These are chance alignments of celestial objects that are close to each
other in the sky. The above is a close approach of Venus and Mercury on
June 27th 2005 when they were about 0.3 degree apart and could be seen in
the same telescope field of view, showing both discs. Although collisions
have occurred in the past, today the moon and planets are in well defined
orbits. The only realistic chance of a collision is between a planet and
asteroid or comet. Indeed, Comet Shoemaker Levy 9 crashed into Jupiter in
1992.
Sometimes one object passes directly in front of another as seen from
the Earth. These events are known as occultations.
