In 2013, NASA's Cassini spacecraft captured this image of Earth from Saturn. Seen here, our planet is 898 million miles away (1.44 billion kilometers) and appears as a blue dot at center right. NASA/JPL-Caltech/Space Science Institute hide caption

itoggle caption NASA/JPL-Caltech/Space Science Institute

The Cathedral Spires in the Black Hills of South Dakota are just one of innumerable formations across the planet that speak to the Earth's ancient history. K. Scott Jackson/Ohio Water Science Center/USGS hide caption

itoggle caption K. Scott Jackson/Ohio Water Science Center/USGS

That little blue dot is how Earth will likely appear in a photo shot from a spacecraft that is studying Saturn. NASA/JPL-Caltech simulation hide caption

itoggle caption NASA/JPL-Caltech simulation

Will we need to tread lightly, in deference to the locals, when finally do make it to Mars? Pat Rawlings/SAIC/NASA hide caption

itoggle caption Pat Rawlings/SAIC/NASA

Many of life's building blocks can be found in the objects bombarding Earth from outer space. Does that mean that life, too, developed elsewhere before arriving here? Mary P. Hrybyk-Keith/NASA hide caption

itoggle caption Mary P. Hrybyk-Keith/NASA

On its way to Jupiter, the Galileo spacecraft looked back and captured this remarkable view of Earth and the moon. The image was taken from a distance of about 3.9 million miles. NASA hide caption

itoggle caption NASA

The planet Venus is seen crossing the sun in June 2004 as photographed through a telescope at Planetarium Urania in Hove, Belgium. The earliest known observation of such a transit was in 1639 by English astronomer Jeremiah Horrocks. Geert Vanden Wijngaert/AP hide caption

itoggle caption Geert Vanden Wijngaert/AP