Phobos (Moon of Mars)
26 km (16.16 mi)
Longest axis (twice the subplanetary radius)
A lumpy little moon. Measured along its longest side. Longest axis: about 26 km.
Published
How big is that?
26 km is 2.9 times Mount Everest and 0.75% of the Moon.
Phobos is the 16th smallest of the 39 Cosmic Scale objects, and the 24th largest of the 109 objects across all three SizeItUp games.
Size comparison
To scale, smallest to largest: Phobos in orange beside the Cosmic Scale objects nearest in size, in teal.
| Object | Size | vs Phobos |
|---|---|---|
| Halley's Comet (Nucleus)The icy core of the famous comet, measured along its longest side. | 15 km long | 58% of the size |
| Olympus Mons (Mars)The tallest volcano in the Solar System. Its real slopes are far gentler; only the height counts here. | 21.2 km tall | 82% of the size |
| Phobos (Moon of Mars) | 26 km long | – |
| Grand Canyon (Earth)Arizona's canyon, seen from above and measured along the Colorado River. | 446 km long | 17× bigger |
| Vesta (Asteroid)One of the biggest asteroids, with a giant crater near its south pole. | 522.77 km across | 20× bigger |
In other units
| Metres | 26,000 |
|---|---|
| Kilometres | 26 |
| Feet | 85,300 |
| Yards | 28,430 |
| Miles | 16.16 |
How it's measured
Longest axis (twice the subplanetary radius). NASA Mars Fact Sheet: subplanetary radius 13.0 km.
Fact sheets list a radius; the diameter here is twice that.
Sources
- nssdc.gsfc.nasa.gov: Satellites of Mars, Phobos: subplanetary axis radius 13.0 km.
Checked against the source on .
Could you have guessed it by eye?
Phobos is one of the objects in Cosmic Scale, a daily size comparison game: you stretch a shape to the size you think is right beside one you know.