by Will Sager
Meteor observers were recently excited about the possibility of an out-of-nowhere meteor shower, called the Tau-Herculids, that offered the possibility of a “storm” at the end of May. Many years ago, my uncle described seeing the 1966 Leonid meteor storm in Virginia and said meteors were falling too fast to count all over the sky. Later I read in Sky and Telescope about observers at Kitt Peak, who got the best display. They witnessed a rain of meteors estimated at 10-40 per second. I have been observing meteors for 45 years and have never seen such a storm. The closest I came to it was the 2001 Leonid shower, when the Earth passed through a rich stream of debris from comet Temple-Tuttle. That night, under dark skies in Arizona, I logged over 1200 over the course of the night. It was awesome, but not quite the storm that my uncle saw. Would the Tau Herculids be my storm?
Meteor showers arise from debris shed by comets or asteroids. The debris expands along the parent body orbit and if the Earth encounters that debris stream in its yearly swing around the Sun, there will be a meteor shower. Owing to perspective, the meteors seem to radiate from a single spot in the sky – like the like the snowflakes when you drive into a snowstorm (Texans will have to take my word on this). The shower is named by the location of this radiant. The Leonids from from Leo, the Perseids from Perseus, and the Tau-Herculids from a point near the star tau-Hercules. Most meteors are tiny grains, dust to sand size, and they cause streaks because they run into the atmosphere at astonishing speeds (more than 50 kilometers per second) and this causes impacted upper atmosphere atoms to be ionized and glow briefly. Every now and then, a larger fragment plows into the atmosphere, causing a bolide (aka fireball), which is an exceptionally bright meteor (usually causing the observer to hoot in delight). The most reliable annual meteor showers have debris all along the parent body orbital path, which happens to intersect the Earth’s orbit. Every year, the Earth runs through the stream, producing the annual shower.
There are a couple of dozen meteor showers every year, but only a few are regular and exceptional. You probably know them, the August Perseid shower, a product of comet Swift-Tuttle and the Geminid shower, a product of asteroid 3200 Phaeton. In dark skies, an observer watching these showers usually sees more than one meteor a minute. Most other meteor showers are minor, meaning their production is far less. The aforementioned Leonid shower is notable because debris is concentrated near the parent comet, so the shower strengthens at 33 year intervals (its orbital period) causing the occasional meteor storm. Scientists have become much more adept at predicting streams of debris shed by comets as they pass by the Sun. That was why I got a double dip with the last Leonid enhancement. The storm was predicted for 1999 and I observed it from Prude Ranch, where it was good, but not exceptional (the shower peak was over Europe that time). By 2001, scientists predicted that the Earth would encounter a rich Leonid stream, so I went to Arizona to catch that outburst.
Fast forward to 2022. Early this year, astronomers reported that there could be a meteor storm from the normally-weak Tau Herculids. The parent body is periodic comet Schwassmann-Wachmann 3 (SW3), which swings around its orbit every 5.4 years. Near perihelion in 1995, the Sun’s heat caused it to break up into smaller bodies, releasing copious debris. Recently astronomers predicted that the Earth might encounter the 1995 breakup debris on the night 30-31 May 2022. There were several unknowns that added uncertainty. Most comet debris trails the parent comet, but in this case the Earth would cross SW3 orbit before the comet. To encounter debris, the breakup would have had to fling material forward in the orbit. Another problem is that the debris would be approaching the Earth from behind, so the collision velocity would be low. Fast meteors smash into the Earth’s atmosphere head on, so the relative velocity is high. For example, the zippy Leonids hit the atmosphere at about 72 km/s. The Tau Herculids would be like a car overtaking your car on the highway, with the relative velocity low. For the Tau Herculids, this was estimated to be about 16 km/s. The particle sizes were also not known. Perhaps the particles would be small and moving too slow to cause visible meteors. Despite this uncertainty, there was the possibility that this year’s shower could be a storm, something like the Leonid outbursts of 1833 and 1966, the Lyrid storm of 1803, or the Andromedid storm of 1872, which was also caused by comet disintegration.
With this conflicting information, what to do? Do I drive a long way to really dark skies? That’s where I got snookered by the Texas weather. My Astrospheric app and the Cleardarksky web site both said that the night would be clear at the dark site until well after the predicted peak at midnight, so that’s where I went. If the meteors were a big nothing, I would not have wasted a lot of time and money. At the dark site I met fellow hopefuls Bram Weisman and Simon Tan, who was accompanied family members. The night started clear with a nice sunset (and you know how this ends don’t you?). I started logging meteors at 10 pm because the sky was dark and the radiant high in the sky. The radiant was in the constellation Boötes, near the star Arcturus, because of kick of splitting up pushed the meteors to a different spot in the sky. While waiting for twilight, I saw a few Tau Herculids, letting me know that they were definitely showing up. Not long after I began logging, the sky started to cloud up intermittently. The clouds would form within a minute and often they would leave just as swiftly. Over time, the cloud cover became heavier and overcast for longer periods. For the night, I logged 19 Tau Hercuilds, with most between 10 – 11 pm. After that, even when I got a nice hole in the clouds, I didn’t see any meteors. Most Tau Herculid meteors were slow, as predicted, often lasting a second or more. The trains were often short, less than 5 degrees, and the colors were usually white to yellow, but sometimes shaded with red. From what I saw, it was clear that the Tau Herculid shower turned out to be a good one, but not a storm. Although the prognosticators predicted faint meteors, most that I saw were bright, between second and zero magnitude, but with some brighter and some dimmer. It also appeared that the peak was sharp, somewhat after 11 pm (0400 UT on March 31), and fell off rapidly afterwards.
Other local observers shared their impressions. From Fredericksburg, Kenneth Drake reported 11 Tau Herculids between 10 – 11 pm and 17 from 11 – 12 pm. Bill Spizziri described several fireballs, including one 25 degrees long with a fire-engine red trail. Fighting the clouds in Katy, Walt Cooney saw only a handful, but he said that the Texas CAMS (Cameras for Allsky Meteor Surveillance) network picked up about 100, using data from stations that Walt runs, as well as others contributed by Don Selle, Fred Cyrway, and Joel and Becky Brewer. CAMS director Peter Jenniskens reported that the global network picked up 2244 Tau Herculid meteors. On the International Meteor Organization web site, an article by Robert Lunsford shows a peak ZHR (zenithal hourly rate: the number of meteors an observer could theoretically see in a dark sky at the zenith) of about 45 at 0445 UT on March 31 (see first figure). In the e-zine Meteor News, Hiroshi Ogawa tells of radio meteor observations (observers record enhancement of radio signals by meteor ionization) that give a similar picture, with a main peak ZHR of about 35 at solar longitude 69.4° (31 May 0330 UT; see second figure). The radio record also shows two minor peaks at solar longitude 68.8° (30 May 1330 UT) and 68.9° (30 May 1530 UT). Ogawa interpreted these smaller peaks as being from sub-streams within the comet debris. When you consider that the annual Perseid and Geminid showers have ZHR of 90 and 100-120, respectively, it is clear that the Tau Herculids were not at storm level or even the level of our finest showers. Nevertheless, many observers were treated to a fine meteor shower. Unfortunately, predictions for the next encounter with a rich SW3 stream is 2049. Put it on your calendar so you don’t forget.
Figure 1. International Meteor Organization ZHR curve.
Radio meteor ZHR curve from Ogawa (2022)
Jenniskens, P., 2022. 2022 Tau Herculids outburst observed by CAMS. Meteor News. https://www.meteornews.net/2022/06/04/2022-tau-herculids-outburst-observ...
Lunsford, R., 2022. Observed meteor outburst of tau-Herculids. International Meteor Organization. https://www.imo.net/update-possible-meteor-outburst-on-may-3031/
Ogawa, H., 2022. A meteor outburst of the T-Herculids 2022 by worldwide radio meteor observations. Meteor News. https://www.meteornews.net/2022/06/05/a-meteor-outburst-of-the-%cf%84-he...
Rao, J. 2022a. Update on a possible outburst of meteors. Sky and Telescope, May 28. https://skyandtelescope.org/astronomy-news/update-on-a-possible-outburst...
Rao, J., 2022. The greatest meteor storms of all time. Space.com. https://www.space.com/greatest-meteor-storms-in-history