Weighing an asteroid with no moon
Why Interamnia’s mass estimates range from 2.7 to 7 × 10¹⁹ kg, and how astronomers combine them.
Published 2026-10-23
Some asteroids make weighing easy: if they have a moon, its orbit gives the mass directly. Interamnia has no known moon, so astronomers must use a harder method.[2]
Gravity tugs on neighbours
When a small asteroid passes near a large one, its orbit is nudged slightly. Measuring those nudges over years lets astronomers estimate the larger body’s mass — but only if they model the other asteroids pulling on the same objects.[2]
A spread of answers
- 2001: about (7 ± 2) × 1019 kg — an estimate that depended on the masses of Fortuna, Amphitrite and Psyche.[2]
- 2011: about (3.9 ± 0.2) × 1019 kg.[2]
- 2014: a reanalysis gave about (2.7 ± 0.1) × 1019 kg.[2]
- 2019: a combination of 21 selected estimates from 1992–2017 gave (3.8 ± 1.3) × 1019 kg.[2]
Some of the individual error bars do not overlap, a reminder that the quoted uncertainties depend on the assumptions in each model.
Why mass matters
Mass divided by volume gives density, and density is one of the best clues to what an asteroid is made of. Combined with the VLT shape, the mass gives a density near 2 g/cm³ — pointing, with other evidence, toward an icy, Ceres-like body.[2] Interamnia is thought to hold roughly 1.2% of the mass of the whole asteroid belt.[2]
Numbered citations link to the sources list on the Interamnia overview page.