Cool Movie: SDO Destroys a Sundog

Last week, on Feb. 11th, the Solar Dynamics Observatory (SDO) lifted off from Cape Canaveral on a five-year mission to study the sun. Researchers have called the advanced spacecraft the "crown jewel" of NASA's heliophysics fleet. SDO will beam back IMAX-quality images of solar explosions and peer beneath the stellar surface to see the sun's magnetic dynamo in action.
SDO is designed to amaze—and it got off to a good start.

"The observatory did something amazing before it even left the atmosphere," says SDO project scientist Dean Pesnell of the Goddard Space Flight Center.

Moments after launch, SDO's Atlas V rocket flew past a sundog hanging suspended in the blue Florida sky and, with a rippling flurry of shock waves, destroyed it. Click on the image below to launch a video recorded by 13-year-old Anna Herbst at NASA's Banana River viewing site—and don't forget to turn up the volume to hear the reaction of the crowd.

SDO has a close encounter with a sundog. Movie formats: 10 MB Quicktime, 1 MB mpeg-4.

"I couldn't believe my eyes," says Anna. "The shock waves were so cool." Anna traveled with classmate Amelia Phillips three thousand miles from Bishop, California, to witness the launch. "I'm so glad we came," says Amelia. "I've never seen anything like it!"

Sundogs are formed by plate-shaped ice crystals in high, cold cirrus clouds. As the crystals drift down from the sky like leaves fluttering from trees, aerodynamic forces tend to align their broad faces parallel to the ground. When sunlight hits a patch of well-aligned crystals at just the right distance from the sun, voila!--a sundog.

"When the Atlas V rocket penetrated the cirrus, shock waves rippled through the cloud and destroyed the alignment of the crystals," explains atmospheric optics expert Les Cowley. "This extinguished the sundog."

Videos by other photographers at Banana River show the shock waves particularly well. Here's one from Romeo Durscher of Stanford, California, and another from Barbara Tomlinson of Beachton, Georgia.

In the past, says Cowley, there have been anecdotal reports of atmospheric disturbances destroying sundogs—for instance, "gunfire and meteor shock waves have been invoked to explain their disruption. But this is the first video I know of that shows the effect in action."

Sundogs are formed by the refracting action of plate-shaped ice crystals.

The effect on the crowd was electric.

"When the sundog disappeared, we started screaming and jumping up and down," says Pesnell. "SDO hit a home run: Perfect launch, rippling waves, and a disappearing sundog. You couldn't ask for a better start for a mission."

SDO is now in orbit. "The observatory is doing great as the post-launch checkout continues," he reports. "We'll spend much of the first month moving into our final orbit and then we'll turn on the instruments. The first jaw-dropping images should be available sometime in April."
Believe it or not, Pesnell says, the best is yet to come.

Sky Tonight [February 2010]

Sky Info for February 2010

The image above shows the position of the planets, stars, constellations and selected Deep Sky Objects (DSO's) in mid-February (2010) at about 7:00pm MST from Las Cruces, New Mexico. It is also valid for late January at 8:00pm and early March at 6:00pm. Clicking on the map will bring up a larger printable (inverted) image. To use the chart, hold it over your head with the direction indicators pointing in the appropriate direction. Many features will be impossible to see in a location polluted by poorly-designed lighting. Most of the DSO's will require binoculars or a telescope.

Mercury

Mercury is visible during the first half of February in the pre-dawn East.






Venus

Venus is lost in the Sun's glare for much of February. It emerges toward the end of the month as the evening star (low in the West).





Mars

Mars rises before sunset. It is now past opposition, but will remain a good viewing target for a few months.





Jupiter


Jupiter is very low in the Western sky at dusk. By mid-month, it will be lost in the Sun's glow.




Saturn



Saturn rises around 9:00 during January.




Uranus/Neptune


Neptune is lost in the Sun's glare. Uranus is still high enouth to view for an hour or so after dark.

Are TGFs Hazardous to Air Travelers?

Instruments scanning outer space for cataclysmic explosions called gamma-ray bursts are detecting intense flashes of gamma-ray energy right here in the friendly skies of Earth. These terrestrial gamma-ray flashes, or TGFs, blast through thunderstorms close to the altitude where commercial airliners fly.

In fact, they could be too close for comfort.

In a recent study,* scientists estimated that airline passengers could be exposed to 400 chest X-rays worth of radiation by being near the origin of a single millisecond blast. Joe Dwyer of the Florida Institute of Technology took part in that research, which used observations from NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager, or RHESSI, to estimate the danger TGFs pose.

"We believe the risk of encountering a TGF in an airplane is very small," says Dwyer. "I wouldn't hesitate to take a flight. Pilots already avoid thunderstorms because of turbulence, hail, and lightning, and we may just have to add TGFs to the list of reasons to steer clear of those storms."
But, he stresses, "it's worth looking into."

Lightning might not be the only reason to avoid thunderstorms. TGFs sometimes come blasting out of these clouds, too.

NASA's Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Telescope will help evaluate the hazards.

"GBM provides the best TGF data we have so far," says Dwyer. "It gets better measurements of their spectra than any previous instrument, giving us a more accurate idea of just how energetic they are."

Although TGFs are quite brief (1-2 milliseconds), they appear to be the most energetic events on Earth. They belch destructive gamma-rays packing over ten million times the energy of visible light photons – enough punch to penetrate several inches of lead.

"It's amazing," says Jerry Fishman, a co-investigator for the Gamma-ray Burst Monitor. "They come blasting right through the whole Fermi spacecraft and light up all of our detectors. Very few cosmic gamma-ray bursts manage to do this!"

The origin of TGFs is still a mystery, but researchers know this much: TGFs are associated with thunderstorms and lightning. "We think the electric field in a thunderstorm may get so strong that the storm itself turns into a gamma-ray factory," says Dwyer. "But we don't know exactly how or why or where inside the storm this happens."

So no one yet knows how often, if ever, planes end up in the wrong place at the wrong time.

A cartoon sketch of electric and magnetic fields in a thunderstorm and some of the phenomena they produce. TGFs may be just one aspect of thunderstorm activity in addition to elves, sprites, blue jets and ordinary lightning.

It's possible that lightning bolts trigger TGFs. Or maybe TGFs trigger lightning bolts. Researchers aren't sure which comes first. GBM's excellent timing accuracy – to within 2 microseconds – will help solve this riddle.

"For some of the TGFs, we've pinpointed the associated lightning," says Dwyer. "This information along with the spectrum should help us figure out how deep in the atmosphere a TGF source is and how many gamma-rays it's emitting. Then we can determine the altitude and location they're coming from in the thunderstorm."

Fishman offers some good news: "If TGFs originate near the tops of thunderstorms and propagate upward from there, airline passengers would be safe."

By looking closely at a TGF's life cycle, that is, how quickly it turns on and off, GBM may also help researchers calculate how large and concentrated the gamma-ray source is. If the gamma-rays are emitted over a large region, the radiation dose would be diluted and much less harmful.
"But if the source is compact and the gamma-rays originate close to an aircraft, then that could be a problem," says Fishman.

The radiation dose from an ordinary lightning leader vs. the dose from a TGF. Both phenomena are associated with electron beams. Tighter, more compact beams deliver a greater effective dose. Details of this model may be found in an upcoming issue of the Journal of Geophysical Research (Atmospheres). Look for "Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft" by J. Dwyer et al. (in press).

"Of course the smaller the source the lower the odds of a plane ending up close to it," adds Dwyer.

GBM wasn't designed to look for TGFs, but GBM co-investigator Michael Briggs has greatly enhanced its sensitivity to them by writing new software.

"TGFs have really been an afterthought for missions so far," says Dwyer. RHESSI, for example, points at the sun, but the RHESSI team figured out a way to measure TGFs by detecting gamma-rays coming in through the satellite's backside. "All these instruments have been pointing across the universe, while right over our heads these monsters are going off!"

"Now the whole field of TGFs is on fire," says Fishman. "People are jumping on the bandwagon to try to figure them out."

Hubble Sees Suspected Asteroid Collision

NASA's Hubble Space Telescope has observed a mysterious X-shaped debris pattern and trailing streamers of dust that suggest a head-on collision between two asteroids. Astronomers have long thought that the asteroid belt is being ground down through collisions, but such a smashup has never been seen before.

The object, called P/2010 A2, was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) sky survey on Jan. 6. At first, astronomers thought it might be a so-called "main belt comet"--a rare case of a comet orbiting in the asteroid belt. Follow-up images taken by Hubble on Jan. 25 and 29, however, revealed a complex X-pattern of filamentary structures near the nucleus:

"This is quite different from the smooth dust envelopes of normal comets," says principal investigator David Jewitt of the University of California at Los Angeles. "The filaments are made of dust and gravel, presumably recently thrown out of the nucleus. Some are swept back by radiation pressure from sunlight to create straight dust streaks. Embedded in the filaments are co-moving blobs of dust that likely originated from tiny unseen parent bodies."

Hubble shows the main nucleus of P/2010 A2 lies outside its own halo of dust. This has never been seen before in a comet-like object. The nucleus is estimated to be 460 feet in diameter.
Normal comets fall into the inner regions of the solar system from icy reservoirs in the distant Kuiper belt and Oort cloud. As comets approach the sun and warm up, ice near the surface vaporizes and ejects material from the solid comet nucleus via jets. But P/2010 A2 may have a different origin. It orbits in the warm, inner regions of the asteroid belt where its nearest neighbors are dry rocky bodies lacking volatile materials.

This leaves open the possibility that the complex debris tail is the result of an impact between two bodies, rather than ice simply melting from a parent body.

"If this interpretation is correct, two small and previously unknown asteroids recently collided, creating a shower of debris that is being swept back into a tail from the collision site by the pressure of sunlight," Jewitt says.

Asteroid collisions are energetic, with an average impact speed of more than 11,000 miles per hour--five times faster than a rifle bullet. The main nucleus of P/2010 A2 would be the surviving remnant of this so-called hypervelocity collision.

A full-context view of P/2010 A2.Photo No. STScI-2010-07

"The filamentary appearance of P/2010 A2 is different from anything seen in Hubble images of normal comets, consistent with the action of a different process," Jewitt says. An impact origin also would be consistent with the absence of gas in spectra recorded using ground-based telescopes.

The asteroid belt contains abundant evidence of ancient collisions that have shattered precursor bodies into fragments. The orbit of P/2010 A2 is consistent with membership in the Flora asteroid family, produced by collisional shattering more than 100 million years ago. One fragment of that ancient smashup may have struck Earth 65 million years ago, triggering a mass extinction that wiped out the dinosaurs. But no such asteroid-asteroid collision has been caught "in the act"--until now.

At the time of the Hubble observations, the object was approximately 180 million miles from the sun and 90 million miles from Earth. The Hubble images were recorded with the new Wide Field Camera 3 (WFC3).