Zapping the Moon

Earth’s Magnetic Field – A Hazard for Lunar Astronauts?

For four days every month the Moon passes through the magnetic field of the Earth and parts of the lunar surface are charged with static electricity. Dr. Mike Hapgood of the Rutherford Appleton Laboratory will present a model at the Royal Astronomical Society National Astronomy Meeting in Preston, which suggests that this charging may increase after the year 2012 and become an important issue for future lunar explorers.

Once in every orbit around the Earth the Moon moves through the magnetic tail – the region on the nightside of the Earth where the magnetic field is drawn out into a million or more kilometre long tail pointing away from the Sun. In the middle of the tail there is a region full of energetic electrons and other charged particles (the plasmasheet). When the Moon passes through the plasmasheet these electrons can collect on parts of the lunar surface and charge them with static electricity. Observations from NASA’s Lunar Prospector spacecraft during 1998 confirm the existence of this charging.

An animation of the Moon (the small white circle) crossing the magnetospheric tail (the blue circle) around Christmas 2007 (the time is shown at the upper left). The view is from the Earth looking away from the Sun. The Moon crosses from right to left and occasionally encounters the plasmasheet (red). The two distance scales are in units of multiples of the radius of the Earth (Re). Click an image to see the animation.
Credit: Dr. Mike Hapgood/Rutherford Appleton Laboratory

Dr. Hapgood’s model suggests that the exposure of the Moon to plasmasheet charging varies markedly over an 18-year cycle linked to changes in the Moon’s orbit. This exposure was low at the time of the Apollo landings in the early 1970s and is low again today – but it was high in the 1990s and will rise again after 2012. The United States, Russia, India, Japan and China have all announced plans to send astronauts back to the Moon around the year 2020 – at the time when lunar surface charging is predicted to be high.

Lunar surface charging may be an important issue for future lunar exploration because it increases the risk of electric discharges, which can interfere with and damage sensitive electronics. It may also affect the behaviour of lunar dust, which is a recognised hazard for lunar astronauts as it can easily enter spacesuits, living quarters and equipment.

A graph of the predicted lunar exposure to charging by the plasmasheet over the years 1960 to 2030. The red trace shows the monthly exposure, which also varies with the seasons (peaking in June and December). The blue line is a smoothed curve that highlights the 18 year cycle. Click the image to enlarge.
Credit: Dr. Mike Hapgood/Rutherford Appleton Laboratory

Dr. Hapgood comments, "Electrical charging is one of the less well-known natural hazards of spaceflight. It’s important to understand it how this affects the Moon so spacecraft designers can use scientific knowledge to protect future explorers."

Astronauts on lunar missions will perform a number of experiments important to astrobiology. For instance, long-duration missions on the Moon will help scientists understand how life can adapt and evolve beyond its planet of origin.

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