After enduring the scorching temperatures needed to explore our Solar System's innermost planet, NASA's MESSENGER spacecraft will finally succumb to old age and end its mission with a bang.

MESSENGER - or the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft - is now out of fuel and under the spell of Mercury's gravity.

Mission engineers from NASA predict that the probe will impact the planet's surface on April 30, travelling at a speed of around 3.9 kilometres per second (or about 14,040 km/h).

The consensus from scientists and engineers is that the mission was a success. 

"MESSENGER had to survive heating from the Sun, heating from the dayside of Mercury, and the harsh radiation environment in the inner heliosphere," said principal investigator, Sean Solomon, from Columbia University, in a press release. 

"The clearest demonstration that our innovative engineers were up to the task has been the spacecraft's longevity in one of the toughest neighbourhoods in our Solar System." 

The mission was approved in 1999, and the spacecraft was launched on August 3, 2004. After 6.5 years, it entered into orbit around Mercury on March 18, 2011.

The original plan was to orbit the planet for one Earth year, collecting data to answer six critical questions about the rocky planet nearest to our Sun. Scientists were interested in why the planet was so dense; what was the structure of its core, whether it had a magnetic field; and what the material was at its poles.

But new questions raised by early findings motivated two extensions of orbital operations, pushing the total time spent circling Mercury to roughly four years.

Solomon praised the mission engineers and the design, saying, "all of the instruments that we selected nearly two decades ago have proven their worth and have yielded an amazing series of discoveries about the innermost planet."

One of the most intriguing outcomes of the mission has been the evidence it has provided to support the hypothesis that Mercury harbours abundant water ice and other frozen volatile materials in its permanently shadowed polar craters.

Solomon says the water stores in these ice deposits was likely delivered via the impacts of comets and volatile-rich asteroids.

"Those same impacts also likely delivered the material in the dark layer discovered by MESSENGER to cover most polar deposits and interpreted, on the basis of its sublimation temperature and low reflectance, to be carbonaceous," he said. "By this interpretation, Mercury's polar regions serve as a witness plate to the delivery to the inner Solar System of water and organic compounds from the outer Solar System, a process that much earlier may have led to prebiotic chemical synthesis and the origin of life on Earth. 

The mission also yielded some incredible technological innovations, including a way to extend the mission by preserving on-board propellants, and a heat-resistant and highly reflective ceramic cloth sunshade, which protected the spacecraft's instruments and electronics from extreme solar radiation.

The latter was particularly important, given the front side of the sunshade routinely experiences temperatures in excess of 300 degrees Celsius, explained MESSENGER project manager Helene Winters from Johns Hopkins University in the US. 

It's actually quite amazing and humbling to sift through all the highlights of a completed mission, and to see just how much coordinated effort, over how many years, goes into pulling this stuff off. 

NASA, and MESSENGER, we salute you!