What causes the Perseid meteor shower?

It’s that time of year when the Perseid meteor shower, an annual celestial event eagerly awaited by skywatchers around the world, makes its appearance, peaking just before dawn on Sunday, August 13th, when observers might see one meteor per minute. But what are we actually seeing? Diana Hannikainen is from Sky and Telescope Magazine…

Diana - The Perseids is a meteor shower. And meteor showers are what are commonly called shooting stars. They're not really stars. In fact, they are little particles of dust and debris left behind by the passages of comets when they come into the inner solar system. Comets are far out in the outer edges of the solar system, and every now and then one of them comes close to the sun. And in doing so it starts to shed material, leaving behind bits of it. And when the earth passes through the stream of rubble left behind some of these comets, then what we see is a display of meteors.

Chris - Why, just at this time of year, because this happens around this time every year. So why is it just in this patch of our solar system?

Diana - This happens to be where the earth's orbit around the sun intersects with previous passages of the comet. And this particular comet that gave rise to the Perseids is known as Swift-Tuttle. Swift and Tuttle were two people who first discovered the comet in 1862, just a couple of days apart. And every August the earth happens to cross where the comet has passed in the past.

Chris - So has it left this debris trail just floating in space. And periodically our orbit crosses it. And that's why we get this display in the sky at this time of year?

Diana - Correct. After it comes through, after the comet comes through and comes into the inner solar system, it leaves behind a rather thin trail of gas and dust behind it. But with time, this trail spreads out, but it's always in the same place. The comet has a fairly steady orbit, and it follows the same trajectory.

Chris - It's in an orbit, this comet, why does it not cross our orbit twice? Why do we only see one meteor shower per year from this comet?

Diana - That is an excellent question, and that is because the orbit of this comet is not in the same plane as the earth and the other planets. It is tilted. You can picture it as zipping in from above and going below the plane of the orbit of the planets before heading back out into the outer reaches of the solar system. So it sort of comes in at an angle.

Chris - And the particles that it's left behind, how big are they? What do they look like?

戴安娜-他们真的,小灰尘颗粒粒ns of sand. If you go to the beach and you grab a fistful of sand, there are some really tiny little bits of sand in there. There are some slightly bigger ones. None of the particles is bigger than let's say a small pebble. And in fact, a lot of people think that the rubber left behind by comets comes just from what we see as the tail when comets approach the sun. But in fact, the comet, if you think of it as a dirty snowball, a little ball of ice that comes in from the outer reaches of the solar system, it is subjected to a lot of stress and strain. And so what happens is it cracks and there are these little fissures on the surface, and it's bits of pieces of the comet that come out of there that then form the debris trail.

Chris - It's extraordinary though that something as tiny as that can produce such an amazing firework display.

Diana - Oh, absolutely. And the reason it does that is because these particles hit the atmosphere, the upper atmosphere, at tremendous speeds. They're coming in really, really fast. The Perseids specifically, the speeds of the particles have been measured at around 200 times the speed of a jet plane. They are slamming into the atmosphere and they're releasing a lot of energy that way. And that is what we see as these beautiful streaks and trails in the night sky.

Chris - Now, talking of things incoming, it's a bit ironic, isn't it, that we heard the report this week that NASA had this DART mission. The idea being to see if we could fend off something that was bigger than a grain of sand. If a big asteroid were coming in. For example the DART mission was all about firing a projectile at something deep in space and deflecting it. And it was announced this week that this experiment has been a success. They've slammed an impactor into an object, but it has unfortunately led to the creation of a whole load of potential impactors now.

Diana - Yes, that's a real oopsie specifically because as you've said, the whole point of the mission was just to test this scenario - 'Can we blow up an asteroid?' In this case, it was the moonlet of an asteroid. Can we direct an explosive across space to such a focused target? And like you said, all of that was successful. So it's incredibly ironic to hear now that yeah, maybe some of that is heading towards us.

Chris - Do we know when it might arrive?

Diana - I'm not sure about that, but I don't think it will be anytime too soon. I think we'll all enjoy meteor showers, the regular meteor showers before we start to see any of that.