Flying by Phoebe

Narrow angle field-of-view artist’s rendering from the bottom of a large ice crevasse on the surface of Phoebe, the least known of all the Saturnian satellites. Image Credit: JPL/ David Seal

On June 11, after an interplanetary journey of nearly 7 years, the Cassini-Huygens spacecraft will begin the most extensive exploration ever conducted of the planet Saturn and its system of rings and moons. The first moon Cassini will encounter is Phoebe, the outermost of the planet’s 30 known satellites.

Phoebe is a tiny moon, only 110 kilometers (68 miles) in diameter. Earth’s moon is more than 30 times as wide. Yet Phoebe is an intriguing target of scientific investigation, for two reasons. First, compared to most other moons in the solar system, Phoebe’s surface is very dark. Second, Phoebe’s orbit is retrograde: it travels around Saturn in the opposite direction to that of the planet’s other moons. These two factors have led astronomers to speculate that Phoebe was captured by Saturn’s gravitational pull after the planet formed. Saturn’s other moons, scientists believe, formed along with the planet itself.

According to Charles Elachi, JPL director and team leader for Cassini’s radar instrument, Phoebe "might be a captured asteroid or Kuiper Belt object." If so, he says, studying the tiny moon, "will give us some insight into Kuiper Belt objects." The Kuiper belt is a vast band of asteroids, or planetesimals, that form the outer fringe of our solar system. They circle the sun out beyond the orbit of Neptune, between 30 and 1000 AU. (An AU, or astronomical unit, is the average distance between the Earth and the sun, about 150 million kilometers, or 93 million miles.)

Many astronomers believe that Kuiper Belt objects represent material that, in the 4.5 billion years since our solar system formed, has never been altered. Material that accreted to form planets and larger moons was chemically altered by the pressures and elevated temperatures that occurred when planetesimals smashed into each other and stuck together during the accretion process. Studying Phoebe could provide scientists with a window into the primitive materials that existed at the solar system’s birth.

Cassini will acquire the most detailed images ever taken of Phoebe. At its closest approach, the spacecraft will pass within 2000 kilometers (1240 miles) of the moon, roughly the distance from New York City to Miami, Florida. Voyager 2, which held the previous record for close-up snapshots of Phoebe, came to within only 2.2 million kilometers (1.4 million miles) of the tiny world. In addition to taking images of Phoebe, Cassini will train its spectroscopic, radar and other instruments on the moon. The radar data will provide insight into Phoebe’s density; the spectroscopic information will help scientists analyze its chemical composition. The spacecraft will also study Phoebe’s interaction with the solar wind and examine the dust that surrounds the moon.

In this artist rendition, the Huygens probe is about to reach the surface of Titan, Saturn’s largest moon. Because it’s in a deep freeze state, Titan’s atmosphere is thought to contain chemicals similar to those found on Earth’s early days. Data from the Huygens probe combined with Cassini’s measurements may offer clues about how life began on Earth.

But Cassini’s Phoebe flyby is merely a prelude to the show the orbiter will put on later this month. At the end of June, the spacecraft will fly up through a gap between Saturn’s F and G rings and perform a 90-minute engine burn to slow itself down, allowing itself to be captured by Saturn’s gravity. The gas giant will then pull the spacecraft back down below the rings, where it will become the first artificial object ever to orbit Saturn. The pictures Cassini takes during this burn – the most detailed images ever of Saturn’s rings – are expected to be nothing short of spectacular.

Once the maneuver is complete, Cassini will begin orbiting the ringed planet, photographing and probing it with a host of scientific instruments that will return a far more detailed portrait of the Saturnian system than any previously developed. "Basically," says Elachi, "Saturn is a complete planetary system." Its rings are of particular interest, he says. "What you want to understand is what they are made of, what’s their dynamic, how old they are and also to shed some light on how dust disks lead to the formation of planets. In a sense, Saturn’s system is a miniaturized solar system and we want to understand that formation history."

And then there are Saturn’s moons, 30 of them in all. As Cassini makes its broad swings around the planet over the next several years, it will repeatedly fly past many of Saturn’s satellites. Of these, Titan is the most interesting scientific target. Titan is second largest moon in the solar system, larger than Pluto and Mercury. Only Jupiter’s Ganymede is larger. And Titan is the only moon in the solar system with its own atmosphere.

Previous flybys of Titan have revealed that its atmosphere is mostly nitrogen and methane. Meteorologically, Elachi says, the methane in the atmosphere plays a role similar to that of water vapor in Earth’s atmosphere. "So there are clouds of methane, there is the possibility of precipitation, rain of methane, and then on the surface itself [there may be] rivers, lakes, oceans which are a mixture of methane, ethane and other hydrocarbons." Scientists speculate that Titan may be rich in more complex organic hydrocarbon compounds, containing a mixture similar to the brew of organics that existed on early Earth. Studying Titan, says Elachi, "will give us a past picture of our own planet, before biology actually got started." However, he adds, "we don’t expect biology to be active today on Titan, because the temperature is about minus 179 degrees Centigrade" (minus 290 degrees Fahrenheit).

This image of Phoebe was acquired by the Voyager 2 spacecraft in 1981 from a distance of 2.2 million kilometers (about 1.4 million miles). Image Credit: JPL

Titan is the target for the second major component of the Cassini-Huygens spacecraft, the Huygens probe. On Christmas Day of this year (Christmas eve in some parts of the world), Cassini will release the Huygens probe and send it hurtling toward the giant moon. About three weeks later, Huygens will reach Titan and begin a two-hour drift down through Titan’s atmosphere, its descent slowed by a pair of parachutes. Along the way, it will take more than 1000 photographs and measure the chemical composition of the atmosphere and the surface. Although not designed as a lander, Huygens should impact Titan’s surface at about 5 meters per second (about 16 feet per second). The probe may survive its gentle crash landing and continue to send data for as long as a half an hour after it reaches the surface.

Related Web Pages

Arrival and Orbital Insertion