Week's Best Space Photos: Cosmic "Dragon," Alaska Auroras

The SpaceX Dragon capsule successfully linked up with the International Space Station, as pictured on April 20.

Launched from Cape Canaveral, Florida, on April 18, the capsule, photographed by an astronaut aboard the space station, is loaded with 5,000 pounds (2,200 kilograms) of supplies. (See "Future of Spaceflight.")

Those include boring things like food, but there's also a laser, an experimental space garden, and a set of legs for Robonaut 2, a humanoid robot that lives on the space station.

The capsule will remain linked with the space station until May 18, at which point it will fall to Earth, splashing down in the Pacific Ocean off southern California.

Stars shuffle in arcs above an abandoned Soviet building in a long-exposure picture taken in Tallinn, Estonia, on April 21.

Submitted to National Geographic's Your Shot photo community, the picture shows so-called star trails, which are the result of a long exposure and Earth's rotation. (See another picture of star trails.)

Four galaxies shimmer in a photograph released April 23 by NASA's Chandra X-ray Observatory, the result of a new collaboration between amateur photographers and NASA.

Clockwise from the upper left, the galaxies are M101 (the "Pinwheel," in the constellation Ursa Major), M81 (also in Ursa Major), Centaurus A (home to a massive jet erupting from its central black hole), and M51 (the "Whirlpool," in Canes Venatici).

Data from Chandra (pink-purple) and the infrared Spitzer Space Telescope (red) have been overlaid onto optical images from two astrophotographers.

Engineers get ready for a test flight of NASA's Morpheus lander at the Kennedy Space Center in Florida on April 24.

Designed for vertical takeoff and landing on the moon or asteroids, the lander runs on green propellants, oxygen and methane.

During the April 24 test flight, this prototype soared to a height of 800 feet (240 meters) before simulating a hazard-avoidance maneuver and autonomously touching down on a dedicated pad at the north end of the shuttle landing facility. The flight lasted 98 seconds. (See "Rare Pictures From the Dawn of NASA Spaceflight.")

Northern lights blaze in the skies over Alaska on April 18 in a photo submitted to Your Shot. (See National Geographic's aurora viewer's guide.)

Auroras are caused by charged particles smashing into Earth's atmosphere; because they're charged, these particles travel along Earth's magnetic field lines, ending up at the North and South Poles. There, the particles collide with atmospheric atoms, including oxygen, which produces the green lights.

SpaceX's Falcon 9 rocket pulls away from the launch pad in Cape Canaveral, Florida, on April 18.

The rocket delivered the Dragon supply capsule to Earth's orbit. Dragon later docked with the International Space Station. (See pictures of the space station's observation deck.)

Moonlight illuminates a gnarled, ancient bristlecone pine in Aurora, Colorado, on a picture submitted to Your Shot on April 22.

"My goal this night was to juxtapose the twisted limbs that were gradually sculpted by winds over countless centuries against ephemeral clouds that were shaped by the same force in mere seconds," photographer Joseph Thomas said with his submission.