Messenger did its job. Now its mission is over.
Messenger is the only spacecraft ever to orbit Mercury. Its four-year tour ended with a crash landing. Mercury is a planet in our solar system.
Messenger plunged from orbit as planned. It slammed into the sun's closest planet at about 8,750 mph. The impact created a crater an estimated 52 feet across.
Messenger became the first spacecraft to orbit hot, little Mercury. That was in 2011. It circled the solar system's innermost planet 4,105 times. It collected more than 277,000 images.
"Today we bid a fond farewell to one of the most resilient and accomplished spacecraft ever to have explored our neighboring planets," lead scientist Sean Solomon said April 30. Solomon is director of Columbia University's Lamont-Doherty Earth Observatory.
Solomon noted in a statement that Messenger set a record for planetary flybys. It went once past Earth, twice past Venus and three times past Mercury. Then it entered Mercury's orbit. The spacecraft survived "both punishing heat and extreme doses of radiation." Scientists said it surpassed expectations.
Flight controllers managed to keep the spacecraft going a few extra weeks. To do so, the scientists used helium gas. The gas was not originally intended as fuel. But the gas tank finally emptied and gravity's relentless tug did Messenger in.
Mercury is the last of the rocky inner planets in our solar system to be littered by mankind. The other rocky inner planets are Mars and Venus.
Messenger's crash occurred on the side of Mercury facing away from Earth and telescopes. Several minutes passed before confirmation was received. Controllers received no signal from Messenger when it was supposed to be back in the coverage zone. It was a sign that the spacecraft succumbed to gravity.
How large was Messenger? It measured 10 feet solar wingtip to wingtip.
"Well I guess it is time to say goodbye," the Messenger Twitter feed stated as the end drew near.
Then after the impact: "On behalf of Messenger, thank you all for your support. We will continue to update you on our great discoveries. We will miss it."
Astronomers called it an end of an era. Messenger detected Mercury's frozen water-covered poles and significantly off-center magnetic field. Some of its other discoveries were volcanic deposits. They are evidence of the planet's eruptive past.
"It has been an amazing journey of discovery," said the University of British Columbia's Catherine Johnson. She is a senior scientist at the Planetary Science Institute. Data analyses will continue for at least another year.
Messenger's $427 million mission began with a launch from Cape Canaveral, Florida. It took off in 2004. Johns Hopkins University handled everything for NASA.
What is NASA? It has been responsible for the civilian space program in the United States.
Until Messenger, the only spacecraft to visit Mercury was NASA's Mariner 10. That was back in the 1970s. Mariner was only a fly-by mission.
The Europeans and Japanese are teaming up to create Mercury's next guests. The countries are building a pair of satellites. They will be known as BepiColombo. Their launch is scheduled in 2017. They would arrive in Mercury's orbit in 2024.
Critical thinking challenge: How could Messengers mission be considered a success if it ended in a crash?