PARIS, March 2 (UPI) -- The European Space Agency says its Mars Express spacecraft is ready to make a close flyby of Mars' moon Phobos, passing just 41 miles above its surface.
The Wednesday 3:55 p.m. EST event will involve precise radio tracking that will allow researchers to peer inside the mysterious moon, the ESA said.
Mars Express is currently engaged in a series of 12 flybys of Phobos. Wednesday's event will be its closest flyby.
"From close range, Mars Express will be pulled 'off-course' by the gravitational field of Phobos," the space agency said. "This will amount to no more than a few millimeters every second and will not affect the mission in any way. However, to the tracking teams on Earth, it will allow a unique look inside the moon to see how its mass is distributed."
The ESA said the tracking teams will turn off all data signals from the spacecraft. The only thing the ground stations will listen for is the carrier signal -- the pure radio signal that is normally modulated to carry data.
With no data on the carrier signal, the only thing that can modulate the signal is any change in its frequency caused by Phobos' mass tugging the spacecraft. That allows researchers to study the moon's interior.
After Wednesday's close flyby, Mars Express will sweep past Phobos seven more times.
