Astronomy Picture of the Day 08-18-04

Explanation: Nobody knows what causes lightning. It is known that charges slowly separate in some clouds causing rapid electrical discharges (lightning), but how electrical charges get separated in clouds remains a topic of much research. Nevertheless, lightning bolts are common in clouds during rainstorms, and on average 6000 lightning bolts occur between clouds and the Earth every minute. Above, several lightning strokes were photographed under a starry sky behind Kitt Peak National Observatory near Tucson, Arizona. Lightning has also been found on the planets Jupiter, Saturn, and Uranus. NASA launched the TRMM mission in 1997 that continues to measure rainfall and lightning on planet Earth.

Out from the shadows: Two new Saturnian moons
CASSINI PHOTO RELEASE
Posted: August 16, 2004

With eyes sharper than any that have peered at Saturn before, the Cassini spacecraft has uncovered two moons, which may be the smallest bodies so far seen around the ringed planet.

This image shows the tiny 'worldlet,' temporarily dubbed S/2004 S1, as it makes its way around the planet. A white box frames the moon's location in the image. Credit: NASA/JPL/Space Science Institute

The moons are approximately 3 kilometers (2 miles) and 4 kilometers (2.5 miles) across -- smaller than the city of Boulder, Colorado. The moons, located 194,000 kilometers (120,000 miles) and 211,000 kilometers (131,000 miles) from the planet's center, are between the orbits of two other saturnian moons, Mimas and Enceladus. They are provisionally named S/2004 S1 and S/2004 S2. One of them, S/2004 S1, may be an object spotted in a single image taken by NASA's Voyager spacecraft 23 years ago, called at that time S/1981 S14.

"One of our major objectives in returning to Saturn was to survey the entire system for new bodies," said Dr. Carolyn Porco, imaging team leader, Space Science Institute, Boulder, Colo. Porco planned the imaging sequences. "So, it's really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons, skipping unnoticed around Saturn for billions of years until just now.²

The moons were first seen by Dr. Sebastien Charnoz, a planetary dynamicist working with Dr. Andre Brahic, imaging team member at the University of Paris. "Discovering these faint satellites was an exciting experience, especially the feeling of being the first person to see a new body of our solar system," said Charnoz. "I had looked for such objects for weeks while at my office in Paris, but it was only once on holiday, using my laptop, that my code eventually detected them. This tells me I should take more holidays."

The smallest previously known moons around Saturn are about 20 kilometers (12 miles) across. Scientists expected that moons as small as S/2004 S1 and S/2004 S2 might be found within gaps in the rings and perhaps near the F ring, so they were surprised these small bodies are between two major moons. Small comets careening around the outer solar system would be expected to collide with small moons and break them to bits.

This shows the second new 'worldlet,' temporarily dubbed S/2004 S2. Credit: NASA/JPL/Space Science Institute

The fact that these moons exist where they do might provide limits on the number of small comets in the outer solar system, a quantity essential for understanding the Kuiper Belt of comets beyond Neptune, and the cratering histories of the moons of the giant planets.

"A comet striking an inner moon of Saturn moves many times faster than a speeding bullet," said Dr. Luke Dones, an imaging team member from the Southwest Research Institute in Boulder, Colo. "If small, house-sized comets are common, these moons should have been blown apart many times by cometary impacts during the history of the solar system. The disrupted moon would form a ring, and then most of the material would eventually gather back together into a moon. However, if small comets are rare, as they seem to be in the Jupiter system, the new moons might have survived since the early days of the solar system."

Moons surrounding the giant planets generally are not found where they originally formed because tidal forces from the planet can cause them to drift from their original locations. In drifting, they may sweep through locations where other moons disturb them, making their orbits eccentric or inclined relative to the planet's equator. One of the new moons might have undergone such an evolution.

Upcoming imaging sequences will scour the gaps in Saturn's rings in search of moons believed to be there. Meanwhile, Cassini scientists are eager to get a closer look, if at all possible, at their new finds. Porco said, "We are at this very moment looking to see what the best times are for retargeting. Hopefully, we haven't seen the last of them."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

Nova in Scorpius — 1722-31 N SCO 04#2

Akira Takao of Kitakyushu, Japan, made this discovery on August 3.583 UT according to IAUC 8380. It the time of discovery the nova was magnitude +7.4 on unfiltered CCD images taken with a 120-mm telephoto lens.

Nova position: á = 17h 29m 18.81s ä = -31° 46' 01.5" (2000.0)

A preliminary AAVSO chart may be found here.

Maurice Gavin provided Astronomy Now with this image of the object taken on August 12th with his 12-inch f/6.3 SCT and MX9 CCD camera:

Supernova 2004dj in NGC 2403

Discovered on July 31.76 UT by Koichi Itagaki of Yamagata, Japan, this supernova lies near the eastern edge of NGC 2403 (Caldwell 7) in Camelopardalis. On first detection it was visual magnitude +11.2, but has now faded slightly.

Supernova position: á = 7h 37m 17.02s ä = +65° 35' 57.8" (2000.0)

The following general location chart for NGC 2403 shows the view to the north at midnight for U.K. observers in mid-August:

Maurice Gavin of the Worcester Park Observatory in Surrey kindly provided Astronomy Now with the following CCD image captured with a 12-inch Schmidt-Cassegrain telescope and MX9 camera on August 7th:

Maurice also obtained the following spectrum with the same instrumentation plus a grating on August 6th:

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.