In 2013, a meteor exploded over Chelyabinsk, Russia injuring over 1000 people. But it could have been much worse - just ask the dinosaurs. For years, people have been pondering how we might avoid a potentially civilisation killing asteroid.


To test one potential method, NASA is launching the Double Asteroid Redirection Test on 24 November which will smash into the asteroid Didymorphos to change its orbit, a trial run for a future planet saving plan.

Listen to more episodes of the Radio Astronomy podcast.


Announcer: Hello and welcome to Radio Astronomy, the podcast from the makers of BBC Sky at Night Magazine. You can subscribe to the print edition of the magazine by visiting or to our Digital Edition by visiting iTunes or Google Play.

Ezzy Greetings, listeners. It's time for the December episode of Radio Astronomy. I'm news editor Ezzy Pearson and I'm joined in the studio today by production editor Neil McKim.

Neil Hello.

Ezzy Coming up later, we'll be telling you how to see the Geminids meteor shower in our stargazing tip of the month. But first, we'll take a look at a topic that we learnt all about was putting together this month's issue. Now on the 23rd of November, which is actually just a few days after this podcast is due to go out, NASA is due to launch the latest space mission, one that I think you'll agree is going to be a little bit more than dramatic.

Neil The mission is called DART, which stands for the Double Asteroid Redirection Test spacecraft, and it's basically an NASA driven test of defence technologies for preventing an impact on Earth by a hazardous space rock. His current launch window stretches from the 24th November 2021 to the 15th February next year, and the intention of the mission is to try and deliberately collide a spacecraft with a space rock to change its orbit. In order to do this a SpaceX Falcon nine rocket will send the DART spacecraft on a collision course with a small moon of the asteroid Didymos, which was discovered in 1996. The 360 kilogram DART spacecraft about the size of a golf cart, would deliberately crash itself into the space rock as a velocity of 6.6 kilometres a second in the autumn of 2022, when DART smashes into the object telescopes back on Earth will measure the resulting change in the Moon's orbit, and the event will be witnessed up close by the tiny LICSIA, which stands for the Light Italian CubeSat for Imaging Asteroids. DART itself is a basic spacecraft, built by the Johns Hopkins Applied Physics Laboratory, and its power will be provided by to roll out solar arrays. As DART approaches Its target on board is a high resolution camera, Draco, and this will navigate the spacecraft and take measurements of the target as it approaches, including its size and shape. Then DART will be intentionally destroyed less than a year from now when it slams into the rock. The target in question, Dimorphos measures about 160 metres across and, as mentioned, orbits the asteroid Didymos – Greek for twins – which is almost five times larger. Didymos itself is a typical near-Earth asteroid discovered in 1996 by the Space Watch project at KITT Peak National Observatory. It Rotates every 2.6 hours and it's 2.11 year path around The Sun takes it from the main asteroid belt just outside Earth's orbit. Astronomers discovered the small moon Didymorphos in 2003. Dimorphos itself orbits the main asteroid every 11.9 hours at just one point two kilometres, moving at a leisurely pace, which has being compared to the walking speed of a tortoise. And it's hoped that the destructive impact of the DART spacecraft will change this velocity by less than a millimetre a second. So the DART mission will shorten the orbit of Dimorphos taking, but it takes around asteroid the Didymos by about 10 minutes. And scientists aren't exactly sure what will happen, but it will be very interesting to see the results.

Ezzy Yeah. It's just this incredible when you think about it, they're just going to send the spacecraft all the way across this huge distance to then smash it into an asteroid just to see what will happen, basically. But it's not just DART by itself that they're going to be sending. There is also in a couple of years going to be the Hera follow up mission, which is actually being done by the European Space Agency, ESA. Initially, it was supposed to be called a mission called AIM or the Asteroid Impact Mission, and it was going to get there before DART reached it and be able to watch things as they unfolded. Unfortunately, back in 2016, the ESA council who decides exactly you know where money's going to go, decided that they weren't going to fund the AIM mission and instead scaled that back to be the Hera mission that would come along a couple of years later, which was a shame because it really would have been a massive advantage to be able to see this... To monitor the asteroid beforehand and then see how it changed afterwards. But still, Hera would be able to do a lot. It's now planned to launch in October 2024 and will have then eventually rendezvous with the double asteroid in October 2026, possibly 2027. If things don't go, if the launch gets delayed. Once it gets there, it will create really get to know the asteroid system, the binary system, as well as it possibly can. It will have a highly advanced cameras on board. It will use something called lidar, which is basically radar, but with light – hence li – instead of radio waves, and really get to know that the surface topology of the two bodies specifically, it will be really focusing in on that crater that DART formed. This is, of course, assuming that, you know, the impact doesn't cause the asteroid to break apart entirely. That doesn't seem likely. You know, these asteroids are I think it's 160 metres across, Is the Moon something like that? And DART is only, you know, like a tiny little thing, so it probably won't cause it to completely fracture, even though these things sometimes I just, you know, loosely held together bags of pebbles. But that's also, you know, one of the things that I'll be looking for is like, if you do throw something at an asteroid, does it break apart completely or does it just form a crater? It will also be really accurately measuring the mass of these two things, because that's one of the things they want to know. They can accurately measure how the timing change so they can accurately measure how much they've managed to slow down Didymorphos as it's going in orbit around Didymos? But in order to know how much of a change you've managed to make, you really need to know what the mass of the object is, and that's something that Hera will be able to provide. So we'll be doing lots of investigation after the fact, really getting to grips with this, this and also it will just really help us investigate a binary asteroid system. It will be the first time that a space mission has got a close up view of a binary asteroid system, but they make up about 15 percent of the asteroid population out there. So if we want to understand, you know, how these things formed, you know, why are these two close together? Did they originally seem to be the same thing and then a bit broke off? Or were they two completely separate items that just happened to get close together and ended up in orbit? All of these things will really help us understand a lot more about how the asteroid belt operates and even, you know, going back into how our Solar System was formed, because that's one of the real reasons why people are scientifically interested in asteroids, they have a kind of like the grab bag that's left over from the formation of the Solar System. And so by understanding them, we can understand much more about our solar system. And that's why, in fact, DART and Hera aren't the only missions that are going to be heading to asteroids.

Neil Yes. Lucy is a NASA's spacecraft that will explore the asteroids orbiting alongside Jupiter. It launched on 16 October and is already on its way to study the Trojan asteroids. These are asteroids that share Jupiter's orbit around the Sun, even 60 degrees ahead or 60 degrees behind the giant planet. And it's thought to be at least a million of them larger than one kilometre in diameter. The aim is for Lucy to conduct a flyby of the asteroid DonaldJohanson of the main belts in April 2025 and then in 2027, Lucy will arrive in the leading cloud of Trojans, studying four of them in detail next in 2031. The spacecraft orbit the second largest Trojan Patroclus which is accompanied by a smaller companion known as many. It will study the composition and surface characteristics of these dark, puzzling objects, hopefully shedding light on their origin and on the early evolution of our Solar System. Then there's Psyche, which is another NASA mission to learn more about asteroids. But this will just focus on one – its namesake, a 220 kilometre diameter space rock made mostly from iron and nickel, a so-called M-type asteroid. The launch of Psyche is scheduled for August 2022 on a Falcon Heavy rocket, with an expected arrival date at its target in 2026. The asteroid Psyche, named after the Greek goddess of the soul, is one of the 10 most massive known asteroids due to its metal content, and its surface is around 90 percent metal. It is thought that Psyche is likely to be the exposed metallic core of a protoplanet that lost its silicate mantle in a collision with another asteroid billions of years ago. If this theory is correct, then the mission will offer a unique way to look at the exposed core of a Solar System body.

Ezzy So of course, we have all of those missions heading towards distant asteroids, which have made and a respectful distance from Earth. But not all asteroids do that sometimes. Asteroids do come much closer to Earth, and in fact, there's about to 27,000 what are called near-Earth objects known. And these are space rocks that come within about 1.3 AU, or one point three times the Earth sun distance, about 99 percent of those are asteroids. And the reason why we know how many of those are is because a lot of people have been paying a lot of attention to them and trying to make sure that we know as many of them as possible. And in fact, in 1998, NASA's stated that it wanted to discover 90 percent of near-Earth objects that are larger than a kilometre, and by 2005, Congress had actually changed that to 90 percent of the ones over 140 metres. But why those particular sizes? Well, it's because if they are coming that close to Earth, there is a risk that they might one day hit Earth. And the size that an asteroid is, the amount corresponds to how much damage it could potentially do to us here on Earth. And we don't particularly want to go the way of the dinosaurs. We would quite like to if we if we can find a way to avoid being hit by an asteroid, I think we should. And what is generally said is that asteroids over about a kilometre, that sort of size, those are the ones where you're talking about global consequences, massive changes to climate being thrown into a nuclear winter, that sort of thing. That was about the size that that killed the dinosaurs was a kilometre asteroid. However, an asteroid that's over about 140 metres, ones of that sort of order, those are big enough to take out a city. You know, they would cause significant localised damage. So we definitely want to know where those ones are. And so what a lot of these searchers have been doing is taking regular pictures of the night sky and then if you compare a picture that's taken one night the next night because asteroids are fairly slow moving, but they do move across this night sky. It's one of the things we talk about quite often in the sky guide is how you can find these asteroids and track them progressively overnight. And that's what these surveys do. Things like the Catalina Sky Survey in Arizona, PanSTARRS in Hawaii and even the Space Watch that actually found Didymos in the first place. They take all of these repeat wide-field pictures of the night sky and look for bright spots that are moving from one night to the next. And those are asteroids, and we now think that there are about 2000 what are called potentially hazardous asteroids. So these are ones that come with within 1.5 million kilometres of Earth so close enough that it's a bit worrying and we want to make sure we know where they are and are over 140 metres because that's the sort of size when are you going to start causing significant damage. That said, you know, smaller asteroids can still be a problem. In Chelyabinsk in 2013, you might remember on the news that there was a massive explosion heard overhead, and that was an asteroid that was probably only about 20-30 metres across, exploding in the atmosphere. However, trying to find all of those is probably a bit of a big ask. But the larger ones we can definitely find. But finding an asteroid is just one part of the problem, there's not much point. Finding these, you know, potentially hazardous or if you can't do anything about it and there's lots of people who've been thinking about how you might possibly be able to slow down an asteroid, there's been entire movies written about it. You know, Armageddon, Deep Impact. All of those sorts of things are looking at how you might possibly, you know, use nuclear bombs and stuff to blow them, of course. But actually, we think the most effective way is to do what DART is doing, which is just slow it down. If you managed to catch an asteroid in enough time when it's far enough away from Earth, even slowing it down, by, I think it was a millimetre per second that DART is going to slow down Didymoon. I'm sorry, I called it Didymon, which was what Didymorphus used to be called. And I think it's a much better name, but it's not. It's prominent, so it's going to slow down, Didymorphus by about a millimetre per second and over about 10 years even that tiny change would be enough that by the time the asteroid got to Earth, Earth had been moved to a different place of its orbit. So Earth isn't there anymore, and there's no risk of a collision. And there's, in fact, a project that the thinking about so building something called HAMMER, which would be a much larger version of DART that would be ready to just send off and go and smash into a big asteroid that's on its way. And it would be capable of deflecting a large asteroid with a couple of years notice or a 30 metre asteroid if there was only a couple of months notice. Because, you know, it's easier to move smaller things than it is to move large things. So it's definitely it's one of those things that people have been worried, quite rightly, about for a while. It's probably even though the risk of it happening today is low. Eventually, the Earth is going to get impacted by an asteroid. But thankfully, because of people like DART and Hera and all of these space watch missions out there, the risk is significantly lower than it might be. Otherwise that will at least know it's coming.

Announcer Now it's time for the Stargazing tip of the month

Neil The Geminid meteor shower is amongst the year's favourite observing events. As with any meteor shower, it success is dictated by the weather and the face of the Moon. This year, the shower reaches its peak around seven a.m. on the 14th of December, making the nights of the 13th, 14th and 14th and 15th. The best ones for viewing it. The Moon is in an advanced waxing phase around this time. 78 percent illuminated in a waxing gibbous phase on the evening of the 13th and setting around three a.m. on the 14th. Although this isn't ideal, the long, dark December nights mean that there will still be around three hours of darkness left to enjoy what the shower has to deliver in the early morning. The Geminids has an excellent zenith hourly rate of 140 to 150 metres per hour. The best of observing advice to spot the Geminid this year will be to get some sleep on the night of the 13th–14th. Setting your alarm clock for about two a.m. in the morning to wake up, then prepare yourself with some warm clothing and perhaps a hot drink and a flask and head out and a sun lounger is an ideal observing location.

Ezzy So that's it from us this month. You can read more about the DART mission in the December issue of BBC Sky Night magazine, where we also take a tour around the Midwinter Milky Way. Find out what quantum soup and nuclear pasta are and how they relate to neutron stars. And take a look at how the constellation Boötes has got his hunting dogs. And that's not forgetting our regular sections that will help you unlock the wonders of the night sky, find the right equipment to observe it with and discover the best things to see after dark this month from all of us here at BBC Sky at night magazine. Goodbye.


Announcer: Thank you for listening to this episode of the Radio Astronomy podcast from the makers of BBC Sky at Night Magazine, which was produced in our Bristol studio by Brittany Colley. For more of our podcasts. Visit our website at or head to Acast, iTunes or Spotify.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.