
Explanation: What created this great cliff on Mars? Did giant waterfalls once plummet through its grooves? With a four-kilometer drop, this high cliff surrounding Echus Chasma, near an impressive impact crater, was carved by either water or lava. A leading hypothesis is that Echus Chasma, at 100-kilometers long and 10-kilometers wide, was once one of the largest water sources on Mars. If true, water once held in Echus Chasma likely ran over the Martian surface to carve the impressive Kasei Valles, which extends over 3,000 kilometers to the north. Even if initially carved by water, lava appears to have later flowed in the valley, leaving an extraordinarily smooth floor. Echus Chasma lies north of tremendous Valles Marineris, the largest canyon in the Solar System. The above image was taken by the robotic Mars Express spacecraft currently orbiting Mars.

Explanation: What would it be like to explore the Moon? NASA's Apollo missions gave humans just this chance in the late 1960s and early 1970s. In particular, the Apollo 15 mission was dedicated to better understanding the surface of the Moon by exploring mountains, valleys, maria, and highlands. Astronauts David Scott and James Irwin spent nearly three days on the Moon while Alfred Worden orbited above in the Command Module. The mission, which blasted off from Earth on 1971 July 26, was the first to deploy a Lunar Roving Vehicle. Pictured above in this digitally stitched mosaic panorama, David Scott examines a boulder in front of the summit of Mt. Hadley Delta. The shadow of James Irwin is visible to the right, while scrolling to the right will reveal a well-lit and diverse lunar terrain. The Apollo 15 mission returned about 76 kilograms of moon rocks for detailed study. In the future, NASA and other space agencies plan to continue to lead humanity's exploration of the Moon, Mars, and beyond.







Explanation: Where's the astronaut? Somewhere in this impressive array of International Space Station (ISS) hardware, astronaut Steve Bowen can be found upgrading and cleaning key parts of Earth's most prominent orbital outpost. Astronaut Bowen and Heidemarie Stefanyshyn-Piper (not pictured), part of the Space Shuttle Endeavour's recently ended STS-126 mission to the ISS, spent nearly three hours on this spacewalk hovering high above planet Earth. Bowen progressed toward achieving a key goal of the mission -- servicing of the Solar Alpha Rotary Joints to better allow some solar arrays to track the Sun. In the lower foreground of the above image is the cylindrical Columbus Laboratory, protruding from the right is an impressively large space station truss, while in the background are some of the expansive solar arrays that collect sunlight to power the ISS. Far in the distance, a blue arc of Earth's thin atmosphere is visible on the horizon. The next space shuttle flight is scheduled for 2009 February, when Discovery will deliver elements to further expand the ISS.