Europe's Mars Express orbiter will sweep less than 42 miles from the Red Planet's scarred moon Phobos on Wednesday to peer into the object's porous interior and gather clues about its mysterious origin.

NASA's Mars Reconnaissance Orbiter captured this view of Phobos with the HiRISE camera in March 2008. MRO was about 4,200 miles from Phobos when this image was taken. Credit: NASA/JPL/University of Arizona

Wednesday evening's flyby will be the closest approach to the moon by any spacecraft, providing scientists with their best chance to map the gravity field of Phobos.

The potato-shaped moon, named for the Greek word for "fear," is about the size of a large city, stretching 17 miles along its longest axis. Mars is also host to a smaller orbiting rock named Deimos

"This will be the closest Phobos flyby ever, and we'll perform a so-called gravity experiment, in order to measure the gravity field of the moon," said Olivier Witasse, Mars Express project scientist at the European Space Agency.

Better gravity maps will yield information on the distribution of mass inside the moon, according to scientists.

"The gravity data will help to better understand what can be the interior of Phobos, especially the distribution of masses inside Phobos," Witasse told Spaceflight Now.

The density of Phobos has already been measured by Mars Express. Researchers believe Phobos is made of loosely-collected material with hollow regions inside that may be filled with ice. According to scientists, the porosity is similar to characteristics of fractured D-type asteroids, more popularly known as "rubble piles."

The Phobos surface composition is more typical of carbon-rich C-type asteroids.

Now circling Mars about 3,700 miles above the surface, Phobos may have been an ancient errant asteroid captured by the tug of Martian gravity as it ventured too close to the planet. Other theories are that Phobos formed at the same time as Mars in the early solar system, or the moon was assembled from smaller chunks of debris launched into space by a massive meteorite collision with the Red Planet.

Mars Express will fly closest to Phobos at 2055 GMT (3:55 p.m. EST) Wednesday. Flight controllers at the European Space Operations Center in Germany will be monitoring the flyby.

The fleeting swing by Phobos is part of a series of 12 flybys as the orbit of Mars Express temporarily brings the spacecraft in the vicinity of the moon. The orbiter has approached within 58 miles of Phobos during previous flyby campaigns.

On Wednesday, engineers will turn off all live data streams coming from the spacecraft, except for the carrier signal, an unmodified constant radio tone that will take six-and-a-half minutes to reach Earth from Mars Express.

The spacecraft will be more than 72 million miles from Earth during Wednesday's approach.

Artist's concept of Mars Express. Credit: ESA

The carrier signal will be analyzed on the ground to fish out miniscule variations in the tone's frequency due to perturbations by the gravity of Phobos. The frequency changes are due to the Doppler effect, which triggers the change in pitch of a passing train or ambulance.

Although ESA says the signal variations will amount to just one part in a trillion, a 230-foot-wide dish antenna from NASA's Deep Space Network in Madrid will be listening to detect the smallest blips. Even though the changes are barely perceptible, it will give scientists their clearest data yet on the interior of Phobos.

As Mars Express flies by Phobos, the moon's gravity will pull the spacecraft off course at a nearly immeasurable rate. This small tug will be enough to precisely measure the Phobos gravity field and its disparities caused by the moon's internal structure.

Learning what's inside Phobos is just the first step.

"The density is used to guess what could be the origin of Phobos, by comparing with known asteroids, for example," Witasse said Tuesday.

Computer models derived from data gathered so far predict between 10 percent and 40 percent of Phobos is hollow, according to Witasse. Some of that volume may be occupied by water ice.

Wednesday's flyby will give scientists an idea of where the hollow caverns are located inside Phobos, and possibly even what they contain.

"We should be able to shed new light on the origin then, [whether it is a] captured asteroid or it formed in-situ," Witasse said.

A radar instrument aboard Mars Express has already been delving into the internal structure of Phobos, and the spacecraft's suite of spectrometers are also studying the moon's surface.

The Mars Express high-resolution stereo camera will be activated for a flyby on Sunday, when the probe will pass about 66 miles from the Phobos surface. The camera will image the proposed landing site of the Phobos-Grunt sample return mission, a Russian-led endeavor to launch in 2011 and return samples of the Martian moon back to Earth.

"All the experiments on Mars Express have something to say about Phobos," Witasse said.

Mars Express launched in May 2003 and is now in an extended mission.