Wednesday, February 1

Parts of the world could be in for a meteor storm as the Tau Herculids approaches


If predictions are correct, some places on Earth will see an intense meteor storm at the end of this month. 

An outburst of the annual Tau Herculids shower is expected to last about an hour on May 30 or 31 (the date depends upon where you live in the world). 

It could fill the sky with shooting stars, or it could be a complete and utter fizzer.

But either way, it will be exciting for astronomers like Jonti Horner of the University of Southern Queensland.

“Even if this doesn’t lead to a big storm itself, it means the next time we have a big storm, we’ll be able to predict it much, much better because of what we learnt from this.”

So what is the Tau Herculid meteor shower? And where is the best place in the world to see it (if it happens at all)? 

Not normally something to write home about

The Tau Herculids is not new, but it is a very minor shower.

Each year, the Earth moves through the debris stream from a small comet called 73P/Schwassmann-Wachmann 3, or SW3 for short.

SW3, which was discovered in the 1930s, orbits the Sun every 5.4 years, leaving a trail of debris behind it.

When it was first discovered, astronomers predicted we should see meteors coming from a point near the star Tau Herculis in the constellation of Hercules, which gives it its name.

Tau herculids sky map
The new radiant point of the meteor in relation to the constellations of Boötes and Hercules (Southern Hemisphere perspective).(Stellarium/ABC: Genelle Weule)

Fast forward 90 years and the radiant point has shifted to the neighbouring constellation of Boötes thanks to SW3’s regular close encounters with Jupiter, which have altered the comet’s orbit over time.

The best meteor showers, such as the Geminids, can produce up to 50 meteors an hour (depending upon where you are in the world).

But the Tau Herculids usually barely rates a mention on the list of showers to see because we’ve only ever seen a handful of meteors since the comet was discovered.

“Most of the time it is genuinely pretty rubbish, but there is the possibility of something special this year,” Professor Horner said.

If predictions are correct, the Tau Herculids could ramp up to rates well beyond what we normally see even in the best showers.

So, what’s changed?

Cloud from disintegrating comet

For the first time, Earth may encounter the tip of a thin javelin-shaped filament of debris laid down in 1995, when SW3 split apart.

“In 1995 when the parent of this meteor shower disintegrated, more than 70 big chunks were observed, and lots of dust and debris,” Professor Horner said.

“And in the years since, we’ve continued to see it fall apart.”

Comet Schwassmann-Wachmann-3
Comet Schwassmann-Wachmann-3 disintegrated in 1995.(Supplied:  NASA/JPL-Caltech)

But whether the encounter will be spectacular, so-so, or simply doesn’t happen at all rides on geometry.

“We’re going to come very near to the end of the javelin of dust that was released in 1995 and the question of whether we’ll see anything or not depends on how long that spear is,” Professor Horner said.

That will depend on how fast the dust is travelling.

Instead of passing through the comet debris at a steep angle,  Earth grazes the edge of the comet’s orbit like the gentle merging of cars on a highway travelling at the same speed.

That means Tau Herculid meteors usually move very slowly — about 16 kilometres per second — and are not very bright.

“If the debris was released at the normal speed comets release debris when they travel around the Sun, the javelin will be too short to reach us,” Professor Horner said.

“But because [the comet break-up] was such a violent event … there is the possibility that some of that dust will have been released at higher speed than normal.”

“If it was fast enough, and a lot of dust can reach us, it means we could see a meteor storm.”

If it doesn’t, the next big Tau Herculid storm is not predicted to happen until 2049, and meteor rates may be lower as the dust spreads out over time.

Meteor showers and storms

Meteor storms can produce more than 1,000 meteors per hour if you live somewhere where the radiant point is directly above your head.

In 1872 and 1885 “stars fell like rain” when Earth ran through a stream of debris left by a comet that disintegrated in 1846.

Painting of the 1872 Andromedids or Belids meteor storm
A painting of the Andromedids or Belids meteor storm over France on November 27, 1872.(Wikimedia Commons: Amedee Guillemin)

The event, known as the  Andromedid or Bielid meteor storm, produced thousands of meteors an hour, but hasn’t been seen since 1885.

Another northern hemisphere meteor shower known as the Leonids has ramped up to storm level several times over the past 200 years.

One of the most spectacular of these happened in 1966, when thousands of meteors per minute fell through Earth’s atmosphere during a 15-minute storm.

The last Leonids meteor storm happened in 2002.

It is thanks to the regular outbursts from the Leonids that astronomers are getting pretty good at predicting when new meteor showers and storms are going to happen.

Last year they accurately predicted a new shower known as the Finlayids.

So, who will get to see it?

Unless the timing of the prediction is way out, it’s unlikely we’ll see the Tau Herculid storm from Australia.

It’s expected to peak about 3pm (AEST) on May 31. 

Not only is that during the day in Australia, but the constellations where the meteors radiate from will be below the horizon at that time.

But the peak will be visible in North America and parts of South America and the Pacific (the further north the better the rates), where it is still nighttime.

“It’s perfect for Baja, California because at that time [11pm May 30 PDT], the radiant will be almost overhead,” Professor Horner said.

And because it’s happening during the new moon phase, the night sky will be dark.

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