Phobos zooms around Mars so fast it rises and sets twice a day!
Small lumpy moon of Mars, likely a captured asteroid, orbits so fast it rises and sets twice a Martian day.
Moon: Orbits a planet rather than the star. Every kind of thing there is →
No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.
Phobos is roughly 22 km from side to side -- a two-hour bike ride, if you could pedal in almost no gravity. In weight, about 10.6 trillion tonnes. Its day: tidally locked; same as its orbital period, 7.66 hours.
| Across | 22 km |
|---|---|
| Distance from the planet it orbits | 9 376 km |
| Once round its orbit | 7.7 hours |
| One spin | 7.7 hours |
Nobody flew a tape measure out there. Every number on this page was worked out from the ground:
Not this one. But the sky over your head is full of ones you can see, and the Planetarium knows which.