Chapter 14:

Twelve New-Found Moons of Saturn are Collisional Remnants of Larger Moons

The 12 new-found moons are in irregular orbits that suggest they are the collisional remnants of larger parent moons, once securely captured in, but later blasted from, their saturnian orbits.

Using several medium-to-large sized telescopes, large-format CCD arrays that photograph big areas of sky, and computers that process multiple gigabytes of data each night, teams of astronomers collaborated last fall in a search for so-called "irregular" moons around the gas giant.

Saturn was known to have six relatively large moons and 12 minor moons. All except one minor moon, Phoebe, discovered in 1898, are classified as regular satellites because they move along nearly circular orbits in the planet's orbital plane, revolving in the same direction as the planet spins.

The 12 new-found satellites are irregular - meaning they orbit outside the plane of Saturn's equator - and it appears that their orbits cluster in three, possibly four, distinct groups, said Carl W. Hergenrother of the UA Lunar and Planetary Laboratory (LPL).

"We think we're seeing orbits cluster, that is, orbits of several moons fall in the same general plane, just as asteroids cluster," Hergenrother said. "And with asteroids that cluster, the belief is they are pieces of what once was a big asteroid that got hit by something. It's possible that we're seeing the same thing with the satellites."

Brett Gladman of the Observatoire de la Cote d'Azur in France, J.J. Kavelaars of McMaster University in Canada, and Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., discovered the irregular saturnian moons in August, September and November, 2000, using the 2.2-meter (87-inch) European Southern Observatory in Chile, the 3.6-meter (142-inch) Canada-France-Hawaii Telescope in Hawaii, and the 1.2-meter (48-inch) Mount Hopkins telescope in Arizona.

Hergenrother, Stephen M. Larson and Rob Whiteley - all of the LPL - and Dennis Means of the UA Steward Observatory used the Steward Observatory's 1.5-meter telescope (61-inch) in the Santa Catalina Mountains north of Tucson and the 2.3-meter (90-inch) Bok Telescope on Kitt Peak to observe the moons for more precise information on their orbits.

Others doing this "recovery" work to help define the satellite orbits used the 4-meter Kitt Peak telescope, the 5-meter Palomar telescope and 2-to-3-meter class European telescopes.

The research is reported in the article, "Discovery of 12 satellites of Saturn exhibiting orbital clustering," in the July 12 Nature.

Astronomers in 1997 and 1999 discovered five irregular satellites around Uranus, and in 1999 - 2000 discovered another 12 irregular satellites around Jupiter, previously known to have eight. The UA Spacewatch on Kitt Peak discovered one of the new-found jovian moons.

Almost all of the irregulars discovered since 1997 cluster in easily discernible groupings, the astronomers note in their article.

"The difficult question is whether the disruptions occurred during the capture process itself when the planets formed long ago, or whether intact moons were captured at that time into orbits near the present grouping and these single moons were subsequently shattered and scattered by intruding comets or asteroids during the subsequent (more than 4-billion-year solar system history)," they wrote.

The most probable theory is that each cluster is the remains of a once-intact moon smashed by a collision sometime after the planets were formed, according to their analysis.

Saturn must have captured the original parent moons during planetary formation, as the objects passed through Saturn's surrounding proto-planetary gas cloud, Hergenrother said.

An alternative theory is that the moons were captured when Saturn suddenly increased in mass - in which case the moons would all be prograde, moving around the planet in the same direction as the planet moves around the sun.

"But we are seeing just as many retrograde as prograde irregular moons at Saturn," Hergenrother said. Objects captured as moons would move in either prograde or retograde orbits depending on their direction as they passed through and were slowed by proto-Saturn's gas cloud.

Satellites in orbital clusters could range in size from one to 100 kilometers in diameter, he added.

"Right now, we see irregular satellites as small as 3 kilometers around Saturn, but there may be many smaller than that. These may go on a continuum in size all the way down to the size of dust. "

Still More New Moons of Saturn

S/2000 S 7, S/2000 S 8, S/2000 S 9
The team led by B. Gladman reports the discovery of three more satellites of Saturn. Selected astrometry and ephemerides:

B. Gladman and J. Kavelaars (Mauna Kea). 3.6-m Canada-France-Hawaii Telescope. Measurer Gladman.
UT1 Science Team (Paranal). ESO VLT-UT1 8-m refl. Measurer Gladman.
J. Kavelaars and P. Nicholson (Palomar). 5-m refl. Measurer Kavelaars.

S/2000 S 10
Another Saturnian satellite has been reported by the IAUC 7512 team.

OUTER SATELLITES OF THE GIANT PLANETS
During the past year and a half the number of known outer satellites (or candidate outer satellites) of the giant planets has more than doubled.

4 More Moons of Saturn

[Cornell University Press Release, October 2000]

An international team of eight "satellite hunters," astronomers who pluck tiny specks of light out of the distant solar system, has discovered four new outer moons of Saturn orbiting at least 15 million kilometers (more than 9 million miles) from the surface of the giant planet.

The discovery gives Saturn a total of 22 known moons, surpassing the 21 orbiting Uranus. Nothing is known about the four new moons except for their brightness. Estimates of their size -- between 10 and 50 kilometers (6-30 miles) across -- are based on assumptions of their reflectivity. Observed from Earth-bound observatories, the moons appear as faint dots of light moving around the planet.

Members of the team, including former Cornell University researcher Brett Gladman and Cornell professors of astronomy Joseph Burns and Philip Nicholson, warn that the findings are still preliminary. They also note that they might have discovered several other objects that could be Saturnian moons. Other members of the team include Jean-Marc Petit and Hans Scholl of the Observatoire de la Cote d'Azur, France; J.J. Kavelaars of McMaster University, Canada; and Matthew Holman and Brian Marsden of the Harvard-Smithsonian Center for Astrophysics.

The discovery of the four new moons was made using a technique developed by Gladman while he was a student at Cornell. Gladman, who now works for the Centre National de la Recherche Scientifique in France, obtained his Ph.D. at Cornell. The technique, which also was used in the discovery of the five new Uranian moons, uses light-sensitive semiconductors, called charge-coupled devices, attached to telescopes to detect the distant points of light. Several of these digital images, taken once every hour, are then compared, using computer software to pick out a moving point of light against the known star background of the sky.

Between 1997 and 1999, the same team discovered a total of five new moons of Uranus. All five, like the newly discovered four outer moons of Saturn, are irregular satellites. Burns notes that an irregular satellite's orbit is "long and looping," unlike the orbit of an inner moon, which is nearly circular and lies in the planet's equatorial plane.

The great distance that the moons orbit from Saturn, says Nicholson, indicates that the moons were captured into orbit after the planet formed, unlike the larger regular satellites that are thought to have coalesced from a disk of dust and gas that surrounded the planet as it formed.

Until this latest discovery, Saturn was known to have only one irregular, outer satellite, Phoebe, which was discovered by W. Pickering 102 years ago. Nicholson notes that Phoebe is traveling in a retrograde orbit, that is, in the opposite direction to the spin of Saturn. All regular satellites move on prograde orbits that follow the direction of the spin of their parent planet. "We look for such patterns because it's easier to capture objects from a solar orbit into a retrograde than onto a prograde orbit," says Nicholson. "If you could demonstrate statistically that retrograde orbits were favored, that would help confirm some theories of capture."

The first two candidates for newly discovered satellites of Saturn were spotted by Gladman using the European Southern Observatory's 2.2 meter telescope in Chile on Aug. 7. Gladman and Kavelaars "recovered" the two objects Sept 23 and 24 at the Canada-France-Hawaii 3.5 meter telescope on Mauna Kea, Hawaii. They also found two new candidates. Additional confirming observations were made at other telescopes.

Related World Wide Web sites: The following sites provide additional information on this news release.

Brett Gladman: ( http://www.obs-nice.fr/saturn).

McMaster University: ( http://pinks.physics.mcmaster.ca/Saturn).

Communications Problems Potentially Limit Huygens Probe

The Cassini spacecraft's radio cannot satisfactorily pick up the entire radio band over which the Huygens Probe is to send it signals to relay, Cassini project officials disclosed in October 2000. There is the danger, therefore, that some of the data that the Huygens Probe is to send as it plunges into Titan's atmosphere for 2.5 hours in November 2004 wile be lost. It turns out that the bandpass of the Cassini receiver is not as wide as the design called for. The equipment cannot be fixed or bypassed, but engineers hope that signal processing can find a way to save the rest of the data. They may even have to change the time and angle at which the Probe is deployed. The Huygens probe and the receiver on Cassini were supplied by the European Space Agency, and the problem was discovered in tests they carried out. They hope to have a proposed solution in the summer of 2001.

The good news is that the camera on Cassini is working very well. It supplied a clear, sharp image of Jupiter from its current distance of 52 million miles.

At present, Cassini is to go into orbit around Saturn on July 1, 2004, and the Probe is to be deployed on November 30, 2004.

On October 9, 2000, the Cassini Imaging Team released its first color image of Jupiter seen through the eyes of Cassini. It can be found at the updated Imaging Team website

http://ciclops.lpl.arizona.edu/

Cassini Flies by Venus
NASA/JPL Press Release

The Cassini spacecraft, marking the 617th day of its voyage to Saturn, on June 24, 1999, successfully completed its second flyby of the planet Venus, once again on time and on target.

As planned, Cassini came within 600 kilometers (about 370 miles) of the planet at 1:30 p.m. Pacific time, with Venus' gravity giving the spacecraft a boost in speed to help it reach Saturn more than 1 billion kilometers away. The spacecraft, launched on its voyage October 15, 1997, remains in excellent condition as it travels its nearly seven-year trajectory to Saturn. Most of Cassini's scientific instruments were set to make observations during the Venus flyby. Scientific data from the flyby will transmitted to Earth over coming days.

Four flybys of planets -- two of Venus and one each of Earth and Jupiter -- give Cassini the speed it needs to reach Saturn. Cassini first flew past Venus on April 26, 1998 at a distance of 284 kilometers (about 176 miles). Today's Venus flyby will be followed by a 1,166-kilometer (724-mile) flyby of Earth on August 18 (August 17 Pacific time at 8:28 p.m. PDT), then it's on to Jupiter for a December 30, 2000 flyby. The giant planet's gravity will bend Cassini's flight path to put it on course for arrival into orbit around Saturn on July 1, 2004.

Cassini's mission is to study the ringed planet, its magnetic and radiation environment, moons and rings for four years. Cassini will also deliver the European Space Agency's Huygens probe to parachute to the surface of Saturn's moon Titan. Titan is of special interest partly because of its many Earthlike characteristics, including a mostly nitrogen atmosphere and the presence of organic molecules in the atmosphere and on its surface. Lakes or seas of ethane and methane may exist on its surface. The Cassini mission is a joint effort of NASA, the European Space Agency and the Italian Space Agency. The mission is managed and the Cassini spacecraft built and operated by NASA's Jet Propulsion Laboratory, Pasadena, CA. JPL is a division of the California Institute of Technology.

More information about the Cassini mission is available at: http://www.jpl.nasa.gov/cassini

Cassini Flies By Venus

from the MEDIA RELATIONS OFFICE, JET PROPULSION LABORATORY The Saturn-bound Cassini spacecraft successfully performed a flyby of the planet Venus on April 26, 1998, coming about 284 kilometers (176 miles) from the Venusian surface. The flyby gave the Cassini spacecraft a boost in speed of about 7 kilometers per second (about 4 miles per second) help the spacecraft reach Saturn in July 2004.

"All indications are that the spacecraft did exactly what we expected," said Deputy Program Manager Ronald Draper at NASA's Jet Propulsion Laboratory, Pasadena, CA. One-way light-time to the spacecraft from Earth was about 7-1/2 minutes.

Leaving Venus, the spacecraft was moving at more than 141,000 kilometers per hour (87,000 miles per hour). Science instruments on the spacecraft searched for lightning in Venus's atmosphere during the flyby, and the radar instrument onboard was activated to test a bounced signal off Venus's surface.

In its long trajectory to Saturn, Cassini will perform another flyby of Venus next June, one of Earth in August 1999, and one of Jupiter in 2000. All of the flybys impart more speed to the spacecraft to allow it to reach its final destination of the Saturnian system. After it enters orbit around Saturn in 2004, Cassini will study the ringed planet, its moons and ring system for at least four years. It will also deliver a scientific probe called Huygens to parachute to the surface of Saturn's largest moon, Titan.

Cassini Imaged from Earth

Dear Colleagues and Friends,

As you all know, Cassini was successfully launched in a spectacular display of flame, sound and fury atop a TitanIVB/Centaur rocket from Cape Canaveral early in the morning of October 15, 1997.

I hope you will be as thrilled as I was to hear that Spacewatch, a near-Earth asteroid discovery program here at LPL, has detected Cassini. Visit the web at

www.lpl.arizona.edu/users/jmontani/

and go to the very bottom of the page to find 3 CCD images (each has a vertical white dropout running through it) which show Cassini moving westward (upward) from the leftmost photo to the right. It's already 7 times farther away than the Moon, and truly an interplanetary wanderer.

Carolyn Porco (carolyn@vips.lpl.arizona.edu), Cassini Imaging Team Leader