Today, the Cassini spacecraft captured images of Saturn's moon Phoebe that have more than a thousand times better resolution than existing technology has allowed in the past. In addition, with its wealth of scientific instruments, the spacecraft was able to gather important data, and allow scientists to study Saturn's most distant major moon and possibly gather clues about the early formation of our solar system.
A few weeks before entering the orbit of Saturn, the Cassini spacecraft will fly by Phoebe and begin its landmark scientific mission. The distance from which images of Phoebe will be taken is radically different from the early 1980's, when Voyager 2 embarked on its legendary tour of the solar system. Then, Voyager 2's cameras snapped images of Phoebe from about 2.2 million kilometers (about 1.4 million miles) away. Cassini, on the other hand, will snap images from a mere 2,000 kilometers (about 1240 miles) from the moon's surface. In addition, thanks to significant technological improvements throughout the past twenty years, Cassini's cameras are not only far smaller, but also far superior in image quality than their earlier counterparts.
Yet, the dramatically improved optical capabilities are only part of the excitement surrounding Cassini's flyby of Phoebe. Through its high-tech onboard instruments, Cassini will also collect spectroscopic and radar data that could decipher the composition and origin of this tiny, distant moon.
Average distance from Saturn: 8,048,000 miles
Orbital period: 550.48 days (retrograde orbit)
Rotational period: 0.4 days
Diameter: 220 km (136.7 miles)
Phoebe is .063 times the diameter of the Moon
1898 - William Pickering discovers Phoebe
September 1981 - Voyager flyby
June 11, 2004 - Cassini flyby
July 1, 2004 - Cassini enters Saturn's orbit
The few images we have of Phoebe are vague-they resemble a series of black and white dots arranged in a circular shape. However, in spite of their low resolution, even these existing images supplied the science community with clues about a moon simply too small and too far away to be adequately studied from Earth.
Phoebe is roughly spherical and has a diameter of 220 kilometers (about 136.7 miles), which is equal to about one-fifteenth of the diameter of Earth's moon. Phoebe rotates on its axis every nine hours and it completes a full orbit around Saturn in about 18 months. Its irregular, elliptical orbit is inclined approximately 30 degrees toward Saturn's equator. Phoebe's orbit is also retrograde, which means it goes around Saturn in the opposite direction of most other moons -- as well as of most other objects in the solar system. Its average distance from the ringed planet is 13 million kilometers (about 8 million miles), which is almost four times farther away from Saturn than its nearest neighbor, the moon Iapetus. Phoebe and Iapetus are the only major moons in the Saturn region that do not orbit close to the plane of Saturn's equator.
Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6 percent of the sunlight it receives. Its darkness and irregular, retrograde orbit suggest that Phoebe is most likely a captured object, which is defined as a celestial body that is trapped by the gravitational pull of a much bigger body, generally a planet.
Phoebe's darkness in particular suggests that the small moon comes from the outer solar system, a region known as the Kuiper Belt, where many celestial bodies contain dark material.
Some scientists think Phoebe could be a captured Centaur. Centaurs are believed to be Kuiper Belt objects that migrated into the inner solar system. Centaurs are found between the asteroid belt and the Kuiper Belt, and are considered to be intermediate bodies that exist between the two belts. If Phoebe is indeed a captured Centaur, images and scientific data taken by the Cassini spacecraft will give scientists the first intensive opportunity to study a Kuiper Belt object.
Kuiper Belt objects are of extreme interest to scientists because they are believed to be primordial-- that is, they appear to date back to the formation of the solar system. Known as the "building blocks" of the solar system, Kuiper Belt objects exist as fragments that were never drawn together by gravity into a planet.
And, because of its small size and poor reflective properties, Phoebe might not have generated enough heat since its formation to alter its original chemical composition. This is another major factor that increases the scientific value of the study of this mysterious moon.
We do know that Phoebe's density is low, but its internal structure and composition are basically unknown. So are its geology, history, and surface morphology. Did it ever melt? Does it have evidence of past interior melting? Was it ever an icy body? Why is Phoebe in such an odd orbit?
Another issue that might be explained by the data returned by the flyby is if Phoebe might come from the dark material found on Hyperion and Iapetus, two other intriguing moons orbiting the ringed planet.
Discovered more than 100 years ago by American astronomer William Pickering, Phoebe is a source of extensive interest by scientists. As it gears up for its four-year grand tour of the Saturn region, the Cassini Mission will gather as much sensitive data and high-resolution images as possible from its monumental flyby of this tiny moon. The event hopes to answer questions-and perhaps generate new ones, about Phoebe and its possible role in determining how the solar system was formed.