It's old. It's craggy. It's barely even a candidate for life. In fact things are so dismal for the moon Callisto, one scientist calls it the ugly duckling of Jupiter's satellites.
But on Friday, Callisto will get a day in the space spotlight as the Galileo spacecraft makes its closest approach ever to the Jovian moon. But alas, it's all part of a scheme to propel Galileo to a more alluring destination -- the colorful, volcanic, pizza-like moon Io.
"The main reason we're flying so close to Callisto is to set up flybys of Io," said Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory in Pasadena.
But can't we turn an engineering necessity into a scientific opportunity?
"Since we have to go close to Callisto anyway to get to Io, we'll take advantage of the opportunity for studying Callisto," said Torrence Johnson, Galileo project scientist. It is Johnson who yesterday said, "Callisto is sort of the ugly duckling of the moons."
C'mon, folks. Can't you say anything nice about Callisto? Surely it has something to offer that the other moons don't. What about all those craters -- more than any other object in the solar system?
"It's the one we need to look at to get the bombardment history of the Jovian system," admitted Johnson. "The craters on Callisto are the visible record of[the]sizes of comets and other objects [that] have pelted Jupiter and its moons, [and] with what frequency over the past 4 billion years."
Ah, now that sounds exciting!
Skim the surface
So at 7:24 a.m. EDT (11:24 GMT) on May 25, Galileo will skim just 76 miles (123 kilometers) above Callisto's surface.
Unlike the planet's other three Galilean moons (named after Galileo, the philosopher-mathematician who discovered them, not the spacecraft) Callisto is still covered in craters that are billions of years old, signs of ancient impacts. The fact that these scars remain means that unlike Io, Callisto is not very active, volcanically or otherwise.
This has to do in part with proximity and stability. Io, being the closest Galilean moon to Jupiter and having an orbit whose distance from the gas giant varies constantly, gets its insides constantly tugged at by Jupiter's immense gravity. Callisto, being the farthest out of the four, and having a more stable orbit, does not get such a wrenching ride around the planet.
But when planning studies of such distant places, it is the search for water, and the life it might harbor, that most whets the appetites of scientists and the institutions that fund them. And although earlier studies show Callisto may have a sea of saltwater deep beneath its surface, Europa is considered a much stronger candidate for having water near the surface.
So Galileo's time at Callisto will be spent looking at big dents.
The craft will snap some high-resolution photographs in an effort to map the density of small craters and to reveal whether and how erosion has taken its toll on the features.
"Some earlier imaging of Callisto has shown fewer small craters than expected," said Duane Bindschadler, who leads the mission's science planning team.
It will be two months before all the data is transmitted back to Earth.
On to Io
The close pass over Callisto will alter Galileo's trajectory and set the craft off toward Io, the most volcanic body in the solar system. This fall, researchers will explore whether Io generates its own magnetic field.
Scientists also plan to snap new pictures of Io, looking to see if a volcanic plume detected near its north pole five months ago is still active. On Aug. 5, Galileo will pass directly over the plume, less than 220 miles (350 kilometers) above its source.
Meanwhile, Galileo will also spend some time this week mapping Jupiter's clouds in infrared light. Scientists will be looking for mysterious features dubbed "brown barges," a type of dark cloud that was prominent on Jupiter when the two Voyager spacecraft flew by in 1979, but has not been seen during the years since Galileo began orbiting Jupiter in 1995.
Death plunge
Galileo's mission was originally scheduled to end in 1997, but it has been extended twice. The most recent extension came in March, when an ultimate death plunge plan was outlined.
In November 2002, Galileo is set to swing closer to Jupiter than ever before, dipping within about 500 kilometers (about 300 miles) of the moon Amalthea, which is less than one-tenth the size of Io and less than half as far from Jupiter.
Scientists will use Galileo's measurements to determine the mass and density of Amalthea. They will also study dust particles as Galileo flies through Jupiter's gossamer rings, which are similar but less noticeable than the rings of Saturn. The craft will also seek new details of the magnetic forces and the densities of charged particles close to the planet.
Galileo's final orbit will take an elongated loop away from Jupiter. Then in August 2003, the spacecraft will head back for a direct impact and burn up as it plows into solar system's biggest planet.
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