Noctilucent clouds form between 76 to 85 kilometers (47 to 53 miles) above the Earth’s surface, near the boundary of the mesosphere and thermosphere.

This 10.5-billion-year-old globular cluster, NGC 6496, is home to heavy-metal stars of a celestial kind! The stars comprising this spectacular spherical cluster are enriched with much higher proportions of metals — elements heavier than hydrogen and helium are curiously known as metals in astronomy — than stars found in similar clusters.

NASA’s New Horizons spacecraft took this stunning image of Pluto only a few minutes after closest approach on July 14, 2015. The image was obtained at a high phase angle –that is, with the sun on the other side of Pluto, as viewed by New Horizons. Seen here, sunlight filters through and illuminates Pluto’s complex atmospheric haze layers.

NASA's OSIRIS-REx spacecraft is revealed after its protective cover is removed inside the Payload Hazardous Servicing Facility at Kennedy Space Center, on May 21, 2016. OSIRIS-REx, targeted for a Sept. 8 launch, will be the first U.S. mission to sample an asteroid, retrieve at least two ounces of surface material and return it to Earth for study.

Flames from the solid rocket boosters lit up the clouds of smoke and steam trailing behind shuttle Atlantis on May 19, 2000, as it lifted off on mission STS-101. It was the shuttle program's third space station assembly flight, and first space flight for astronaut Jeff Williams, currently aboard the station as a member of the Expedition 47 crew.

Astronauts aboard the International Space Station (ISS) see the world at night on every orbit — that’s 16 times each crew day. An astronaut took this broad, short-lens photograph of Earth’s night lights while looking out over the remote reaches of the central equatorial Pacific Ocean.

Together with irregular galaxies, spiral galaxies make up approximately 60 percent of the galaxies in the local universe. However, despite their prevalence, each spiral galaxy is unique — like snowflakes, no two are alike. This is demonstrated by the striking face-on spiral galaxy NGC 6814.

ESA astronaut Tim Peake photographed the SpaceX Dragon cargo craft as it undocked from the International Space Station on May 11, 2016. Scheduled to splashdown in the Pacific Ocean at 2:55 p.m. EDT, the spacecraft will return the final batch of human research samples from former NASA astronaut Scott Kelly’s historic one-year mission.

The water-world Enceladus appears here to sit atop Saturn's rings like a drop of dew upon a leaf. Even though it appears like a tiny drop before the might of the giant Saturn, Enceladus reminds us that even small worlds hold mysteries and wonders to be explored.

A prototype 13-kilowatt Hall thruster is tested at NASA's Glenn Research Center in Cleveland. This prototype demonstrated the technology readiness needed for industry to continue the development of high-power solar electric propulsion into a flight-qualified system.

A sinuous feature snakes northward from Enceladus' south pole like a giant tentacle.

An Air Force Test Pilot School T-38C passes in front of the sun at a supersonic speed, creating shockwaves that are caught photographically for research. NASA is using a modern version of schlieren imagery to visualize supersonic flow phenomena with full-scale aircraft in flight. The results will help engineers design a quiet supersonic transport.

On March 16, 1966, command pilot Neil Armstrong and pilot David Scott successfully docked their Gemini VIII spacecraft with the Agena target vehicle, the first-ever linking of two spacecraft together in Earth orbit. This crucial spaceflight technology milestone would prove vital to the success of future moon landing missions.

The distinctive blue bubble appearing to encircle WR 31a is a Wolf–Rayet nebula — an interstellar cloud of dust, hydrogen, helium and other gases. Created when speedy stellar winds interact with the outer layers of hydrogen ejected by Wolf–Rayet stars, these nebulae are frequently ring-shaped or spherical.

NASA's Lunar Reconnaissance Orbiter captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon.