What’s the difference between a comet and an asteroid? The distinction is hazy at best. Thanks to the recent discovery of ice on an asteroid in the main belt, the lines have been further blurred.

Comets are typically thought to be small masses of rock and ice. They originate in the outer Solar System and are characterized by vapor tails that stream out as the comet ventures close to the Sun. Most asteroids, on the other hand, are relatively large and waterless rocks that typically orbit in the inner Solar System. They have no observable tails. Astronomers admit that these are loose definitions, and in recent years scientists have uncovered exceptions to both.

In 2006, astronomers observed tails of vapor and dust coming from a few small objects in the main asteroid belt. Never before had they seen this type of comet-like behavior from objects that were thought to have formed so near to the Sun. In April 2010, two independent research teams reported in the scientific journal Nature that the asteroid 24 Themis is completely covered by a thin layer of ice.

The discovery of ice on 24 Themis does more than muddle the lines between comet and asteroid. The astronomers responsible for the discovery say that it’s now conceivable that an icy asteroid similar to 24 Themis may be responsible for delivering water to our planet. The researchers don’t yet know how water ice found its way to 24 Themis, but they believe it has been there for at least four billion years. At that time, asteroids were regularly bombarding the young Earth—and perhaps, making a lasting impression that characterizes our blue planet today.

For other recent discoveries in astronomy, check out the Science Bulletins website.

On April 21, 2010, NASA released the first collection of images taken by its newest solar spacecraft, the Solar Dynamics Observatory (SDO). Launched on February 11, SDO has already captured two significant events—a prominence and a solar flare—in unprecedented detail at extreme ultraviolet wavelengths. See these solar events in motion in AMNH's latest Astro Bulletin.

Even though you’ve been told never to gaze directly at the Sun, the SDO satellite allows researchers to do just that, at a resolution and with coverage never before achieved. With an array of four telescopes called the Atmospheric Imaging Assembly (AIA), which observe at different wavelengths, SDO constantly monitors the Sun’s corona by taking one image every 10 seconds. Astronomers will use these images, along with data from other instruments onboard the satellite, to better understand the dynamics of solar activity and apply that knowledge to other star systems across the Universe.

During solar flares, coronal mass ejections, and other eruptive events, the Sun ejects charged particles into the Solar System. As these particles near Earth, they can interact with Earth’s magnetic field and trigger space weather. Typically, space weather manifests itself as colorful auroras near the poles, but it can also disrupt radio communications and global positioning systems (GPS) on Earth. By using SDO to examine how eruptive events evolve, astronomers should be better able to predict space weather and help people prepare.

SDO’s work has just begun. Preliminary image collection began on March 30, and the mission is expected to run until 2015. SDO’s first crop of high-resolution images already gives an indication of the beautiful and valuable data still to come.

To learn more about other recent astronomy news, take a look at the Science Bulletins website.

Planck and Herschel, a pair of ESA satellites, have begun returning vibrant new images of the Milky Way Galaxy. The satellite duo is scanning the sky at infrared wavelengths that have not been well-studied in the past. Working in tandem, they will help astronomers explore the forces that have shaped our galaxy and the Universe from its early moments. Planck sweeps the entire sky to collect information on the coldest—and oldest—areas of the Universe. When astronomers have identified areas of particular interest using Planck, they use Herschel to hone in to get a detailed look at slightly different wavelengths.

The ESA recently released some of Planck's images that focus on our own galaxy. The above example, which was compiled with data from Planck and another infrared-detecting satellite, IRAS, captures tendrils of cold dust in a section of the Milky Way. Dark patches represent cooler clumps of matter. The brighter areas are much warmer mixtures of gas and dust, which is where stars are likely to form.

Both Planck and Herschel will continue to study the sky's coldest regions in the coming years, and together they will give astronomers an idea of how the Galaxy as we know it was formed.

To see what else astronomers are learning about our Universe, check out the Science Bulletins website.