A Solar Sentinel's Stunning Debut

Solar prominence on March 30, 2010
Twisting streamers of ionized gas tower over the Sun in a large prominence captured by the Solar Dynamics Observatory on March 30, 2010.

A Solar Sentinel's Stunning Debut

When NASA's Solar Dynamics Observatory roared into space on February 11th, mission scientists breathed a sign of relief. Launch is (almost) always the riskiest part of any space mission.


Solar Dynamics Observatory
Three instruments aboard NASA's Solar Dynamics Observatory will keep the Sun under constant scrutiny throughout an 11-year-long solar cycle.

Once SDO safely reached its final orbit, a geosynchronous circle inclined 28° to the equator, the flight team got to work readying the craft to monitor the Sun as never before. Commands beamed up and data beamed down through the mission's dedicated ground station in New Mexico.

(Just to clarify, an orbit that's geosynchronous has a 24-hour period, matching Earth's rotation; if a spacecraft's orbit is geosynchronous and circular, from the ground the craft appears to nod up and down in declination but keeps the same right ascension. A geostationary orbit has, in addition, 0° inclination — telecommunications satellites use such orbits because they appear fixed in the sky, so that receiving antennas don't need to track them.)

After two months of waiting, we've finally gotten a glimpse of what SDO can do. Yesterday afternoon NASA officials unveiled the first images and videos from the craft's three instruments.

The Sun's disk has always fascinated me in a kaleidoscopic way. Seething and churning, its surface is constantly changing. Anyone who's kept tabs on the Solar and Heliospheric Observatory (SOHO) knows what I mean. But SDO takes the visualization of the Sun to a whole new level. The frequency, or cadence, of the observations is much faster, and the videos released yesterday are breathtaking. They're too big to show here — the little clip you can view at right is just a tease, really. To get a real eyeful, head over to the project's special website set up for these first-light releases. (The mission's main website is here.)

Simply put, SDO's 5-year mission is to characterize how the Sun changes and how those changes affect Earth. It's not the first spacecraft with that objective, but the three experiments aboard are designed to identify those changes as never before. Each day the craft will transmit some 1½ terabytes of data — the equivalent of 380 full-length movies.


Full-disk view of erupting Sun
This false-color image, produced by combining three images from the Solar Dynamics Observatory's AIA instrument, shows the Sun's appearance on April 8, 2010.

I think the instrument you'll get to know best is the Atmospheric Imaging Assembly (AIA), a quartet of telescopes that record full-disk images at ultraviolet and extreme-ultraviolet wavelengths. Whereas SOHO takes images at some of these same wavelengths every few minutes, the AIA imagers will snap away every 10 seconds.

SDO's Extreme Ultraviolet Variability Experiment (EVE) will focus on the solar photons that primarily heat Earth's upper atmosphere and create our ionosphere. Other spacecraft have measured the Sun's extreme-ultraviolet variability, but EVE will do it better and faster, in some cases sampling the Sun's output every ¼ second.

The job of SDO's Helioseismic and Magnetic Imager (HMI) is to monitor the magnetic fields at the solar surface and to probe the goings-on deeper down. Our star is constantly quivering, and physicists can use the strength and frequency of these vibrations to map the Sun's interior — in much the same way that geophysicists use earthquakes to probe Earth's mantle and core.

It's great that solar scientists now have a powerful new tool to explore our star. But I'm equally thrilled that the spacecraft will be producing some gorgeous eye candy to captivate and engage the public.

Sparks on Saturn

Several dark storms (arrowed) are confined to a region near 30° south latitude in Saturn's atmosphere. This turbulent region produced quite a few storms as the Cassini spacecraft approached Saturn in May 2004, when this image was taken.

Sparks on Saturn

One of the "flashier" results from planetary exploration is that titanic bolts of lightning routinely zap the atmosphere of Jupiter. It stands to reason that bright discharges should occur on Saturn as well.

Both the Voyagers (in 1980-81) and more recently the Cassini orbiter have recorded circumstantial evidence for Saturnian lightning. The ringed planet has a dynamic cloud band near 30° south, dubbed "storm alley," where localized spots often come and go, and the spacecraft have picked up radio static presumably propagating upward from the flash points.

But capturing the lightning flashes themselves has proved challenging at Saturn. For one thing, the planet's colder temperatures cause its cloud deck to lie deeper down and to be capped by more high-altitude haze (which explains why Saturn's cloud features are more muted than Jupiter's). Those beautiful rings reflect sunlight onto the planet's night side, confounding attempts to take long-exposure images.

Fortunately, Saturn's recent equinox has minimized interference from the rings, and yesterday Cassini scientists announced that they'd recorded a series of powerful lightning bolts in the planet's southern hemisphere. As noted in a Jet propulsion Laboratory press release, Cassini's camera first saw flashes last August, during a prolonged storm that lasted nine months.


Lightning on Saturn
Cassini's narrow-angle camera captured these lightning flashes on Saturn over a 16-minute period on November 30, 2009. The light-toned storm cloud is about 1,900 miles (3,000 km) long. This short clip is looping continually; NASA's original animation is 10 seconds long.

Subsequently, the imaging team zeroed in on a briefer storm at the end of 2009 that yielded enough observations to create the video seen here.

More than spectacle, the lightning reveals much about the state of Saturn's atmosphere. In an article to appear soon in Geophysical Research Letters, a team led by Ulyana Dyudina (Caltech) report that the flashes witnessed last August probably came from a single large cloud at 36° south popping off about once per minute. Given that the bright spots appeared about 120 miles (200 km) across, the researchers estimate that lightning was 80 to 160 miles (125 to 250 km) below the planet's ammonia-ice cloud tops. This would most likely place the storm cell in the middle cloud layer (ammonimum hydrosulfide, NH4SH) or possibly lower down in the water-ice cloud at the bottom of the layered deck.

These bolts zap the Saturnian sky with more than a billion joules of energy, comparable to lightning bolts on Earth and Jupiter. (Cloud-cloaked Venus is thought to have lighting too, though it's never been observed directly.) But why the lighting on Saturn is confined to its storm alley, for now at least, remains a mystery.

Spotlight on Messier 66

Spiral galaxy Messier 66
Roughly 100,000 light-years across, Messier 66 is the largest member of the Leo Triplet. It displays asymmetric spiral arms (dotted with pinkish star-forming regions) and an apparently displaced core. Click here for a larger version.

This time each year, as Leo rises higher in the northern spring sky, backyard observers often zero in on a tidy trio of galaxies collectively known as the Leo Triplet. Its three majestic spirals — Messier 65, Messier 66, and NGC 3628 — lie about 35 million light-years away.

Today the Hubble Heritage Project released a sweeping portrait of M66, which at 100,000 light-years across ranks as the group's largest member. Also known as NGC 3627, M66 spans a generous 9 arcminutes and glows at magnitude 8.9.

The Hubble view combines images taken by the Hubble Space Telescope in 2004 and 2009, along with a hydrogen-alpha image by amateur astrophotographer Rob Gendler to accentuate the red star-forming regions.

Astronomers are intrigued by this galaxy's unusual structure. Its asymmetric arms seem to be climbing up and out of the main disk, and the bright core is offset from the disk's center. These oddities are most likely due to the gravitational pull of the two neighboring galaxies.

According to the press release that accompanies the image, Messier 66 has hosted three supernovae since 1989, the latest one occurring in 2009. That's not surprising, given that the spiral arms teem with star-forming regions.

A Blast from the Past

Apollo rocket crash site  Located between Oceanus Procellarum and Mare Cognitum, this is what resulted when a Saturn IV-B rocket stage slammed into the Moon on April 14, 1970. The view is roughly 1,300 feet (400 m) wide. Click here for a larger version.

With the cancellation of plans to return Americans to the Moon, at least anytime soon, you might think that NASA's Lunar Reconnaissance Orbiter is a spacecraft without a mission. After all, LRO's two key objectives were to scan the lunar landscape for places where future crews could safely land and to assess how much water might be stashed in the Moon's polar regions. But don't worry: the global observations being returned by this highly capable orbiter will keep lunar specialists busy for decades. Here's a guide to the spacecraft's seven instruments.

Much ado has been made of LRO's views of landing sites from previous missions. Most famous among these is the revered Apollo 11 site, Tranquility Base, snapped a couple of times last year. The stunningly detailed views are made possible by LRO's high-resolution cameras, dubbed LROC. The twin sets of 8-inch (195-mm) f/3.6 optics, viewing from an orbit only 30 miles (50 km) up, resolve the lunar terrain at about 2 feet (0.5 m) per pixel.

In the past few months, the LROC team has chronicled a host of other spots where landers set down. For example, there's now complete coverage of the six Apollo sites. But the image cache also includes the robotic Surveyors 3, 5, and 6, as wel as the Soviet Union's lunar rovers (including Luna 21) and the automated sample-return missions Luna 20 and 24.

Diehard space cadets will have to excuse me, but I can only get excited to a point about these trips down memory lane.

However, a view released last week caught my eye. It's the site where, on April 14, 1970, NASA controllers forced a Saturn IV-B rocket stage (the one used by Apollo 13) to slam into the surface. The spent rocket, 58 feet (18 m) weighing 14 tons, hit at 1½ miles (2½ km) per second. That wallop — equivalent to the explosive power of 10 tons of TNT — created a distinctive crater about 100 feet (35 m) across. The energy of the impact created small tremors picked up by a seismometer left behind by Apollo 12's astronauts.

Saturn IV-B rocket
The Saturn IV-B rocket stage stood 58 feet tall and had a dry mass of about 14 tons.

Located at 27.9°W, 2.6°S (southwest of Copernicus between Oceanus Procellarum and Mare Cognitum), the crater has bright mounds in its interior, and the surrounding veneer of ejecta includes bright rays that extend for more than a mile. LROC took this view with the Sun well up in the sky, which brings out subtle differences in the albedo (reflectivity) of the surface. Look carefully, and you'll spot some dark rays among the bright ones.

You might be wondering how this crater compares with the one excavated last October by the Centaur rocket used to carry LRO (and its kamikaze impactor, LCROSS) to the Moon. Although the impact speeds were comparable, the Centaur is much smaller and weighs only 2 tons. Still, LCROSS scientists estimate the resulting crater at 70 to 100 feet across. That blast, famously, took place in the permanently shadowed floor of Cabeus crater — so we might never have views of it showing this kind of detail