January 21, 2015

The photos are fuzzy, but astronomers eagerly studying the first images of the dwarf planet Ceres taken by NASA’s Dawn spacecraft think they may be seeing craters. Astronomers wouldn’t be surprised to learn that Ceres, like many planets and moons in the solar system, has been whacked by space rocks during its billion-year history. Still, the images are the clearest yet of Ceres–three times better than the previous best, which were taken by the Hubble Space Telescope. The new images were taken when Dawn was about 238,000 miles (383,000 kilometers) away from Ceres, about the distance between Earth and the moon. Over the next six weeks, as Dawn speeds closer and closer, astronomers will be able to determine if the surface of Ceres really is dotted by craters. And if all goes well, that finding will be only the first of many discoveries about this largely mysterious object. Astronomers expect to learn much more, especially after Dawn enters orbit around Ceres on March 6. Launched in 2007, Dawn was designed to study both Ceres and Vesta, the third-largest asteroid in the solar system.

The dwarf planet Ceres appears in both visible and infrared light in some of the first images of Ceres taken by the Dawn spacecraft. The craft was 238,000 miles (383,000 kilometers) away at the time. In the infrared image, which is really a temperature map of Ceres, white is warmer and red is colder. (NASA/JPL-Caltech/UCLA/ASI/INAF)

Ceres, which is shaped like a slightly squashed sphere, is about 590 miles (950 kilometers) in diameter. It is so large that it accounts for more than one-fourth of the total mass (amount of matter) of all of the millions of asteroids in the Main Belt, a region between the orbits of Mars and Jupiter. In fact, Ceres has enough mass to be considered a dwarf planet.

On entering obit around Ceres, Dawn will become the first spacecraft to circle two deep-space objects. The craft reached Vesta for a one-year study in 2011. While on this mission, Dawn revealed that Vesta has three layers–an inner core, middle layer called the mantle, and an outer crust. Scientists think both Ceres and Vesta probably formed early in the solar system’s history from many smaller bodies that collided and stuck together, a process called accretion. However, the gravitational pull of Jupiter likely prevented the two from combining with other objects and growing to the size of a planet.

Additional World Book articles:

• Space exploration
• Exploring the Suburban Solar System (a special report)

May 14, 2012

Vesta, the third most massive object in the asteroid belt, is not an asteroid at all but a unique baby planet that has survived in the rough-and-tumble environment of the Main Belt since the formation of the solar system more than 4 billion years ago. That finding is one of several surprising discoveries about Vesta announced by scientists studying data collected by NASA’s Dawn space probe. According to the scientists, Vesta is the solar system’s only surviving planetesimal. Also known as protoplanets, planetesimals were asteroid-sized objects that collided and stuck together to form the planets, moons, and other solid or mostly solid objects of the solar system.

Vesta’s true identity was revealed after scientists discovered that Vesta has a unique internal structure. Much like the common rocks and minerals found on Earth, asteroids are usually made of the same materials throughout. However,  measurements made by Dawn indicate that Vesta has three layers–a core, a mantle, and a crust. In this way, Vesta is more like the inner planets of the solar system than the asteroids. The inner planets, also called the terrestrial planets, are Mercury, Venus, Earth, and Mars. All of them, along with Earth’s moon, have a mostly iron metal core. They also have at least two more layers (Mercury may have three) called the mantle and the crust that consist of rocks and minerals. These layers were formed very early in the history of the solar system. Vesta has a metal core along with a rocky mantle and crust, indicating it, too, was formed in the early solar system.

Vesta has an irregularly shaped surface, as shown in this color-coded map of surface elevation. Red indicates higher regions; blue represents lower areas. An enormous crater lies near Vesta’s south pole. It measures about 285 miles (460 kilometers) in diameter and features a central peak.  (NASA/Ben Zellner, Georgia Southern University/Peter Thomas, Cornell University)

The three layers also show that Vesta was on its way to becoming a planet when it stopped near its present size. Many scientists believe that the gravitational pull of Jupiter, the largest planet, probably prevented Vesta from combining with other planetesimals to form a full-sized planet. Common theories state that protoplanets in the solar system either combined to become full-sized planets or were destroyed in a collision with another object. Vesta’s amazing survival is a stroke of luck for scientists studying the formation of the planets and the early history of the solar system.

Launched in September 2007, Dawn is currently nearing the end of the first part of a two-part mission. The probe arrived at Vesta in July 2010 and will observe the asteroid for one year. Dawn is scheduled to leave Vesta and begin the journey to Ceres in July. It should arrive at the largest asteroid in 2015.