Don't shoot the Messenger: NASA's suicide probe to punch hole in Mercury

The little probe that could

Mercury is the closest planet to the Sun, and also the fastest orbiting, and presented some unique challenges for NASA engineers looking to study it.

When Messenger launched on August 3, 2004, more than half of the 1,100kg probe's weight was fuel. The probe had to burn long and hard, and use six gravity assists from other planets, to get going fast enough to catch up with Mercury.

Even so the trip took seven years, and NASA engineers were able to use some novel forms of propulsion along the way, including using the probe's body as a solar sail.

The probe is shrouded in a sunshield of ceramic cloth that can withstand 600 degrees centigrade on one side while remaining at room temperature on the other. This was deployed shortly after launch, and the engineers found that by aiming the probe's blanket, and the twin solar cells, they could harvest propulsion from the solar wind.

Youtube video made by NASA about Messenger

They were helped by the fact that the rest of the probe is so light. With so much space required for fuel, only about 47.2kg was left for the payload of instruments. NASA cut materials to the bone: the spacecraft's titanium fuel tanks are one millimeter thick, and the body is a lightweight composite material that was specially developed for the job.

You may think that, being so close to the Sun, power wouldn’t be a problem at least. But because it is an orbiter, Messenger spends some time in Mercury's shadow, so battery packs and heaters had to be built in to stop the electronics and propellant freezing and running out of juice.

Even getting solar power was a problem. Only a third of the solar arrays were actually filled with power-generating cells; the rest was taken up by an elaborate system of mirrors to deflect the Sun's rays away, and the arrays themselves had to be hinged so that they could be turned side-on to cool down.

The extreme temperatures, and reduced payload, also meant that Messenger was a dish-less probe. Usually there's a satellite dish pointed back at Earth for communications, but the gimbals needed to keep it in position couldn’t survive the temperature extremes Messenger goes through, so NASA borrowed a new type of flat radio aerial from the US military and used that instead.

All this brilliant engineering is now going to be smeared across the surface of Mercury. Jim Green, director of NASA's Planetary Science division, said the spacecraft was the second stage of the three-step approach the agency takes with exploration.

"If you look at NASA's history of exploration, the first stage with a new planet is to get a flyby sorted out," he explained. "The next stage is to put a probe into orbit around it – as Messenger has done – but the third stage is to land on the surface."

NASA doesn't have plans to do that any time soon. Budgets are tight and Mars is a more promising target but Mercury will be getting another visitor soon. In 2017 the BepiColombo spacecraft, a joint mission of the European and Japanese space agencies, will blast off, and seven years later will start adding to our knowledge of the Sun's closest planet. ®