1 May 2015

Crash landing on Mercury's surface

10:34 am on 1 May 2015

NASA's Messenger has reached an explosive end after 10 years in space and four in orbit around Mercury.

Images from NASA's Messenger reveal Mercury - not as it would appear to the human eye, but with colours used to show chemical and physical differences. Young craters appear fresh blue or white; tan areas are plains formed by liquid lava.

Images from NASA's Messenger reveal Mercury - not as it would appear to the human eye, but with colours used to show chemical and physical differences. Young craters appear fresh blue or white; tan areas are plains formed by liquid lava. Photo: HO / NASA / AFP

Now fully out of fuel, the spacecraft smashed into a region near Mercury's north pole, out of sight from Earth, at about 8am (NZST).

Mission scientists confirmed the impact minutes later when the craft's next possible communication pass was silent.

Messenger reached Mercury in 2011 and far exceeded its primary mission plan of one year in orbit.

That mission ended with an inevitable collision: Messenger slammed into this solar system's hottest planet at 14,000km/h - 12 times quicker than the speed of sound.

The impact, which will have completely obliterated this history-making craft, only happened because Mercury has no thick atmosphere to burn up incoming objects - the same reason its surface is so pock-marked by impact craters.

According to calculations, the 513kg, three-metre craft will have blasted a brand new crater the size of a tennis court, which is still far too small to be visible from Earth.

Messenger's fuel supply, half its weight at launch, was completely spent weeks ago but four final manoeuvres were conducted, to extend the flight as long as possible. These were accomplished by venting the helium gas normally used to pressurise actual rocket fuel into the thrusters.

The last of those manoeuvres took place on 28 April.

During its twice-extended mission, Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) transformed the world's understanding of Mercury. It sent back more than 270,000 images and 10 terabytes of scientific measurements.

It found evidence for water ice hiding in the planet's shadowy polar craters, and discovered that Mercury's magnetic field is bizarrely off-centre, shifted along the planet's axis by 10 percent of its diameter.

Skimming the surface

For four years - and 4104 circuits in total - Messenger traced a highly elliptical orbit around Mercury. It regularly drifted out to a distance of nearly twice the planet's diameter, before swinging to within 96km at closest approach.

To maintain this pattern in the face of interference from the sun, it needed a blast of engine power every few months.

The floor of Mercury's Abedin Crater is covered with once-molten rock, melted by the impact that formed it.

The floor of Mercury's Abedin Crater is covered with once-molten rock, melted by the impact that formed it. Photo: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

An image released in April 2015 shows Mercury's gravity anomalies in colors.

An image released in April 2015 shows Mercury's gravity anomalies in colors. Photo: NASA / Goddard Space Flight Centre Science Visualisation Studio / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Messenger made its penultimate pass at a distance of between 300 and 600 metres - one or two times the height of the Eiffel Tower - at about midnight (NZST).

"If you could see that, it would be a real spectacle," said Jim Raines, the instrument scientist on the craft's FIPS instrument (Fast Imaging Plasma Spectrometer) and a physicist at the University of Michigan. "It would cross the horizon in just a second or two, flying low overhead at ten times the speed of a supersonic fighter."

The next time it swung back close to Mercury's surface, eight hours later, it hit - with the impact preceisely modelled precisely using maps produced by Mercury's own data.

As the impact happened on the side of the planet facing away from Earth, the craft was out of contact in its final moments and probably carried more than 1000 unseen images to its final, explosive resting place.

Mercury's twin cameras (Mercury Dual Imaging System - MDIS) used to take hundreds of photos every day. Earlier this month, mission scientists released fresh images (including the one displayed below) which superimposed years of spectrometry data about the chemistry of the planet's surface, illustrated by different colours, onto black-and-white images built up from thousands of smaller MDIS photos.

-BBC

Messenger acquired over 250,000 images during its mission - including this, released in April, which measures ultraviolet through infrared light.

Messenger acquired over 250,000 images during its mission - including this, released in April, which measures ultraviolet through infrared light. Photo: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

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