When meteor showers attack! Forecasters are trying to pin down which comet debris poses a threat to us

Meteor showers are a delight to behold – nature’s own fireworks displays – but their beauty hides a more dangerous side.

Before they enter Earth’s atmosphere, the fast-moving grains and pebbles of interplanetary rock – meteoroids – pose a risk to spacecraft.

Larger meteoroids can penetrate the cabin of crewed space vehicles, and even small impacts can damage satellite components and short-circuit electronics.

For example, ESA’s Olympus 1 communications satellite started tumbling out of control and had to be subsequently deactivated during the 1993 Perseids shower.

There is an ever-present background threat of meteoroids, so spacecraft are built with shielding and redundant systems.

But during a meteor shower, the risk can jump for a short period, forcing spacewalks to be postponed or satellites to be turned to present a less vulnerable side to the shower radiant.

Meteor showers explained

Meteor showers are caused when Earth passes through the stream of debris left along the orbital track of a comet, and so they are broadly predictable.

NASA’s Meteoroid Environment Office, where Althea Moorhead works, issues annual meteor shower forecasts to help mission planners avoid the worst risks.

The difficulty is that the intensity of different showers changes over the years or key parameters are unknown, such as the density of particles or their size range.

Plus new showers are being reported all the time.

Gauging how dangerous a meteor shower is

Moorhead has been working on improving predictive models to make sure that warnings are only issued for the most genuinely hazardous events. 

She started by defining the threshold of what constitutes a potentially hazardous meteor shower.

It’s a lengthy definition, but boils down to when the rate of energetic, potentially damaging, meteoroids is at least 5% that of the background level.

There are over 100 showers with relatively well measured parameters and Moorhead applied her criteria to pick out those that present the worst risks. 

Perhaps surprisingly, the meteoroid showers that result in the most impressive displays of shooting stars can pose minimal risk to spacecraft.

These are produced by unusually large, high-speed meteoroids, but if there’s not a great number of them, the chance of any hitting a satellite is tiny.

The Epsilon Geminids, for example, which occur in late October, are caused by exceptionally fast particles and can produce spectacular fireballs, but pose an utterly insignificant risk to spacecraft.

On the other hand, slow-moving showers dominated by smaller particles can act more like a shotgun blast of particles hazardous to satellites. 

The most dangerous meteor showers identified by Moorhead include the Geminids, which peak in mid-December, and the Perseids in mid-August.

She also keeps a close watch on variable showers, which are typically low-activity and don’t exceed her danger threshold for most years, but can occasionally surge hundreds of times over during an outburst.

The Leonids have frequently produced such outbursts in the past, when Earth passed through a particularly dense stream of meteoroids from the comet Tempel–Tuttle.

It’s strange to imagine space mission planners checking the weather forecast for warnings of rocky rain, but hopefully now they should at least know when they need to take their proverbial umbrella.

Lewis Dartnell was reading The Threshold at which a Meteor Shower becomes Hazardous to Spacecraft by Althea V Moorhead et al.

Read it online at: arxiv.org/abs/2408.04612

This article appeared in the November 2024 issue of BBC Sky at Night Magazine