We’ve all read the advice that no equipment is needed during a meteor shower. All you have to do is sit back and marvel at one of the most spectacular sights the universe has to offer. But that’s about it, and while you’re sitting back on a lounge chair and watching, it can be a wonderfully grounding and relaxing experience. Unless you happen to be on national television and miss a meteor behind your head and just tell the world there’s nothing to see. Not that I’m bitter about that, of course.

It’s easy to get confused; a meteor is a piece of rock that has fallen through the Earth’s atmosphere and been destroyed along the way, a meteorite survives the fall, and a meteoroid is a piece of space rock that drifts through space before hitting the atmosphere. We can see meteors every night of the year and these are called sporadic meteors. However, about 20 times a year we can enjoy a burst of meteor activity in events called meteor showers. There are other showers as well, but they are often weak and barely noticeable.

Individual meteors are seen as they fall to Earth. Their passage through the atmosphere causes the gas to heat up and emit light, which we see as the familiar streak of light. The atmosphere is of great importance to us because it protects us from the countless meteor visitors that would otherwise hit the surface. Instead, only the largest ones get through, but fortunately they are few. Spacecraft and satellites, of course, orbit above the protective shield of the atmosphere and are much more susceptible to damage.

Organizations like NASA take the risk of meteoroid impacts very seriously and their biggest concern is the sporadic meteors. The meteor showers that we all enjoy only increase the risk for a short time and their characteristics are well understood. This means that their risk profile can be calculated very well with NASA’s Meteoroid Environment Office issuing regular forecasts. However, the real risk comes from outbursts, such as one-off unexpected meteors or numerous small showers that are not yet well documented or understood.

This is not just a paper exercise, however. The International Space Station has been in orbit since 1998 and has had to adjust its course several times during that time. There have been times when the occupants had to get into an escape module and move away from the station because of a possible meteorite impact. However, no major damage has been done to date. There are thousands of satellites in orbit and some have sustained damage.

A paper recently published by Althea V. Moorhead and a small multidisciplinary team from NASA’s Meteoroid Environment Office and the University of Western Ontario looks at quantifying potentially hazardous meteor showers and examines existing showers to determine which ones pose a high risk and are of concern. The research is published on the arXiv preprint server.

The team concludes that for a meteor shower to be classified as hazardous, the rate of meteors of a given mass or greater hitting an exposed spacecraft surface in low Earth orbit over a unit of time (known as the meteor flux) would need to increase by 5% over the sporadic rate. This would deliver 105 joules of energy, enough to damage relatively delicate spacecraft components. The team is clear that they are not recommending that spacecraft should reduce this rate, but instead should at least pay attention to studies based on meteor forecast reports.

Once the criteria were established, the team then evaluated 74 of the 110 identified meteor showers listed in the IAU Meteor Data Centre catalogue. They found that six showers met the criteria for being classified as hazardous, exceeding the threshold by a factor of two. The notable showers are the Arietids, Geminids, Perseids, Quadrantids, Southern Delta Aquariids, and the Xi Sagittariids during the day. There are another 11 showers that meet the threshold but do not exceed it by a factor of two.

As the study clearly shows, spacecraft operators need to be very aware of meteoroid activity, not only during their orbital operations, but also during design and planning. We on the ground can still sit back and relax and enjoy the spectacle, of course, but it adds an interesting context that, as we head out into space, those beautiful swarms pose a very real risk to the safety of our explorers.