It’s a big solar system out there, and it’s filled not only with planets, dwarf planets, and moons, but also with hundreds of thousands smaller chunks of rock and ice. Most of these asteroids and comets orbit the Sun without ever coming near Earth, but impacts can happen. For decades, NASA has been on the lookout for any asteroid that could cause our planet harm and, in the thousands of objects found, determined that none poses a threat to Earth. Tune into this week’s Gravity Assist to learn about how we hunt for asteroids and comets that might threaten Earth.
Jim Green: Our solar system is a wondrous place, with a single star, our sun, and everything that orbits around it, planets, moons, asteroids, and comets. What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos, with billions of other solar systems.
Hi, I’m Jim Green, NASA’s Chief Scientist, and this is Gravity Assist.
Today I’m here with Lindley Johnson, NASA’s Planetary Defense Officer. He’s in charge of keeping our home safe from countless numbers of small bodies and debris that hit our planet, or even fly by our orbit. You know, the dinosaurs didn’t have a space program, as we say in this business. And, it’s not a matter of if, it’s only a matter of when.
And, so Lindley, you’re our first ever planetary defense officer. So, for our listeners, what’s the difference between an asteroid, a meteor, and a meteorite?
Lindley Johnson: Okay, well that’s a common question that gets asked. It can be a little bit confusing. But, the difference between an asteroid and a meteoroid is simply the size. The International Astronomical Union just recently laid out the distinction between the two and everything that’s smaller than one meter in size is a meteoroid and everything bigger than a meter in size is called an asteroid.
Now, when a meteoroid, or even an asteroid, enters Earth’s atmosphere, you see this blazing trail across the sky. That’s what was seen in ancient times, but they didn’t know exactly what it was, and that was called a meteor.
Now, once the object passes through Earth’s atmosphere, it breaks up, and pieces fall to the earth’s surface, to the ground, and those rocks, so to speak, are called meteorites, and those are what scientists like to collect as soon as they can after a fall to do a scientific analysis of what is essentially a free sample return from space.
Jim Green: All right. You know, it’s not just scientists. You know, more recently, they looked at a dagger that came from [King Tutankhamun]’s tomb and it was iron. And, they did the analysis of it and it turned out it had a certain iron nickel ratio, which indicated that it was not of Earth origin.
Lindley Johnson: Yeah, that’s true. In fact, archaeologists have found a number of artifacts that look like they were made out of meteorites. The iron meteorites are pretty much able to just about survive entry through the Earth’s atmosphere, and probably to the ancients, that was their one source of metal.
Jim Green: Right, you know, and the connection with space and, perhaps, their religious gods.
Lindley Johnson: Uh-huh.
Jim Green: Yeah, indeed. All right, getting back to today, you know, what’s the first step in protecting Earth from these kind of strikes?
Lindley Johnson: Well, you’ve got to find them first. You’ve got to be looking for them, find out what’s out there or what may come close to Earth’s orbit, and you need to find them well in advance of any close approach or impact to the Earth, ‘cause it will take us time, years in fact, to be able to get out there and do something about them before they become an impact hazard to the earth.
Jim Green: Is that because they’re so small? I mean, they’re so much smaller than planets and moons.
Lindley Johnson: Well, that has--that certainly makes it more difficult to find them, but it’s just simply the time that it takes to get a spacecraft out--to get it built, first of all, and then to get a spacecraft out to the object to be able to interact with it in some way, to change its velocity. That’s the main principle behind mitigating an impact, is just simply changing the velocity of the object so that at the predicted impact point time--in time, the object shows up late.
Jim Green: How are we able to see these relatively small objects? Aren’t they largely very dark and black?
Lindley Johnson: Yes. The--there is certainly a population of these objects that is very dark, as dark as coal, so that makes them very difficult to see in the visible part of the spectrum, which is where we are mainly searching right now using ground base telescopes. But, that’s one reason why we would like to be able to search for them in the infrared part of the spectrum, because these objects are illuminated by the Sun and they absorb the heat from the Sun and then reemit that heat as radiation that can then be detected in the infrared part of the spectrum.
The catch to that, though, is you have to have a sensor that is looking for them in space, because they are so small and dim that the energy can’t make it all the way through the Earth’s atmosphere. It blocks it out.
Jim Green: Now, we currently have one telescope in space that is in the infrared.
Lindley Johnson: Yes, that’s right. We have a spacecraft, which was originally called the Wide Field Infrared Survey Explorer, launched by [NASA’s] astrophysics division, to build up an infrared background of the sky. And, it was constantly imaging the sky to build up this map of the sky.
Well, we quickly figured out that with all the images that were--it was taking, we could look for an asteroid moving across the sky in those images. And, it became a prototype capability, so to speak, for an infrared telescope. After the astrophysicists were done with it, the planetary defense program here at NASA took over operations of the spacecraft and we’ve made it a full time asteroid hunter and for characterization of them as well, of figuring out their size.
Jim Green: You know, with all the telescopes now we have on the ground that are looking for them, and as you mentioned, NEOWISE out there in the infrared band looking for them, how many asteroids are out there that are what we call near earth objects?
Lindley Johnson: Well, so far, we have found a little over 18,000 asteroids--of all sizes that come near Earth’s orbit. But, we think that’s a very small part of the overall population. Right now, our tasking is to find the 140 meter and larger asteroids that come near Earth’s orbit. 140 meters, that’s about 400 to 500 feet across.
So, we have several ground based projects to do that. But, our prediction on the population of those objects is--140 meters and larger in size, is 25,000.
Jim Green: Wow.
Lindley Johnson: So far, we’ve only found a little over 8,000 of those in the 20 years that we’ve been searching so far. So, it’s a very complex and complicated job.
Jim Green: Yeah, so this is a slow and steady process, but it’s one of those that we’ve got to keep doing each and every day, is finding these objects. And, then you analyze their orbits to determine if they’re really threats to Earth. So, how many out there are really threats to the Earth?
Lindley Johnson: Well, that’s right, we have to take several observations over a course of time to be able to determine their orbit and whether they’re, sometime in the future, going to come close enough to Earth to be an impact hazard. Right now, with the known objects, there are none that have a significant possibility of impacting the Earth. There are several that come very close and if their orbits were to deviate off of our current predictions for them, they could be impact hazards.
So, there is a subclass of near Earth objects, which we call the potentially hazardous asteroids, that we have to more closely monitor and keep track of those. We now have about 1,900 of those objects that we closely monitor.
Jim Green: Well, let’s talk about one that flew by the Earth pretty recently. It’s called Asteroid 2010 WC9. Yeah, so tell me a little bit about that.
Lindley Johnson: Okay, well, by the designation, we know that this object was found originally in 2010. That’s the 2010 part of the designation. The WC9, then, is--tells us which month it was found it. It begins with a W, so we know that that is at the latter part of November of 2010, and then 9 is a sequential number. It’s kind of a complicated designation system, but it works for the astronomers.
Jim Green: Do we know if it was observed first by WISE or by ground base, or how do we know?
Lindley Johnson: No, this was first observed by the Catalina Sky Survey in Arizona and just a few observations were able to be taken of it in that time period, in late November, first couple of days in December of 2010. So, we didn’t have a very good orbit on it yet at that time, and in fact, we couldn’t project the orbit out more than a year or so with any degree of certainty.
Jim Green: So, this close fly by is going to really give us a lot more information about it.
Lindley Johnson: Oh, sure. So, we knew it would have another close approach with the earth about this timeframe, but we didn’t know how close. In fact, the uncertainty in timespan of when the close approach would happen was 18 days.
Jim Green: --Wow.--
Lindley Johnson: Yeah, object moving at the speed it does, it covers a lot of distance in 18 days. The Catalina Sky Survey, though, again reacquired this object on the 8th of May and started taking observations on it again. The Minor Planet Center, where all the observations from around the world go to, was able to quickly correlate those observations with the orbit that they had on this object found back in 2010, 2010 WC9.
So, now, we’ve greatly expanded the orbit span--the observation span on this orbit, and can much more accurately predict its orbit.
Jim Green: Yeah, better predict when it comes by the earth again.
Lindley Johnson: Right.
Jim Green: One of the really neat things about this object is that it passed under the earth and the moon. I thought all these objects had pretty much Earth-like, or plane-like orbits.
Lindley Johnson: Well, the vast majority of them do. They’re close to what we call the ecliptic, the orbital plane of the Earth, in the solar system. But, there are quite a few of them that are on a more highly inclined orbit. Their orbit plane intersects the Earth’s orbit plane at an angle that we call the inclination. For this asteroid, the--its inclination of its orbit is 18 degrees, which is fairly high.
Jim Green: Yeah.
Lindley Johnson: It makes it a little more rare.
Jim Green: How close to the Sun does it get?
Lindley Johnson: It actually goes in almost as close as Venus’s orbit to the Sun and then goes back out to the main belt area--
Jim Green: --Wow.--
Lindley Johnson: --beyond Mars.--
Jim Green: --Highly elliptical.--
Lindley Johnson: --That’s probably where it originated at some point and some encounter with Mars or Jupiter along the way put it into this--
Jim Green: --Gravitationally? Uh-huh, right.--
Lindley Johnson: --Yeah, gravitational assist, put it into this more highly acclaimed--inclined orbit.
Jim Green: Yeah, so Mars and Jupiter in particular are throwing these objects out of the asteroid belt. So, we--this is one of the ideas that tells us that we always have to be monitoring our skies and constantly looking, because the source region, the asteroid belt, is full of them.
Lindley Johnson: Yeah, that’s right. It’s probably the remnants of a planet out there that either didn’t form or got pulled apart by Jupiter, one of the two. And, so Jupiter is always causing disturbances in the asteroid belt and either flinging objects out of the belt, and out of the solar system in some cases, or pushing them into the inner solar system.
Jim Green: You know, one of the other ways we use to not only characterize them, but get a better orbit, is through radar. How do we--how does that happen?
Lindley Johnson: Well, we first have to find them optically. The radar is not--we don’t have powerful enough radar that can sweep the skies and detect these objects. We have to have a good enough orbit that we know when to expect the energy back that the radar bounces off of the object, when to know that energy will return, because we’ve got to dig it out of the noise. If we didn’t know where to look, we wouldn’t see it at all.
So, that’s the first thing, is we have to get a good enough orbit on it with optical telescopes so that we know where to aim the radar and when to expect the return.
Jim Green: You know, it also tells us a lot about its size and characteristics.
Lindley Johnson: Right, with enough energy back bounced off the object, we can do what they call radar imaging. It’s a little different than optical imaging, but it’s sort of the same principle, and we can get a good indication of the size, much more precise than we can optically, and also its spin state, how fast is it rotating. And, the other thing that radar does for us is determines whether these objects are binaries or even have more moons. We know of several binary asteroids now, an asteroid with a small moon circling it, and that’s all been done by radar.
Jim Green: What’s really neat about that, too, is if they get close enough to the Earth and the radar hits them, you can actually see features on their surfaces.
Lindley Johnson: That’s right, you can see craters or large boulders on the surface. It’s really very interesting to see these radar images come back.
Jim Green: I mean, the concept that these things are still accumulating material, you know, and having--they’re strewn with boulders on the surface. You know, we have a specific name for that.
Lindley Johnson: Yeah, it’s accretion of matter and these are the remnants of the construction of the solar system. These are the building blocks and, you know, you walk around any construction site, you’re going to see all this debris around. Well, that’s what the asteroids and comets are, debris from the construction of the solar system.
Jim Green: Yeah, actually we call them rubble piles.
Lindley Johnson: Well, that too. Yeah.
Jim Green: You know, what kind of damage, if one of these larger objects would make it through the atmosphere and hit the Earth, would we expect?
Lindley Johnson: Well, it certainly depends on the size of the object. This object, 2010 WC9, is estimated to be between about 50 meters and 120 meters in size. That could be a very damaging impact. For instance, the object believed to have created Meteor Crater in Arizona is estimated at only 50 meters in size. So, that’s at the low end of our estimate of the size of this object. It would devastate a state-wide area if it were to impact.
Jim Green: But, you know, objects that are half that size, maybe 20 meters, do they make it to the surface?
Lindley Johnson: Well, pieces of them certainly will. It depends on how strongly they’re composed. If they are the average rocky asteroid, they will disintegrate in Earth’s atmosphere. That, of course, recently happened in February of 2013 over Chelyabinsk, Russia. Object about 20 meters in size entered about 9 a.m. and detonated about 23 kilometers above the surface. The energy release was equivalent to about a half a megaton of energy.
Jim Green: You know, I remember that day really explicitly. I was being interviewed on a variety of TV shows about another asteroid that was flying by the Earth, completely disconnected, and when this happened, we were all scrambling to figure out what was going on.
Lindley Johnson: Well, I remember that day well too, because, you know, we had predicted that 2012 DA4 was going to have a close pass by the Earth.--
Jim Green: --And, that’s the one we were talking about.--
Lindley Johnson: Yeah, just outside of geosynchronous. And, actually, my community, the planetary defense community, was at the United Nations committee on peaceful uses of outer space meeting that week to provide our recommendations on what we, as an international community, ought to be doing about near Earth asteroids. And, we thought this close approach was going to be our signature event for that week.
Jim Green: Little did you know.
Lindley Johnson: Well, you know, Mother Nature has its ways and Mother Nature just put an exclamation point on our report to the UN.
Jim Green: So, you know, as you go to these meetings that are international and you talk about these as hazards and mitigation strategy, what can we possibly do on Earth to prepare to respond to these objects and over a certain size range?
Lindley Johnson: Right. Well, as I said before, the first thing we’ve got to do is find them. We’ve got to expand our capabilities to find them far enough ahead of time that we have a chance to do something about them out in space. You know, any object that’s larger than about 50 meters in size, we’re going to want to find years in advance so that we have a chance to do something about it in space, because we just don’t want to suffer the impact from those objects.
And, so we also have to be looking at the technology that might be used. As I said, the principle to be used is you just need to change the speed of the asteroid by just a fraction of a percent to--several years in advance to make it a miss instead of a hit. It will not arrive at the same time that the Earth is at that point in space. So, that’s a principle to be used.
Now, if the object is small enough, just hitting it with a spacecraft, what we call a kinetic impactor, would be enough to change that velocity enough to slow it down. And, in fact, we’re working on demonstration of that capability now. It’s called Double Asteroid Redirect Test.
Jim Green: Or, DART.
Lindley Johnson: DART, yes. And, DART is about to enter its full scale development phase.
Radar images of the binary asteroid system Didymos, featuring the main asteroid and its “moonlet.” Didymos is the target for a future NASA mission called the Double Asteroid Redirect Test, or DART.
Credits: Aricebo Observatory
Jim Green: Yeah, that’s a really neat mission. Let’s talk about DART a little bit. You know, this is our first attempt to actually change, intentionally change, the orbit of an asteroid and it’s going to a particular one.
Lindley Johnson: That’s right. We’ve chosen the target asteroid, Didymos, actually the asteroid system, Didymos, because Didymos is a binary asteroid. It is about a half a mile acros—the primary—and it’s orbited by a moon that is only about 300 feet across. So, with the DART spacecraft, what we’re going to do is impact the moon and change its velocity and its orbit about the primary. So, that will change its orbit. And, we can observe that from the ground, from ground base telescopes, observe the change--
Jim Green: --And, radar.--
Lindley Johnson: --of the moon’s--of the moon, yeah both optically and radar. Of course, radar will give us some more precise measurements and provide us the data that more precisely shows how much force we were able to impart on this moon.
Jim Green: Yeah, that’s a really neat idea. If you hit that moon, you ought to be able to detect the change in its orbit much quicker than if that was a single asteroid orbiting the sun.
Lindley Johnson: Yeah, that’s right. And, the other thing about it is that we’re not changing the orbit of this whole system around the Sun, so we’re not increasing the danger to Earth of this asteroid.
Jim Green: You know, there are some things that, you know, we just can’t do much about. You know, climate gives us hurricanes and tornadoes, you know, and there may be things in the future which, like asteroids, that come in and impact the Earth that means we wouldn’t be able to move it in time. What are the things that we’re doing now that could help everyone?
Lindley Johnson: Well, that’s right and we are working with the interagency community, the US government, to better prepare all of our activities if an object were detected on an impact trajectory. Yes, it might be something on very short warning and that we’re not able to do anything about it in space. NASA’s job, of course, would be able--would be to warn of it and predict how bad the effects could be based upon our observations of its size and composition.
And, we provide that information to other agencies, like the Federal Emergency Management Agency, who would then be responsible, if it were to impact--predicted to impact a US territory for the safety of lives and infrastructure.
Jim Green: Yeah, makes a lot of sense that we work with FEMA, who’s a real expert in the ability to put out warnings and get the state and local governments to be able to help in getting the information out and people moved.
Lindley Johnson: Yeah, it’s been a real unique opportunity to work with FEMA. And, FEMA, although that, of course, is their job, this provides--this is a little unique thing for them. This is probably the only natural disaster known of that we could predict, not only minutes but days, weeks, or even years in advance. And, so they sometimes wonder, “What are we going to do with all that time?”
Jim Green: Well, we’re going to get people out of the way if it does happen then. But, of course, that’s just one of several things. You know, as you say, the earlier the warning, the better. Are there other approaches that we’re modeling or thinking about?
Lindley Johnson: Right, well there are at least a couple of other techniques we call them that we think would be effective against an asteroid. Another technique that we’ve done modeling of and some development of the capability is called a gravity tractor. If you have enough time and the asteroid isn’t too large, all you need to do is nestle up to it with a spacecraft and stand off an orbit with it, and the mutual attraction between the spacecraft and the object, by gravity, will slowly tug that asteroid off its natural orbit into a new orbit.
We’re just using nature’s tug rope, gravity, to rug the asteroid into a more benign orbit. That would take several months to years to do that. But, the other nice thing about it is that we can put it into the new orbit, into the exact new orbit, that we want to. We could tug on it a little while and then take measurements to see if we’ve changed the orbit enough, and if we need to do more, then we tug on it longer.
Jim Green: You know, it’s really amazing to think that just a few decades ago, we didn’t really realize that--how big of threats these kind of things could be in our future. But, we’re really evolving.
Lindley Johnson: That’s right. You know, 40 years ago, the astronomers didn’t know that there were very many objects in orbits that could cross the Earth. In fact, when the near Earth object program started at NASA, back in 1998, there were only 500 asteroids that were known that came inside of the orbit of Mars. Now, we have 18,000 in our--
Jim Green: --Right, and still looking.--
Lindley Johnson: --And, still looking, and that’s probably less than 5 percent of the population out there that, you know, could do damage to the earth’s surface, where they impact.
Jim Green: You know, the more we work in this area and the ones we find and then announce, it gets picked up a lot by the press and, of course, we’re interested in being open, transparent in telling everyone what’s happening, but what do you think is the most common misconception that you see that the media or press do when they talk about these asteroids?
Lindley Johnson: Well, I think because it is something that really sparks the imagination and, you know, there have been these movies about asteroid impacts and those kinds of things, is that every asteroid that we announce that’s going to have a close approach, it sets off a concern that, “Oh, is this asteroid going to impact us?” So, I don’t think they understand that once we have observations on the object and can establish its orbit, it’s going to stay on that orbit.
Jim Green: --Right.--
Lindley Johnson: It, you know, is just not going to wander around the solar system randomly. This is all controlled by the laws of nature, the laws of orbital mechanics, and once you’ve established a stable orbit, you can predict where that object is going to be many, many years in advance. Our current capabilities, modeling capabilities, we can confidently give it enough observations, predict the orbit out 100 years into the future, to determine whether the object is ever going to be a threat to the Earth.
Jim Green: If our listeners could walk away with one lasting message, what would it be?
Lindley Johnson: Well, I think the message is that asteroids are--can be a hazard to the Earth, can--are still impacting the planets. But, we now have the technology and capabilities to find out if any of them are going to be a threat to us and the spacecraft technology to go out and do something about it and prevent it. This is the only known natural disaster that we know that we could prevent if we know about it far enough in advance.
Jim Green: Yeah, that’s really neat. You know, one of the things that I ask each of my guests in this program is what was their gravity assist? In other words, that activity or event that really occurred in their past that propelled them forward and allowed them to become the scientist and engineer they are today. Lindley, what was your gravity assist?
Lindley Johnson: Well, gee, I’ve had so many of them in my career. I really have. I’ve been lucky and some might say, been in the right place at the right time, on several occasions. I think the Apollo program, when I was growing up, got me interested into space and being part of the space program. That, in turn, got me into interested in being in the Air Force and being part of the Air Force’s space program. That gave me the capabilities, the skills, because of the opportunities I was given and the trust that was given to me to really develop my capabilities for program management and looking forward into the future with these kinds of things. Then, coming to NASA, 15 years ago now, and being put in charge of this program, this near Earth object program, called back then, and also working with the solar system exploration missions have all given me the skills and the background to be NASA’s first planetary defense officer.
Jim Green: And, Lindley, I’ve known you for a number of years and I can attest that you are absolutely the right person for this and I am so delighted that, indeed, we can look to you as our officer of defense for this planet, and I want to thank you for all the hard work you’ve done over the many years.
Lindley Johnson: Yeah, well thank you for this opportunity and the years I’ve worked with you in the planetary science division. It’s been a real highlight of my life.
Jim Green: Yeah, I’ve enjoyed it, and it has been one of mine, too. Well, join us next time as we continue our exploration of the solar system.
I’m Jim Green, and this is your Gravity Assist.
Last Updated: July 17, 2018
Editor: Gary Daines
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