Mars Express Swings by Phobos Discovery

A European space probe is on track for a close encounter with the Martian moon Phobos, an odd, potato-shaped satellite — origins unknown — that may be partly hollow.
Mapping Phobos’ gravity is among scientists’ top priorities when the Mars Express spacecraft soars as close as 50 kilometers (31 miles) above the moon Wednesday night.
Previous passes of Phobos by Mars Express have raised as many questions as they’ve answered. For example, calculations of the moon’s density led scientists to the surprising theory that parts of Phobos may be hollow. Minute changes in the probe’s flight path — tracked by a radio signal — as it passes over the moon Wednesday will be closely monitored in an attempt to correlate Phobos’ gravitational tugs with internal structural variations.