10 Things: September 25

NASA's Dawn mission to the asteroid belt is no ordinary deep space expedition. Instead of traditional chemical rockets, the Dawn spacecraft uses sophisticated ion engines for propulsion. This enabled Dawn to become the first mission to orbit not one, but two different worlds — first the giant asteroid Vesta and now the dwarf planet Ceres. Vesta and Ceres formed early in the solar system's history, and by studying them, the mission is helping scientists go back in time to the dawn of the planets. To mark a decade since Dawn was launched on Sept. 27, 2007, here are 10 things to know about this trailblazing mission.

1. Ion engines: not just for sci-fi anymore. Most rocket engines use chemical reactions for propulsion, which tend to be powerful but short-lived. Dawn's futuristic, hyper-efficient ion propulsion system works by using electricity to accelerate ions (charged particles) from xenon fuel to a speed seven to 10 times that of chemical engines. Ion engines accelerate the spacecraft slowly, but they're very thrifty with fuel, using just milligrams of xenon per second (about 10 ounces over 24 hours) at maximum thrust. Without its ion engines, Dawn could not have carried enough fuel to go into orbit around two different solar system bodies. Try your hand at an interactive ion engine simulation.

2. Time capsules. Scientists have long wanted to study Vesta and Ceres up close. Vesta is a large, complex and intriguing asteroid. Ceres is the largest object in the entire asteroid belt, and was once considered a planet in its own right after it was discovered in 1801. Vesta and Ceres have significant differences, but both are thought to have formed very early in the history of the solar system, harboring clues about how planets are constructed. Learn more about Ceres and Vesta—including why we have pieces of Vesta here on Earth.

3. Portrait of a dwarf planet. This view of Ceres built from Dawn photos is centered on Occator Crater, home of the famous "bright spots." The image resolution is about 460 feet (140 meters) per pixel. Take a closer look.

4. What's in a name? Craters on Ceres are named for agricultural deities from all over the world, and other features carry the names of agricultural festivals. Ceres itself was named after the Roman goddess of grain and harvests (that's also where the word "cereal" comes from). The International Astronomical Union recently approved 25 new Ceres feature names tied to the theme of agricultural deities. Jumi, for example, is the Latvian god of fertility of the field. Study the full-size map.

5. Landslides or ice slides? Thanks to Dawn, evidence is mounting that Ceres hides a significant amount of water ice. A recent study adds to this picture, showing how ice may have shaped the variety of landslides seen on Ceres today.

6. The lonely mountain. Ahuna Mons, a 3-mile-high (5-kilometer-high) mountain, puzzled Dawn's Ceres explorers when they first found it. It rises all alone above the surrounding plains. Now scientists on the Dawn team think it is likely a cryovolcano — one that erupts a liquid made of volatiles such as water, instead of rock. "This is the only known example of a cryovolcano that potentially formed from a salty mud mix, and that formed in the geologically recent past," one researcher said. Learn more.

7. Shining a light on the bright spots. The brightest area on Ceres, located in the mysterious Occator Crater, has the highest concentration of carbonate minerals ever seen outside Earth, according to studies from Dawn scientists. Occator is 57 miles (92 kilometers) wide, with a central pit about 6 miles (10 kilometers) wide. The dominant mineral of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. This material appears to have come from inside Ceres, and this upwelling suggests that temperatures inside Ceres are, or were, warmer than previously believed. Even more intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geological time. The salts could be remnants of an ocean, or localized bodies of water, that reached the surface and then froze millions of years ago. See more details.

8. Captain's log. Dawn's chief engineer and mission director, Marc Rayman, provides regular dispatches about Dawn's work in the asteroid belt. Catch the latest updates here.

9. Eyes on Dawn. Another cool way to retrace Dawn's decade-long flight is to download NASA's free Eyes on the Solar System app, which uses real data to let you go to any point in the solar system, or ride along with any spacecraft, at any point in time—all in 3-D.

10. No stamp required. Send a postcard from one of these three sets of images that tell the story of dwarf planet Ceres, protoplanet Vesta, and the Dawn mission overall.