Using the team-oriented messaging app Slack, experts from the University of Arizona dissect asteroids. The transcript of their conversation has been lightly edited.
Who are we talking with?
Tim Swindle (tswindle): Tim Swindle is the director of the UA’s Lunar and Planetary Laboratory and a professor of planetary sciences. Swindle’s research focuses on the use of noble gases in extraterrestrial materials—including meteorites—to study the evolution of the solar system.
Bob McMillan (bobmcmill): Bob McMillan is the principal investigator of the Spacewatch Project at the UA’s Lunar and Planetary Laboratory and Steward Observatory. McMillan’s main expertise is in astrometry, the measurement of the positions and motions of celestial objects.
Ed Beshore (ebeshore): Ed Beshore is a senior staff scientist at the Lunar and Planetary Laboratory and the deputy principal investigator of NASA’s OSIRIS-REx, a UA-led mission to visit a near-Earth asteroid and return a sample to the Earth. Beshore is interested primarily in instrumentation and planetary astronomy.
Emily Litvack (elitvack): Emily Litvack is the science writer for the UA’s Office for Research & Discovery. Here, she serves as moderator.
elitvack: Hi guys! Glad to have you with us for our first ever Slack Chat. Gonna be a good one! Let’s get right into it. It’s Asteroid Day, and so I wanted us to address what exactly we think is the real risk of asteroids these days. Should we start with the 1908 Tunguska impact event, since Asteroid Day falls on its anniversary? What happened there, and do we expect any similar impact in the near future?
ebeshore: In 1908, an asteroid or a cometary fragment exploded over Siberia and flattened 80 MILLION trees.
ebeshore: Fortunately Siberia is pretty uninhabited, so it was not a major disaster, but it was a sobering reminder that the world gets hit by asteroids from time to time, and that the consequences could be high.
ebeshore: I'll leave it to Bob to mention if there is anything dangerous looming on the horizon. He is full-time looking for hazardous asteroids.
bobmcmill: Several times per month there are close approaches of small objects, some of which get detected by the surveys like Catalina and Pan-STARRS.
tswindle: Those small objects that Bob mentions are not generally the kind that are going to cause a Tunguska event if they hit, they're the ones that are going to make a spectacular fireball and leave some meteorites behind. There are a lot more of those small ones than the really dangerous big ones.
tswindle: The way I think of it is that the chances of an asteroid injuring someone today are almost zero. The chances of an asteroid injuring someone in the next couple of centuries is almost 100%. The chances of a devastating asteroid hitting in the next few million years and killing millions or billions are nearly 100% if we do nothing, almost zero if we deal with it in a logical way.
ebeshore: I agree with Tim. This is a low-intensity threat. The great success of the asteroid surveys (2 of the leading ones are here at the U of A) is that the RISK from an asteroid impact has been much better characterized.
ebeshore: What I mean by risk is a metric that combines probability and consequence. A 6-mile wide asteroid has a huge consequence, but is a VERY rare event. Fortunately, surveys like Spacewatch and Catalina have served to help us understand that the probability of a large object hitting us in the near future is pretty low.
elitvack: So, to me, it sounds like catastrophic impacts from asteroids aren't something we really need to be worried about any time soon. But why so unlikely? There are millions of asteroids in the asteroid belt.
tswindle: Bob can give a more detailed answer. Quick answer: there's an enormous amount of space out there. When the first spacecraft took an image of an asteroid as it went through the Main Asteroid Belt, it had to be targeted to get close enough to see one, rather than having to dodge asteroids like in Star Wars.
ebeshore: I think the risk (recall probability and consequence) from a small object hitting the Earth is smaller than other risks that the human race faces every day.
ebeshore: Take for example disease. Disease killed 27 million people in 2012 alone.
ebeshore: If you line up the worst natural disasters of each type in the 20th century (Earthquake, heat waves, etc.), those killed 6 million people.
bobmcmill: Another comparison: when you step on commercial airliners, you're betting 100,000 to one that you'll make it on all your flights.
bobmcmill: Clarification of terminology: "retiring" vs. "mitigating" risk: Discoveries & observations of asteroids retire previous statistical estimates of risk by adding specific asteroids to the inventory along with knowledge about their orbits.
bobmcmill: Actually mitigating risk would require doing something to a potential impactor to deflect its path.
ebeshore: The question now is, what do we do? I think we continue to look for asteroids, but from a public policy perspective, the risk from an impact is on the order of other risks that the human race faces. So how do we spend our resources to deal with that risk?
ebeshore: I fear that some of the discussion in recent years has gotten a bit hyperbolic, and we need to place the asteroid impact risk in the context of those other natural disasters.
bobmcmill: The probabilities of impacts listed at the JPL web site are much longer odds, 10 million to 1 billion to one typically.
elitvack: Sounds like low probability, high consequence is a key point when it comes to large asteroids impacting Earth.
bobmcmill: To answer, "what should we do?", I'd say apply the technology we have to inventory the asteroid population down to a practical size limit.
ebeshore: Yep - just remember that one cyclone in 1970 killed 500,000 people. So how do we spend our next dollar of money? Better weather prediction, better evacuation plans, emergency response teams
ebeshore: I agree with Bob - a modest amount of money can continue to discover and monitor asteroids and push down the threat from an object hitting the Earth by surprise.
tswindle: I also agree with Bob on the "what should we do." And remember that in getting that inventory, we're also learning something about the solar system now, and we're finding potential targets for exploration, either by robotic spacecraft or by astronauts.
ebeshore: Yes - the understanding of the distribution of asteroid orbit and sizes has told us some interesting things about the Solar System. We have gotten that knowledge from the surveys that are looking for hazardous objects
bobmcmill: We're betting that the technology to find smaller & smaller asteroids will improve soon enough before a dangerous impact, which is infrequent enough not to get panicky.
elitvack: I want to go back to @bobmcmill's comment about mitigation. Step one is to know what you're dealing with, and step two is doing something about it. Specifically, I'm thinking about Bennu and the OSIRIS-REx mission. In addition to my curiosity about the "mitigation" aspect of it, OSIRIS-REx presents some opportunities to create new knowledge. Wanna hear your thoughts.
tswindle: There have been a lot of mitigation strategies suggested, but until now, they've been in the realm of science fiction or back-of-the-envelope calculations. OSIRIS-REx is providing some real data about what at least one Near-Earth asteroid is like. Other missions will follow testing specific mitigation strategies.
ebeshore: Perhaps most importantly for science is that OSIRIS-REx will bring back a sample from a primitive, carbonaceous asteroid. Getting the opportunity to study that sample in our modern Earth-bound laboratories will be an important milestone.
tswindle: The "primitive" part is important because we really don't know where the "volatiles" on Earth, like water and the organic material, came from, and we're kind of fond of that stuff.
ebeshore: Important point: Our colleagues in Japan, with their Hayabusa mission were the first to bring back a sample, but OSIRIS-REx will bring back a LOT. At least 60 grams, and perhaps as much as a couple of kilograms.
ebeshore: But how will OSIRIS-REx help with understanding the asteroid threat? For one thing, it will investigate the Yarkovsky effect...
ebeshore: One thing we know about Near Earth Objects (NEOs) is that they don't last very long flying around the inner solar system. They eventually impact the Sun or a planet, or get thrown out of the Solar System completely. So where is the fresh supply of asteroids coming from?
tswindle: The Yarkovsky effect is amazing. It's something that hardly anyone knew about 30 years ago, but we now realize that it's part of the answer to that question of fresh supply. Who's going to try to explain the Yarkovsky Effect?
ebeshore: I can try. We believe that asteroids in the asteroid belt somehow “leak” out to become NEOs. But how? The Yarkovsky effect is the result of sunlight falling on the asteroids and being re-radiated as heat. That re-radiation can act as a tiny rocket thrust, pushing asteroids into zones where they can be affected by the gravity of the giant planets Jupiter and Saturn. Once there, the periodic push from the planets, like correctly-timed little pushes on a swing, can throw an asteroid out of the main belt and into the inner solar system.
tswindle: The thing about the Yarkovsky effect is that it's a small effect at the time, but over thousands or millions of years, the effect can get an asteroid into a completely different orbit. We can't usually measure that small effect, unless we have very, very precise measurements of where an asteroid is over a few months or years’ time. With OSIRIS-REx, we're going to park a spacecraft next to it, so we'll be able to get these phenomenally precise measurements of where it is.
ebeshore: Bob, if I recall, the Yarkovsky effect is the top uncertainty that prevents us from predicting the orbits of asteroids far into the future. Is that correct?
bobmcmill: Yes, it's a big effect on long time scales.
ebeshore: So if OSIRIS-REx can help us understand the details of the Yarkovsky effect, then our predictive power may increase, and help us better quantify the threat from specific asteroids.
elitvack: So the asteroids experience heat radiated from the Sun, which occasionally pushes them closer to planets like Jupiter and Saturn, which occasionally push the asteroids out of the asteroid belt entirely. But that all doesn't happen fast?
bobmcmill: The Yarkovsky migration itself is slow, but the impulse kicking out of the gravitational resonance is "fast" by cosmic standards.
ebeshore: Yes - these are things that OSIRIS-REx will allow us to directly measure at Bennu.
elitvack: Alright, so with OSIRIS-REx we're getting pristine sample (and lots of it) from an asteroid to study molecules that are precursors for life AND we'll get some precise measurements of the Yarkovsky effect. Sound about right?
ebeshore: For sure.
elitvack: I'm wondering why we picked Bennu. Lots of NEOs out there.
ebeshore: Bennu had to meet a lot of criteria to get picked. 1) We had to have an asteroid we could get to with available boosters. 2) we had to select an asteroid whose orbit would present a mission opportunity within specific timelines 3) we wanted a carbonaceous asteroid and 4) One that wasn't spinning so fast that might have thrown off its sampleable regolith
elitvack: Okay, I see where you're going. The list of ideal asteroids is quickly shrinking.
tswindle: If you want to go to a carbon-looking asteroid in an orbit that's easy to get to, it turns out that the hundreds of thousands of asteroids quickly collapsed to a list of just a few.
ebeshore: Yes - a goldilocks problem.
elitvack: We first started talking about asteroids today in terms of the kind of risk they do (or don't) present. What's that look like with Bennu?
tswindle: The same things that make it one of the easiest asteroids to get to make it one of the most likely to hit Earth. But the odds are really, really low.
elitvack: Really, really low odds, yet some of the highest of any known NEO? Am I right?
ebeshore: JPL says about 1 in 2,700 chance of an impact over the next couple of centuries, or a 99.96300000% chance the asteroid will miss the Earth
ebeshore: Yes - one of the highest. OSIRIS-REx should help us understand the risk in a much more definitive way.
elitvack: Doomsdayers, did you hear that?
ebeshore: Bennu should be more fun than fearful.
elitvack: Could be a silly question, but I'm going to ask it anyway, because that's my job. Could future efforts to mitigate risk of an impact look something like "blowing up" big asteroids that are on an impact trajectory?
bobmcmill: Blowing an asteroid up would probably be a mistake ...
bobmcmill: You'd have to keep track of all the pieces and where they're going ...
bobmcmill: the conventional wisdom seems to be to deflect it gently in a slow, controlled manner.
tswindle: If you've got one that is going to come close, but will probably miss, blowing it up would virtually guarantee that something's going to hit. If you can make sure that there aren't any pieces big enough to be dangerous, you're OK, but I don't think we'd be able to guarantee that.
bobmcmill: Slowly, so you don't change the shape or distribution of its separate boulders.
ebeshore: I have also heard compelling talks that suggest a nuclear device with a high X-Ray flux would be effective.
bobmcmill: The deflection process could take many years. But yeah, if you're out of time, a stand-off impulse from a nuke might work.
ebeshore: Exploded close to an asteroid, the X-rays would vaporize the top layers off the asteroid, creating a rocket-like thrust that would provide a deflecting impulse.
tswindle: That's just one strategy to deflect it gently, and one that's a little bit out there, at that. I've heard everything from running something into it (to change its speed) to parking a "heavy" spacecraft nearby.
tswindle: My favorite is painting it white or black so that the Yarkovsky effect will change, and move it out of the way for you...
ebeshore: I think Bruce Willis would prefer the nuclear device rather than a 500 year mission using a gravity tractor ;)
elitvack: PAINTING IT?! That's unreal.
ebeshore: Yes - consider the product placement opportunities....
tswindle: I think that one's out there as well, but we've got this effect that's moving things around based on how much light they reflect. Just change that, and the outcome is different. Then you worry about how well the paint will hold up in the sunlight.
elitvack: Wild! I do want to take a step back and ask, aside from impact threats, why we should be interested in asteroids.
tswindle: They're leftovers from the creation of the planets.
tswindle: The planets were made out of the current asteroids' brothers and sisters, so they're made of the same stuff. But unlike the planets, where the interiors melted and formed cores and crusts, and maybe volcanoes, the asteroids have mostly just sat there. That means we can find samples of the things that went into the planets, but got mixed and melted and stirred past recognition on Earth.
ebeshore: I hope that the discussion about the impact threat can lead to larger conversations about the Solar System, its origins, and the origin of life. Asteroids may well hold answers here, because compared to planets, they have led pretty quiet lives. That means they have been relatively undisturbed (perhaps) and may be the right place to answer some of the fundamental questions about our own existence here on Earth.
tswindle: For example, we really don't know where the water on Earth came from. One popular theory was that it came from comets (they're half ice, right?), but the composition of the water in comets is fundamentally different from that on Earth (it's the isotopic composition of the hydrogen, for those who think water is just H2O). Maybe the water is from asteroids like Bennu.
ebeshore: Yes – it’s interesting to consider that answers to some of the most basic questions are still unknown.
ebeshore: It’s also important to consider that some of the most fundamental research on asteroids happens right here at the University of Arizona. And it’s been going on for some time.
bobmcmill: The first observations of asteroids with electronic CCD imaging cameras were made here by Tom Gehrels in 1984.
tswindle: Tom Gehrels also edited the first scientific book about asteroids, in 1979.
ebeshore: Among the first to help raise awareness about the threat of asteroid impacts and the need to look for NEOs
tswindle: Besides finding about half the known asteroids, and having the OSIRIS-REx mission, the Lunar and Planetary Lab at UA has a long history of people studying meteorites as well. The late Mike Drake, who was the driving force behind getting the OSIRIS-REx mission originally, was a meteorite expert who was one of the first to figure out that we have meteorites from the asteroid Vesta, and later became one of the central figures in the discussion of Earth's water.
tswindle: UA has just bought a transmission electron microscope, to enable us to study things like meteorites or the Bennu samples at an atom-by-atom scale. Tom Zega, a relatively new faculty member (five years ago), will be leading that effort.
ebeshore: Arizona knows asteroids…
elitvack: I have one closing question. I think we should finish where we started. How worried should I be about asteroids?
tswindle: I don't lose sleep worrying about whether we're going to get hit by an asteroid. But I do sometimes dream of the things we can do with asteroids as we find them and study them.
ebeshore: In the words of the Hitchhiker's Guide to the Galaxy, don't panic.
bobmcmill: Addressing the impact hazard in a systematic way deserves a reasonable budget but not a panic.
elitvack: This has been AWESOME, you guys! Thanks so much for taking the time out of your day to join me.