As Earth orbits the Sun, it passes through dust and debris left behind by comets and asteroids. These debris gives rise to meteor showers – which may be one of nature’s most amazing sights.
More meteor showers are predictable, reoccurring each year when Earth passes through a particular trail of debris.
Sometimes, however, the Earth passes through a particularly narrow and dense pile of debris. This results in a meteor storm, sending thousands of shooting stars cross the sky every hour.
A minor downpour called Tau Herculids could create a meteor storm for watchers in the Americas next week. But while some sites promise “the most powerful meteor storm in generations”, astronomers are a bit more cautious.
Presentation of Comet SW3
The story begins with a comet called 73P/Schwassmann-Wachmann 3 (comet SW3 for short). First spotted in 1930, it is responsible for a weak meteor shower called Tau Herculids, which today appears to radiate from a point about ten degrees from the bright star Arcturus.
In 2006 (two orbits later), comet SW3 had disintegrated again, several shiny fragments accompanied by numerous small pieces.
Is the Earth on a collision course?
This year, the Earth will cross the orbit of comet SW3 at the end of May.
Detailed computer modeling suggests debris spread along the comet’s orbit like huge, thin tentacles in space.
Has the debris spread far enough to meet Earth? It depends on how much debris was ejected in 1995 and how quickly that debris was thrown outward when the comet collapsed. But the bits of dust and debris are so small that we can’t see them until we encounter them. So how can we get a glimpse of what might happen next week?
Could history repeat itself?
Our current understanding of meteor showers began 150 years ago with an event quite similar to the story of SW3.
A comet called 3D comet/Biela was discovered in 1772. It was a short-period comet, like SW3, returning every 6.6 years.
In 1846, the comet began to behave strangely. Observers saw that its head had split in two, and some described an “arch of cometary material” between the pieces.
By the next return of the comet, in 1852, the two fragments had clearly separated and both fluctuated unpredictably in brightness.
The comet was never seen again.
But in late November 1872, an unexpected meteor storm graced the northern skies, stunning observers with rates of more than 3,000 meteors per hour.
The meteor storm happened when Earth passed through 3D/Biela’s orbit: this was where the comet itself should have been two months earlier. A second storm, weaker than the first, occurred in 1885, when the Earth again encountered the remnants of the comet.
3D/Biela had disintegrated into rubble, but the two large meteor storms it produced served as a suitable wake.
A dying comet, collapsing before our eyes, and an associated meteor shower, usually barely imperceptible against background noise. Will we see history repeat itself with Comet SW3?
What does this suggest for the Tau Herculids?
The main difference between the events of 1872 and the Tau Herculids of this year comes down to when the Earth passed through cometary orbits. In 1872, the Earth crossed Biela’s orbit for several months after the comet was due, passing through matter late where the comet would have been.
In contrast, the encounter between Earth and the debris flow from SW3 next week occurs several months before the comet must reach the waypoint. So the debris must have spread ahead of the comet for a meteor storm to occur.
Could the debris have traveled far enough to meet Earth? Some models suggest we’ll see a loud display of the shower, others suggest debris will fall right next to it.
Don’t count your meteors until they’ve flashed!
Whatever happens, next week’s shower observations will greatly improve our understanding of how comet fragmentation events occur.
Calculations show that the Earth will cross the orbit of SW3 around 3 p.m., May 31 (AEST). If the debris extends far enough for Earth to encounter it, then an explosion of the Tau Herculids is likely, but it will only last for an hour or two.
From Australia, the show (if there is one) will be over before it gets dark enough to see what’s going on.
Observers from North and South America, however, will be in the front row.
They are more likely to see a moderate display of slow-moving meteors than a huge storm. That would be a great result, but could be a little disappointing.
However, it is possible that the shower offers a truly spectacular display. Astronomers travel around the world, just in case.
And the Australian observers?
There is also a small chance that an activity will take longer than expected, or even arrive a little late. Even if you’re in Australia, it’s worth looking up on the evening of May 31, just in case you might spot a fragment of a dying comet!
The 1995 debris flow is just one of many deposited by the comet in recent decades.
In the early morning of May 31, around 4 a.m. (AEST), Earth will pass through debris from the comet’s 1892 passage around the Sun. Later in the evening, around 8 p.m. on May 31 (AEST), Earth will pass through debris deposited by the comet in 1897.
However, debris from these visits will have spread over time, so we expect only a few meteors to grace our skies from these streams. But, as always, we could be wrong – the only way to find out is to have a look!
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