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Tuesday, November 17, 1998 Published at 21:38 GMT

Small and dangerous

SOHO will be put in 'sleep' mode during the shower

By BBC News Online's Science Editor Dr David Whitehouse

Most of the Leonids are tiny with a mass of about a millionth of a gram - not much more than particle of cigarette smoke.

Normally, objects this size would pose no threat to spacecraft. However, when they are travelling many times faster than a bullet from a high velocity rifle, the threat increases.

Larger particles that could knock a satellite out of its normal position are unlikely.

The velocity of the meteor impacts can be anything between 65 and 80 km (40 and 50 miles) per second. This can result in some physical damage to sensitive areas of satellites. A form of sandblasting can erode satellite surfaces such as thermal insulation blankets.

Astronomer Dr John Mason: "No chance of fragments hitting the ground"
"These microparticles could penetrate a fairly weak spacecraft skin," said Professor Tony McDonnell of the Unit for Space Sciences and Astrophysics at the University of Kent in Canterbury.

However, the most likely form of damage is to vulnerable power systems. "Perhaps a handful of satellites could have unusual electrical anomalies," said McDonnell.

Assessment of risk

Past evidence suggests that the risks are fairly low. During the past four decades, only one spacecraft, the European Space Agency's Olympus satellite, is known to have been disabled by a (Perseid) meteor. Furthermore, no spacecraft were damaged by the 1966 Leonid storm.

[ image: The speed of the meteors makes them dangerous]
The speed of the meteors makes them dangerous
However, there are now over 10 times as many as in the mid-1960s.

Spacecraft operators have taken no chances this time. The Space Shuttle mission that carried John Glenn was timed to avoid the Leonid shower. Cosmonauts on the Mir space station were considering moving into the attached Soyuz lifeboat at the peak of the shower.

In the case of the Hubble Space Telescope, its mirror was turned away from the shower. Most of the scientific instruments on the European ERS-1 and ERS-2 Earth observation satellites and the Solar and Heliospheric Observatory (SOHO) were powered down and placed in 'sleep' mode during the shower.

SOHO and another satellite, the American advanced Composition Explorer (ACE), which are located 1.5 million km sunward of the Earth, were particularly at risk since the main stream of meteors was expected to pass much closer to them than any of their Earth-orbiting brethren.

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