The transit of Venus can't be observed with the Hubble Space Telescope because the extreme brightness of the sun would destroy the telescope and its instruments.

CNN: A transit of Venus, an age of wondrous science

In 1962, the Mariner II space probe began sending back to Earth man's first information from another planet, Venus.

CNN: Monday,

Opportunities to photograph Mercury from space are also limited because light sensitive equipment, such as the Hubble Space Telescope, are not allowed to look at objects close to the Sun, such as Mercury or Venus.

BBC: Close up on Mercury