Don Yeomans is a senior research scientist at NASA’s Jet Propulsion Laboratory, where he manages the Near-Earth Object Program Office. His latest book, Near-Earth Objects: Finding Them Before They Find Us, briefly chronicles the major asteroid strikes on our planet and the ongoing search to find others that might be headed our way. A review of his book will be published in an upcoming issue of Air & Space. Yeomans spoke with associate editor Heather Goss about the search for near-Earth objects.
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Air & Space: You explain in the book that this concept of “rocks from space” is relatively new, and that the astronomers who first encountered them called them “vermin of the skies.”
Yeomans: Oh yeah, that’s great.
Can you explain how our awareness of asteroids evolved, especially in the last couple of decades?
The first asteroid was discovered on January 1, 1801. They were expecting to find a planet between Mars and Jupiter, because there’s a bit of a gap and people were convinced there should be a planet there. When they found it they said, “Ah! We were right, there is an extra planet there.” It was called a planet for a while but then a few years later they found another one, and then a few years later yet another, and it became obvious pretty soon that there wasn’t just one “planet” between Mars and Jupiter, but several.
Even so, it was slow going for a long time. The first near-Earth asteroid was discovered in 1898 – Eros – and that was considered a bit of an oddity because back in the 1900s it was thought that the inner solar system was void of any [asteroid] material. Then, I think it was 1932, a couple of near-Earth asteroids were discovered one after another, Apollo and Amor, and so it was obvious that there are a few asteroids near Earth.
That was an era when they were using photography, and that really isn’t the most efficient way to find these objects. When astrophysicists were looking primarily at stars or galaxies, they would see these streaks of light that would be printed on their photographic film when they were taking long exposures, and they would curse them as the “vermin of the sky” because they disrupted their observations.
The discovery of near-Earth objects didn’t really take off until the 1990s, because that’s when [film] photography was replaced by CCD chips, very much like the chips that are in your camera or your cell phone. It wasn’t until the late 1990s that [the era of rapid near-Earth object discovery began when] NASA-supported observatories began using wide-field CCD-equipped cameras.
What is one of the most interesting things we’ve learned from the study of asteroids?
One thing they found is that some asteroids and all comets have organic material – carbon-based material. Comets [and some asteroids] have water-ice, and a lot of asteroids have hydrated minerals. So it’s very likely that the water and organic carbon-based materials that were resident on the early Earth and allowed life to form were probably delivered by impacts of these near-Earth asteroids and comets. Subsequent impacts by these objects, at a much reduced rate, punctuated evolutions, allowing only the most adaptable species to progress further, like mammals, so in a sense we owe our very origin and our position at the top of the food chain to these objects impacting the Earth a few billion years ago.
The subtitle of your book – Finding Them Before They Find Us – refers to this role that asteroids play in the destruction of life. How worried should we be that one of these things is going to come hurtling toward us any time soon?
I don’t think anyone should lose any sleep over this. But as we noted earlier, until the 1990s people just weren’t looking. It’s not as if these things weren’t zipping by every two weeks – they were – we just never saw them. So we were oblivious to the possible danger. Now NASA has been tasked by Congress to actually find and track them. The first goal was to find 90% of the near-Earth asteroids larger than a kilometer and track them, and we’ve done that. The kilometer was selected because the first step was to find the objects that could cause global problems. We’ve found almost 95% of that population so far, and none of them present a threat. Then Congress said, “NASA, good job, now find 90 percent of near-Earth objects 140 meters and larger.” After a few years, we’re 40 percent to reaching that goal. [The diameter] 140 meters was selected because that’s roughly the size of an object hitting the ocean that would cause a tsunami, or hitting land that would cause widespread regional devastation. We’re already starting to move onto the smaller ones, [because] anything above about 30 meters could cause a real problem; anything less than that, it wouldn’t likely survive through Earth’s atmosphere, it would just be a compressive fireball.
How do you make that calculation? If you’ve found approximately 40% of the near-Earth asteroids 140 meters and bigger, how do you know what the total number is in order to get a percentage?
It’s all done statistically. If our NASA-supported telescopes are searching every night for new near-Earth objects, let’s say over a ten-year period they find 900 objects larger than a kilometer. At the end of that ten-year period, it turns out that for every 10 objects they find, nine of them have already been discovered. Then you would say, alright, we’re at a point where we’ve found 90 percent of the population already, and if we’ve found 900 of them, the total population is likely to be 1000.
What is the best way to find near-Earth asteroids?
The ground-based optical observations are finding a lot of these objects. On the other hand, if you’re asking what way would be most efficient, then you would need a near-infrared telescope in space. Because these asteroids are dark, they absorb sunlight and radiate heat, mostly in the infrared, so they’re much easier to see in the infrared than in the optical. Ideally you’d like to have an infrared detector at the back end of a good-sized telescope, in either Earth orbit or one that’s not too dissimilar to the orbit of Venus. That’s the [B612 Foundation] Sentinel approach, and that would be the ideal technique for finding these objects.
A newly discovered object, Comet ISON, has been in the news lately, and it sounds like we’ll be able to see it bright in the sky this fall.
It was found by a couple of Russians at an observatory, International Scientific Optical Network, that’s what the name ISON stands for. This comet is already fairly bright and its out near Jupiter, so if we extrapolate its brightness as it comes in very close to the sun on Thanksgiving Day of this year, it has the potential to be very bright in the morning sky, or actually in the evening sky after sunset. Having said that, I’m old enough to remember Comet Kohoutek back in 1973; we predicted that would be the comet of the century and it turned out to be a bit of a fizzle. Comet brightness and behaviors are notoriously difficult to predict. But, if this comet behaves itself and acts like most comets it will put on a pretty good celestial show come this November.
What seems interesting, and tying it back to your book, is that they only just found it a few months ago. There are still these surprises out there in the sky, so this comet seems like a good argument for why we need to keep looking all the time.
That’s true. Comets, some of them get cut from the Oort cloud, which is at the very limit of our solar system, at 100,000 times the distance between the sun and the Earth. It takes millions of years for those objects to get from the Oort cloud to the inner solar system. ISON is a comet from the Oort cloud, so it’s almost certainly on its first return to the inner solar system. It’s scientifically interesting because it really hasn’t evolved much; it’s been out in the deep freeze of this Oort cloud for so long, retaining the percentage of ices and dust that formed some 4.6 billion years ago. With comets that have been around the sun several times, like Halley or other periodic comets, the volatile ices like carbon monoxide or carbon dioxide usually get depleted, and you’re left pretty much just with water ice. But these new [comets], they come in with their entire suite of volatiles – carbon monoxide, carbon dioxide, methane, ammonia, water – and so they’re the ones that are really the most interesting. I think that’s why astronomers are particularly excited about this comet.
Going back to potentially dangerous near-Earth objects, you write that we’ve started to look for these at the same time that, thankfully, we’ve developed the kind of technologies that could save us should we find one coming our direction. How might we be able to save ourselves from impact?
The key is: find them early, find them early, and find them early. If you find them 10, 20, 30 years prior to a predicted Earth impact, then you do have time to deal with them. So let’s say we find an object that’s 100 meters in diameter that’s predicted to hit the Earth 20 years down the road, then the easiest and simplest [solution] would be to simply run into it with a spacecraft. We demonstrated the navigational technology to do that when the Deep Impact mission purposely ran into Temple 1 on July 4, 2005. Just run into it, slow it down a millimeter per second or two, change its orbital period so that in 20 years time when it was predicted to hit the Earth it would miss by a wide margin. There have been other techniques discussed, everything from a so-called gravity tractor, which is just too wimpy to be a primary deflection device, to paintballs which is cute and colorful, but just nonsense.
Is that the method of putting paint on one side in order to change its reflectiveness, and that would sort of push it a little bit in some direction?
It would change the orbital dynamics of the object just a little tiny bit, but it’s really just ridiculous. I would take the spacecraft carrying these paintballs and run the entire spacecraft into the asteroid [laughs]. There are some other techniques – a solar reflector, a solar sail, laser ablation techniques – but some of these are far more complex, require far more sophisticated engineering. It’s the old principle of KISS: Keep It Simple Stupid. Just do the simplest and easiest technique that works and that would be just running into it with a spacecraft.
In a footnote toward the end of the book you mention that President George W. Bush’s press secretary got an email in 2008 with the subject line, “HEADS UP,” about an asteroid headed towards Sudan. The scientist suggested that the administration warn the Sudanese, but no one did because the U.S. had no formal relations with them. This seems particularly interesting when you write in a later chapter that some body like the United Nations should have an internationally approved plan at the ready. How likely do you think it is that they can all get together and agree on something?
In a couple of weeks I’m leaving for Vienna, where the United Nations Committee on the Peaceful Uses of Outer Space is meeting, and that’s what we’re going to talk about: getting an international plan in place and accepted by the United Nations so that when an object is found on an Earth-impact trajectory, there will be an international agreement that says “this space-faring nation would be charged with deflecting this object for the benefit of the others,” or something like that. The protocols and guidelines have not yet been written, but they are being discussed and hopefully they will be written in not too long, so we won’t have to start this process after we find an object.
There aren’t that many space-faring nations, and it seems like there are endless possibilities for last-minute negotiations -- for example, what if it’s going to hit a region with which the elected asteroid-deflecting nation is at war?
It’s very interesting, and the space lawyers will have to get involved. If the deflection is unsuccessful and it was to hit in Europe, but the deflection didn’t quite make it and now it’s going to hit in the United States: that’s the so-called Deflection Dilemma. You can’t just blast [the asteroid] off the surface of the Earth, it moves slowly from the impact site across the Earth, and then off the edge of the Earth, if you’re successful. But there is a chance you would move the impact point from one country to another if your deflection attempt was not successful.
You wrote that “human exploration is usually driven not by the quest for knowledge but for commercial gain.” Does that mean you support asteroid mining?
It’s not something that would be commercially viable right now, but these objects are rich in platinum group elements. They’re rich in iron and nickel, they’re rich in ices – water ices and hydrated minerals – so they could be processed for water, and water can be broken down into hydrogen and oxygen, which is rocket fuel. You wouldn’t go out and mine these materials to bring them back to Earth – that just wouldn’t be a good business model, because you could do it cheaper with minerals here on Earth. But as launch costs decrease and the pockets of platinum group elements and others that we now have on the Earth’s surface get less and less, it might one day make sense to mine this material and bring it back to Earth. But it would probably make more sense if you’re going to build structures in space. If you’re going to build outposts in space then you’re going to have to look around for resources in space, you’re not going to build structures on the surface of the Earth and then launch them. It’s just too expensive. You’re going to build watering holes and fueling stations full of water, and then you’d like to take advantage of these natural resources that are already there.
There’s one asteroid out there that’s particularly special to you, right? Can you tell us about 2956 Yeomans?
[laughs] It’s actually a pretty garden-variety, ordinary asteroid, I’m afraid. I was hoping it would be a binary or an Earth-threatening object, but it’s just a silicate rock. The advantage, of course, is that this rock will be out there for millions of years, far longer than I’ll be around, so you get a certain immortality.