First as a planet and then as a dwarf planet, Pluto has been a curiosity since American astronomer Clyde Tombaugh discovered it in 1930. Its small size, just 2,300 kilometers wide, and great distance from Earth make it a difficult target for study; when NASA's New Horizons probe approaches Pluto in 2015 scientists will get their first close look at the icy body.

Until that time far-off glimpses from New Horizons, which launched in 2006, and fuzzy images from the Hubble Space Telescope will have to suffice; a new batch from Hubble ranks among the best looks at Pluto to date. The images were generated by synthesizing multiple photos taken in 2002 and 2003 to draw out features of Pluto, which fills only a few pixels on Hubble's Advanced Camera for Surveys.

The new images reveal substantial differences from how Pluto looked when Hubble examined it in 1994. Pluto has gotten redder, and the northern hemisphere has brightened as the south has darkened. These are likely seasonal changes, which proceed relatively slowly due to Pluto's huge, time-consuming orbit around the sun—only one third of a Pluto year has elapsed since Tombaugh's discovery in 1930. (A Pluto year lasts nearly 248 Earth years.)

Pluto's distance from the sun varies from roughly 30 to 50 times the distance Earth keeps from the sun. As such, it receives greatly varying intensities of solar radiation in its different seasons. (The northern hemisphere is heading toward summer solstice in 2029.) The shifting brightness between hemispheres may reflect melting in one location and refreezing elsewhere, a process that could resurface Pluto on a global scale in the course of a single trip around the sun.