, however, the distances would still be approximate.

In an afternoon press conference, Karl Gebhardt (University of Texas at Austin) reported a new measurement that doubles the mass of the supermassive black hole at the center of the giant elliptical galaxy M87. Gebhardt, using a powerful supercomputer called Lonestar, took a larger view of galaxies with black holes and their interaction with dark matter 'halos' at their edges. He is suspicious that previous models used to measure black hole masses were wrong, because they failed to include information that would result in a larger mass of stars in the galaxy, which would influence the perceived masses of the stars orbiting the central black holes.

Gebhardt’s new calculations show larger masses for black holes, with the black hole in M87 tipping the scales at 6.4 billion suns, roughly 2 to 3 times larger then previously thought. This would help to resolve the observed difference in mass between the large and faraway black holes in quasars and those in galaxies closer to us, which have generally appeared smaller.

On Tuesday morning, Jin Koda (State University of New York, Stony Brook) presented observations of colliding galaxies done with the 8-meter Subaru Telescope in Hawaii. Wide-field infrared imaging revealed outlying tidal debris in several well-known systems, such as the Antennae and Arp 220. “We did not expect such enormous debris fields around these famous objects,” says Koda, who likened this debris to skid marks on a road. This allowed Koda’s team to track the past interaction of these galaxies. He also suggested that these outlying collision areas may be ideal for star formation.

Also on Tuesday, Alan Stockton (University of Hawaii) described his study of galaxies that existed when the universe was only 20% of its current age. His team used the adaptive optics system on the 10-meter Keck II telescope to filter out atmospheric disturbances to get a very clear look at these faraway objects. Surprisingly, these galaxies were already filled with old stars, suggesting that most stars had already formed by 500 million years after the Big Bang. Many of the galaxies had disk shapes, which are fragile and would have been disturbed by galaxy interactions. This implies that large galaxies formed very early in the universe’s history.

Valerie Daum is an intern at Sky and Telescope