Year of the Comet
Comet C/2011 L4 (PANSTARRS)

PANSTARRS information

Comet C/2012 S1 (ISON)

ISON information

Ask Astro

Published: September 27, 2010
Earth and Moon
Earth's Moon doesn't have its own satellite (called a "sub-moon") because Earth's gravity would quickly destabilize the sub-moon's orbit, either ejecting it from the system or sending it into orbit around Earth.
Photo by NASA /JPL / Northwestern University
If a sub-moon could form around (or be captured by) a satellite, the gravity from the parent planet would quickly destabilize the sub-moon's orbit, either ejecting it from the satellite system or sending it into orbit around the planet.
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