Phobos: Facts About Martian Moon

NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos, from a distance of about 6,800 km (about 4,200 miles).
CREDIT: NASA/JPL/University of Arizona

Facts about Phobos:

• Radius of moon: 6.9 miles (11.1 km)
• Semi-major axis around Mars (distance from planet's center): 5,826 miles (9,376 km)
• Closest approach: 5,738 miles (9,234 km)
• Farthest approach: 5,914 miles (9,518 km) 
• Orbit eccentricity: 0.0151
• Orbit inclination: 1.075 degrees
• Time to make one orbit: 7.65 hours
• Mass: 1.0659 x 10^16 kg
• Density: 1.872 g/cm^3
• Surface gravity: 0.0057m/s^2
• Escape velocity: 25 mph (41 km/h)