Composite image of the Smith Cloud, as it might look from Earth if we could see it. The cloud itself is in false color, radio data from the Green Bank Telescope. The background image shows the cloud's actual location, with the Milky Way stretching from top to bottom right. (credit: Saxton/Lockman/NRAO/AUI/NSF/Mellinger)

Seventy million years ago, some unknown force blasted a tremendous amount of gas out of our galaxy. Known as the Smith Cloud, that gas is now arcing back toward the Milky Way, pulled in by its gravity. Thirty million years from now, it will return to our galaxy once more.

The cloud and its trajectory were already well-known, but the new study confirms its origin was inside the Milky Way.

The Smith Cloud was originally an amorphous blob of gas, but the forces it has been subjected to have shaped it into a comet-like form. It’s a whopping 11,000 light-years long and 2,500 across. At that size and its current distance, if it could be seen in visible light, it would appear 30 times bigger than the Moon in the night sky.

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