NASA's Messenger spacecraft, in orbit around Mercury the past four years, will come to an abrupt end on Thursday.
Messenger's path will intersect with the surface of the planet. The impact of the 1,100-pound spacecraft at 8,750 miles per hour is expected to gouge a crater some 50 feet wide.
That will bring to a close a mission that has painted an unexpected portrait of Mercury, once thought a boring round rock not much different from Earth's moon. Mercury, the smallest planet in our solar system, is only slightly larger than the moon, although it has more drastic temperature swings - 800 degrees Fahrenheit during the day, minus 300 degrees at night.
"It's really been exciting to see a planet unfold, a planet that is one is our neighbors," said Sean C. Solomon, the principal investigator for the mission. "Almost every aspect of Mercury has had its share of surprises."