Washington: Forty years after Apollo's moon mission, a scientist has figured out why moon dust is so sticky, ruining scientific equipment and endangering astronauts' health.

He concluded that its stickiness changes as the sun moves higher in the sky, eventually allowing the very weak lunar gravity to pull the dust off.

Lunar dust causes havoc: from destroying scientific equipment deployed on the lunar surface to creating blinding dust clouds that interfere with lunar landings.

It also may be a health hazard to space travellers since dust clinging to space suits detaches when astronauts re-enter their lunar module. It then floats free in zero gravity, ready to be inhaled, during the three-day journey back to earth.

"Before you can manage the dust, you have to understand what makes it sticky," said NASA geophycisist Brian O'Brien, who authored the paper.

His analysis is the first to measure the strength of lunar dust's adhesive forces, how they change during the lunar day - which lasts 710 hours - and differ on vertical and horizontal surfaces.

O'Brien used data from the matchbox-sized Dust Detector Experiments deployed on the moon's surface in 1969 during the Apollo 11 and Apollo 12 missions.

Lunar dust particles are minuscule, with an average size of 70 micrometers, the thickness of a human hair.

The particles get positively charged by photoelectric effects caused by powerful solar ultraviolet radiation and X-rays - generating adhesive forces that compel the specks of dust to cling to surfaces of scientific instruments and space suits.

In his new study, O'Brien analysed the behaviour of dust on horizontal and vertical solar cells in one of the Apollo dust-detecting experiments.

O'Brien found that later, as the sun rose and the angle of incidence of the sun's rays on the dusty vertical surface facing east decreased, the electrostatic forces on the vertical cell weakened, said a NASA release.

The tipping point was reached when the sun was at an angle of about 45 degrees: then the pull of lunar gravity counteracted the adhesive forces and made the dust start falling off. All dust had fallen by lunar night.

The study is slated for publication in the Geophysical Research Letters. IANS