Members of the New Horizons science team react to seeing the spacecraft's image of Pluto on Tuesday, before its closest approach. Bill Ingalls/NASA via AP hide caption

itoggle caption Bill Ingalls/NASA via AP

An image of Pluto that was taken by New Horizons shortly before its flyby Tuesday shows a heart-shaped pattern on the planet's surface. NASA says Pluto "sent a love note back to Earth." NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute hide caption

itoggle caption NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute

A portrait of Pluto (right) and its moon Charon in a colorized, composite image made July 11 during the final approach of the New Horizons spacecraft. Astronomers are eager to get a much closer view of the icy world Tuesday. NASA-JHUAPL-SWRI hide caption

itoggle caption NASA-JHUAPL-SWRI

NASA's New Horizons mission will be the first ever to visit Pluto and its moons. This artist's conception shows the probe as it passes the dwarf planet. JHUAPL/SwRI hide caption

itoggle caption JHUAPL/SwRI
NASA/ESA/M. Buie (Southwest Research Institute)

An artist's concept shows the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015. Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute hide caption

itoggle caption Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Artist concept of New Horizons spacecraft. The Hubble Space Telescope is being pressed into service to help scientists look for a post-Pluto target for the space probe. Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute hide caption

itoggle caption Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute