This month could see arguably the most spectacular astronomical event in two decades, a complete no-show, or something in between. Meteor storms are inherently unpredictable events, so there's no way to know for sure if the Tau Herculids will deliver. If you like to believe in trying your luck, however, you might want to make plans to get away from city lights on May 30, particularly if you live in North or South America.
On a clear night under a dark sky, you'll probably get to see a few meteors on any given night. At certain times of the year, the pace picks up, with what is known as a meteor shower. Along with the random meteors (sporadics), nights with a meteor shower involve extra flashes of light that all seem to come from the same point in the sky.
Meteor showers are measured by the Zenith Hourly Rate (ZHR), the number of shooting stars one might see without clouds or light pollution if the point of origin was directly overhead. The best meteor showers have ZNRs of 100 but most are in single figures. Very occasionally, however, events occur where the ZNR gets into the thousands, earning the name "meteor storm".
Officially meteor storms involve a ZHR of more than 1,000. There hasn't been one since the Leonid storms of 2001/2002. So even the possibility of one created a buzz in astronomical circles years ago that has built up as the day approaches.
Meteors occur when bits of dust or gravel hit the Earth's atmosphere, burning up in the process. Depending on their size and speed this can produce anything from a barely visible flash to a fireball like the one that shook three states last month.
When comets or asteroids break up, bits of rock are left scattered across a patch of the Solar System. When the Earth plows through, meteor showers or storms result, depending on the density of debris.
After a comet's dissolution, these patches start small and concentrated. One year the Earth may pass through a rich area and get an abundance of meteors, the next it may miss almost everything. As time goes on patches spread out under the influence of planet's gravity, the solar wind, and varying dispersal speeds. This produces reliable annual showers that can go on for days, but never achieve the peaks they can in the immediate aftermath of a comet's demise.
The Tau Herculids (named after a star in the direction from which they initially appeared to come) are among the youngest meteor showers, left behind as Comet 73P/Schwassmann–Wachmann 3 broke up in 1995. Most years they are so sparse that lists of showers ignore them, but there's a much denser patch and some astronomers believe the Earth is heading right for it this month.
Calculating the exact location of such patches is tricky, although it has improved a lot in recent years. It depends on estimating the influence of complex uncertain factors. Several people have produced slightly different estimates of when the Earth will hit the main Tau Herculid stream. If they're right, it should be visible over much of North America around 1 am EDT on May 31 (twilight May 30 on the west coast).
South Americans will see fewer meteors, since the radiant point will be low in the north, but can be more confident the Sun will have set when the storm occurs. If the calculations are out by a few hours one way or another, it could be Europe and Africa, or Asia and Australia that get lucky instead.
“This is going to be an all or nothing event. If the debris from SW3 was traveling more than 220 miles per hour [350 kilometers per hour] when it separated from the comet, we might see a nice meteor shower. If the debris had slower ejection speeds, then nothing will make it to Earth and there will be no meteors from this comet,” NASA's Bill Cooke said in a statement.
Meteor showers or storms lose much of their dazzle in moonlight, but so soon after a new Moon that won't be a problem for this event. More concerning is that their orbits relative to the Earth mean the Tau Herculids are among the slowest meteors. That makes larger objects easier to spot, but smaller ones never get bright enough to be seen. No one knows what the size mix of the 1995 break-up is like.
The 2001 Leonid storm lasted several hours, but the Tau Herculids are expected to be much shorter – perhaps only 15 minutes – if there's a storm at all. However, take it from someone who was lucky enough to witness a storm once – even a quarter of an hour of meteor storm will create memories that last a lifetime.