Phobos and Deimos

Students learn how Asaph Hall discovered Phobos and Deimos in 1877, how the two small moons differ in size and orbit, why Phobos is slowly spiraling toward Mars, and the evidence for and against the captured-asteroid and giant-impact ideas for their origin.

Grades 5-8About 15 minutes of narrated flight, plus discussion stops8 stops

The full guide, with the 40-minute plan, discussion key points, exit ticket, activities, transcript and PDF, comes with a teacher license. Already licensed? Use Open the full guide.

What students will see and do

Students start at Earth with Hall's 1877 search, fly to Mars for several stops about Phobos and Deimos, visit the potato-shaped asteroid Ida for comparison, return to Mars for the giant impact idea, and end at Earth with Japan's planned sample return mission. Aetherion pauses for discussion at the first Earth stop (how discoveries really happen), at Mars's 'Phobos, the Speedy Moon' stop (predicting Phobos's future), and at Ida (weighing the origin clues).

Learning objectives

• Describe the sizes, shapes and orbits of Phobos and Deimos and compare them with Earth's Moon.
• Explain why Phobos rises in the west and is slowly spiraling inward while Deimos drifts outward.
• Compare the evidence for the captured-asteroid idea and the giant-impact idea for the moons' origin.
• Explain how a returned sample could help scientists test competing hypotheses.

NGSS connections

Stops on this journey

1. A Search in Eighteen Seventy Seven (discussion stop)
2. Meet the Red Planet's Moons
3. Phobos, the Speedy Moon (discussion stop)
4. Craters and Grooves
5. Deimos, the Distant One
6. A Potato from the Asteroid Belt (discussion stop)
7. Born from a Giant Impact?
8. Bringing Home a Sample

Vocabulary

• moon
• orbit
• captured asteroid
• giant impact
• tidal force
• impact crater
• sample return
• hypothesis