SkyMatters: Dr Niall Smyth guides us through the November skies

Mon, 01 Nov, 2021 - 06:00

Dr Niall Smyth

November is the month of the Leonids meteor shower. The shower results from the collision between the Earth and the debris laid down by the passage of a comet first observed in 1865 and called Tempel-Tuttle after its discoverers.

Over the course of 33 years the comet orbits from deep out in the solar system to eventually rounding the Sun at which time the rise in its temperature causes evaporation of dust that ultimately form the meteors we see.

This year the full Moon will, unfortunately, drown out many of the Leonids on its peak on the night of the 17th. This will be true even from pristine dark sky locations because particles floating high up in our atmosphere scatter moonlight and brighten the whole sky. Not so much a case of light pollution as one of moonlight pollution.

The Leonids hold the record for producing the largest meteor storm ever witnessed. The renowned Irish astronomer and populariser, Agnes Mary Clerke (born in Skibbereen) wrote of the 1833 Leonids that “the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm”.

She estimated the number of meteors to be around 200,000 per hour, or around 55 per second compared to a typical, but good meteor shower rate of around 1 every few seconds. This enormous rate is without equal, but almost all of the meteors were so small that they burned up harmlessly in the earth’s atmosphere.

Not the experience for Ruth Hamilton in the small town of Golden in British Columbia, Canada. She was sleeping when, around 11:30 pm on the night of 3rd October last, a meteorite with the mass of a bag of sugar crashed through her roof and landed on her pillow. This particular meteorite likely came from the asteroid belt – a region between Mars and Jupiter that contains trillions of objects of similar size.

Why it left the asteroid belt is unknown, but the constant changes in gravitational forces between Mars and Jupiter and other asteroids, and even the odd comet, mean every now and again an asteroid is ejected from the asteroid belt and plunges towards the Sun, or in the case of Ruth Hamilton the Earth.

The asteroid belt itself is of great interest to astronomers as it contains some of the original building blocks of the solar system from 4.5 billion years ago. It’s also of interest to space miners, or it will be when space mining becomes mainstream.

That’s because asteroids contain many precious metals. A recent report in the Planetary Science Journal discusses two asteroid which are 85% metallic – one is estimated to contain more cobalt, nickel and iron than can be mined from the Earth. And yes, that means it’s worth a small – actually extremely large – fortune.

Being struck by a meteorite is extremely rare – indeed there’s only one reliable case of this happening, to Ann Hodges of Alabama on November 30th 1954. But we know that BIG meteorites can cause a lot of damage, as would be attested to by dinosaurs if they hadn’t been wiped out by one. The chances of a really dangerous collision are extremely small, but not zero. So the launch of NASA’s DART mission on November 24 to the double asteroid Didymos is significant.

It will be the first time an object in space has been impacted by a spacecraft with the sole intention of deflecting it. The DART mission will use an experimental ion propulsion system that doesn’t burn fossil fuels, unlike most conventional rockets. If successful, DART will give us the first glimpse of a future where space can no longer threaten us with the same fate as that of the dinosaurs.