Travelling at 3.91 kilometres per second, the MESSENGER spacecraft will collide with Mercury's surface, creating a crater estimated to be 16 meters (52 feet) in diameter.
A spacecraft that transformed scientists' understanding of Mercury, the solar system's innermost planet, ended its mission Thursday by crashing into the world it revealed so vividly.
NASA's Messenger mission was launched in August, 2004. Equipped with a special thermal shield to protect it from the sun's intense radiation, the probe made three close passes before settling into orbit around the planet in March of 2011.
Messenger has provided reams of data on the parched and desolate world, revealing that Mercury is a far more complex and dynamic planet than researchers once suspected.
"It's been an amazing journey," said Catherine Johnson, a planetary scientist at the University of British Columbia and a participant in the mission. "It will be strange not to be going on the Web every day to download the latest data."
Over the course of more than 4,000 orbits of Mercury, Messenger radioed back more than 250,000 images, revealing details about the planet's volcanic past and using indirect means to probe its massive core, which is nearly as large as Earth's even though Mercury is only one-third Earth's total diameter.
The mission was time-limited by the probe's finite supply of fuel, used for making adjustments to its orbit. Over the past year, scientists opted to use the mission's final reserves by having Messenger circle closer and closer to Mercury's surface. This improved measurements of the planet's gravitational and magnetic fields, which will help scientists to elucidate the planet's inner workings, include a molten outer core.
The spacecraft hit the surface in the midafternoon, Eastern Time, on Thursday. It marks one of the last missions to see an unexplored planetary surface in the solar system up close for the first time. Along with the New Horizons mission, which flies by Pluto in July, it brings to a close the first chapter of humanity's exploration of other worlds.
Named after the mythological messenger of the gods, Mercury is by far the most elusive of the planets that can be seen with the unaided eye. Because it hugs the sun so closely, it can be seen from Earth only at twilight and near the horizon. The invention of the telescope did little to improve the view, since Mercury is too small to see clearly.
That changed with the coming of spaceflight. A few fleeting glimpses of the planet by NASA's Mariner 10 spacecraft in 1974 and 1975 revealed a heavily cratered surface, superficially resembling the moon.
But Mariner 10 saw only about 45 per cent of the planet and left many questions unanswered. As space exploration turned to focus on Mars and other destinations, Mercury had to wait for another generation to pass before scientists were given a second and far more penetrating look.
Earlier this month, mission scientists assembled a top-10 list of the probe's key discoveries. Topping the list was the discovery that Mercury is much richer in light, volatile elements such as potassium and sodium than scientists expected, altering theories of planet formation.
"The ideas for how the inner planets got assembled … are all being changed by Messenger's results," said Sean Solomon, the principal investigator for the mission at a recent news briefing.
Messenger also confirmed the presence of ice hidden in permanently shadowed craters near Mercury's pole, even though the surface temperature on the day-lit side of the planet is about 430 degrees C.
That withering heat gives the mission a broader purpose. Mercury is the closest analogy astronomers have in our solar system for some of the many hundreds of planets that have in recent years been discovered circling even closer to the stars they orbit.
"We at least need to understand what's in our backyard to try to understand these other places too," Dr. Johnson said.
Ivan Semeniuk