Smith’s Cloud is the approximate size of an elongated dwarf galaxy—about 11,000 light-years in length, about 2,300 light-years in width—and is about 8,000 light-years from the Perseus arm of the Milky Way galaxy. The Persius arm, the expected impact point for the Cloud, is about one-fourth of the way around the Milky Way from Earth's position in the galaxy.

Smith’s Cloud was discovered in 1963 by Gail Smith (now Gail Smith Bieger), at the time an astronomy student at Swarthmore College, Swarthmore, Pennsylvania. Bieger now lives in The Hague, in the Netherlands.

It is conjectured that Smith’s Cloud was left over from the birth of the Milky Way galaxy, possibly traveling the same way that comets and meteors are still roaming around our galaxy. However, until this discovery, nothing was much known about its specific motion in relation to Earth and the Milky Way galaxy.

U.S. astronomer Felix Jay Lockman, associated with the National Radio Astronomy Observatory in Green Bank, West Virginia, and fellow colleagues used the NRAO Robert C. Byrd Green Bank Telescope to make their observations and calculations of the impending collision.

Smith’s Cloud is made up of almost entirely of cold hydrogen gas that emits radiation only in the radio wavelengths. As far as it is known, there are no stars within the cloud.

When the collision occurs in 20 to 40 million years, it will likely produce many new stars because of the large amount of hydrogen gas contained within it. In fact, it is estimated that the cloud contains about one million solar (Sun) masses.

The Lockman team made their announcement at a Friday, January 11, 2008 meeting of the American Astronomical Society in Austin, Texas, United States.

Related stories on Smith’s Cloud is found at: Astronomy magazine (“Cosmic cloud races toward Milky Way”) and MSNBC (“Space cloud to collide with our galaxy”).

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