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Solar system
Mars' moon, Phobos

4.5 billion years, about the same age as Mars


Solar system, in orbit around Mars

Avg. distance from Mars

9,377 km (5,827 mi)


The non-spherical moon has the dimensions of 27 x 22 x 18 km (17 x 14 x 11 mi).


1.1 x 1016 kg

Orbital period around Mars

11.1 hours

Distinguishing features

Phobos is a heavily cratered, nonspherical, potato-shaped moon with a prominent crater near one end. The crater is named Stickney, the maiden name of Asaph Hall's wife. Hall discovered Mars' moons in 1877 using the U.S. Naval Observatory's 26-inch (66-centimeter) refractor.

Mars' moon, Phobos

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"Fast Facts: Mars' moon, Phobos" is a table that lists the moon's age, location, average distance from Mars, diameter, orbital period around Mars, mass, and distinguishing features. A picture of Phobos is included.

Printer-friendly web page
Adaptable, at teacher's discretion
How to use in the classroom

Use this resource as:

A source of information. Read the table to find out about this object.

A large-number recognition and ordering activity. Have students review several Fast Fact moon tables, including this one. Have students read the tables to find each object's mass. Then ask them to arrange the objects according to their mass, from smallest to largest. Students also can arrange the objects according to their diameter.

A math activity. Use Newton's law of gravitation, the distance between Io and Jupiter, and the masses of the two objects to determine the gravitational force between them. (Recommended for grades 10-12)

An inquiry tool. Have students write down questions they would like answered about the image and the information in the Fast Facts table.

An engagement tool. Involve students in a discussion.

Related materials

Amazing Space resources by topic: Solar system

HubbleSite: Press releases on Moons