How do we know how big Iocaste is?

Why many small-moon sizes come with the phrase “assuming an albedo of 0.04”.

Published 2026-10-23

No spacecraft has imaged Iocaste up close. So how can NASA say it has a radius of about 2.6 km?^[2]

Brightness isn’t size

A distant moon is just a point of light. How bright it looks depends on its size and on how reflective its surface is (its albedo).

The assumption

For many small irregular moons, astronomers assume an albedo. NASA’s figure for Iocaste assumes 0.04 — a dark surface reflecting 4% of sunlight.^[2]

How wrong could it be?

If the real surface is darker, the moon must be bigger; if brighter, smaller. For Jupiter’s moon Kalyke, infrared data gave an albedo of 2.9%, a reminder that assumed values are estimates.^[6]

Shape

Ananke-group moons are too small to pull themselves into spheres, so they are probably irregular in shape.^[2]

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