Dimorphos by the numbers: name, size and orbit

Why it's called “having two forms,” how big it really is, and the numbers behind its 33-minute orbit change.

Published 2026-10-23

Dimorphos is tiny by planetary standards, yet it's one of the best-measured small bodies in the Solar System. Here are the key numbers.

Name

• Discovered: November 20, 2003, by Petr Pravec and colleagues at Ondřejov Observatory.^[5]
• Provisional designation: S/2003 (65803) 1; formal designation (65803) Didymos I.^[5]
• Named: June 23, 2020, by the IAU. Dimorphos means “having two forms”; it was suggested by Kleomenis Tsiganis of the Aristotle University of Thessaloniki.^[5]
• Nickname: “Didymoon,” used before the official name.^[5]

Size

NASA gives a diameter of about 170 meters; it measures about 177 meters along its longest extent.^[1][5] ESA describes it as a 151-meter target.^[3] Differences like these reflect different measurements and ways of describing an irregular shape.

Mass

Dimorphos's mass is not yet confirmed. Assuming a density of about 2.17 g/cm³ like Didymos, it would be about 5 × 10⁹ kg — roughly the mass of the Great Pyramid of Giza.^[5] Measuring the real value is one of Hera's goals.^[8]

Orbit

• Before DART: about 11.92 hours per orbit around Didymos.^[5]
• After DART: 33 minutes shorter.^[1]
• NASA's success threshold: 73 seconds — DART beat it by more than 25 times.^[1]

Spin

Sunlight is slowly changing Dimorphos's rotation through the YORP effect, with an estimated rotation-period doubling time of 86,000 years.^[5]

Numbered citations link to the sources list on the Dimorphos overview page.