Phoebe is about 137 miles (220 kilometers) wide. That's about one-fifteenth the diameter of Earth's Moon.
"The last time we had observations of Phoebe was by Voyager in 1981," said Torrence Johnson, former Voyager imaging team member and now part of the Cassini imaging team. "This time around, the pictures of the mysterious moon will be about 1,000 times better, as Cassini will be closer."
Cassini will gather photographs, radar observations and other data of Phoebe from just 1,240 miles (2,000 kilometers) away.
Lots to learn
Scientists don't know much about Phoebe's composition and structure. It is dark, as moons go, and it orbits backward to the rotation of Saturn and most of the planet's other satellites. That orbital motion suggests it is a captured asteroid or, more likely, it was snagged from a region beyond Neptune known as the Kuiper Belt, Thomas told SPACE.com.
Thing is, astronomers have never seen a Kuiper Belt Object up close, so they may not even know Phoebe's origins after the flyby.
"We really don't know what we're going to see, and that's why this is exciting," Thomas said in a telephone interview.
If Phoebe looks like other asteroids, scientists will have to consider that it might be one even though it would have had to make an unlikely migration, doubling its distance from the Sun while moving from the asteroid belt out to Saturn. But if Phoebe looks radically different from asteroids, then the chances are it is indeed something that was born beyond Neptune.
The images released this week already hint at sunlit ridges and deep craters, common to asteroids and other moons. The closest view was made June 7 from 1.5 million miles (2.5 million kilometers) away.
Exciting close-up
"We anticipate Phoebe will be heavily cratered in the higher resolution images we expect to see in the next few days," Thomas said. "The hints of different brightness also suggest the highest resolution images, several hundred times better, will show a variety of materials."
He added that Phoebe is proving to be roughly spherical, but not perfectly so. Impacts from other objects have likely carved its shape over the eons.
"It's like chipping things off of bricks," Thomas said. Larger moons tend to emerge collisions with their roundness intact.
One of the most telling observations Friday will be a gravity measurement. Cassini will record how the tug of Phoebe alters the spacecraft's course during the flyby. That will provide a firm estimate of the moon's mass and density, something astronomers know for only a handful of small objects in the solar system.
This will be Cassini's only chance to examine Phoebe up close. The far-out satellite will be well out of range of the spacecraft after it begins looping around Saturn for the first time at the end of this month.
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