I’m talking about one like the one that created the K-Pg boundary, and is thought to have caused the extinction of the dinosaurs. That one is thought to have had a diameter of about 10 km, and may have been put on an Earth trajectory after an impact in the asteroid belt. Either way, it’s thought to have thrown enough ash into the air to darken the world for years afterword, and to have precipitated earthquakes, worldwide fires, and volcanic eruptions, killing off photosynthesizers and everything that depended on them.
Anyway, I realize such events are very rare, but if (or when) it happens again, are we likely to have much warning? Is it possible there would be none?
Do we have any sense of how many such rocks are out there, compared to how many we’re actively watching? If one changed course suddenly, because of an impact (for example) could it hit before we knew it was coming?
10km is pretty large by the standards of Earth-crossing asteroids; I’d be surprised if there were any that large that haven’t already been found and known to pose no threat. The real wild cards are comets. A previously unobserved comet could come from the outer solar system and conceivably impact the Earth. We’d know 2-3 years in advance that it was coming “near” (1,000,000 km or less) the Earth, maybe 8-12 months that it would be coming “very near” (about lunar distance) and maybe 4-6 months warning that “Holy Shit, that’s too close for comfort!”. A big if is how closely the comet might “clip” the Earth: the more dead-center its course was, the longer beforehand that would be certain, whereas a close graze might keep us guessing almost until the day it happened. The SF novel Lucifer’s Hammer portrays such a scenario. Short answer, we’d probably know it was a serious possibility 2-3 months ahead of time.
You may be interested in this video. It shows a time progression of asteroids as they’ve been discovered and tracked in the past few decades. It really gives you a good idea of how much stuff there is out there for us to track and worry about.
The green ones are "safe" in that they aren't currently in an earth-crossing orbit (that could obviously change if something disturbs them). The red ones are earth-crossers, though none are on a known earth-collision orbit.
From the Large Synoptic Survey Telescope project, their estimate of potential damage to Earth from NEO impact. Their estimate of incidence of impact of a 10 km object is on the order of 50 million years. However, you should understand that this is at the ‘swan’s tail’ type of event. Predicting when such events will occur or the interval between them is a fool’s errand. Although seemingly improbable, such events could follow each other within a few days with almost as much probability as separated by millenia.
Well, if I’m reading the chart right, the dino-killer comes around every 100 million years or so, and the last one was only about 60 million years ago, so we should be safe for 40 million years or so. So that’s good news. (J/k, I know it doesn’t really work that way.)
In answer to the query about how likely we would be to observe the object for a significant time prior to impact, it depends. More specifically, it depends on the trajectory (a very long period or non-recurring object will not be observed in advance of passing the orbit of Saturn), the albedo (a non-reflecting object will not be readily observable unless it happens to occlude a planet or bright star), and the inclination to the plane of the ecliptic (a highly inclined trajectory will be less likely to be seen because we don’t generally look there for interplanetary bodies). A rogue planetoid–one that originates from outside the solar system completely–will be unpredictable and almost completely unobservable until just weeks or even days prior to impact, simply because we have no model for where or how to look for it. Fortunately, such objects appear to be extremely rare, and the probability that it would impact the Earth is almost infinitesimal.
Virtually all searching for potentially hazardous objects is being done from ground-based observatories today, which is extremely limiting (only half the sky can be observed at any given time from a fixed position on Earth) and affords little opportunity to observe and track objects based upon instantaneous parallax. A solar orbiting constellation of satellites–say, located at L1, L4, and L5 libation points, or any other orbit with at least one observatory within Earth orbit) would provide substantially greater data and capability, at a cost that is a pittance compared to potential cost of an unmitigated impact.
And what, exactly, might we do to mitigate such an impact? If we knew that a dino (and rino ) killer was coming in 6 months or 1 year, do we just put our heads between our knees and kiss our asses goodbye? Would there be mass global panic of Kryptonian proportions? What more could we do about it?
As I understand it, right now at the end of 2013, we need at least 10 years warning. Anything less and we just ride it out. IMHO we are to focus on Dino killers. Something much smaller could still ruin your day. Doing what SOAT suggest would be a first step.
You might want to look at these two SDMB threads, which discuss the utility of various means for moving large masses in space, via nukes and otherwise. In them, Stranger does yeoman work as usual. In one of the threads, the B612 Foundation is mentioned. They are an advocacy group for spreading awareness of the threat of NEO, as well as fundraising for an orbital telescope to try and find all of them. Papers from the 2004 Planetary Defense Conference may be found here, and quite a few of them deal with the task of deflecting a NEO.
Not quite dino-killer-sized, but there has been at least one asteroid big enough to really ruin our day that passed closer than the Moon, and wasn’t even discovered until after closest approach. So no warning at all is perfectly plausible.
And it would probably take a decade of advance warning to divert an object, but there are a lot of other preparations we could make. It’d take at most one year to stockpile food (and seeds and breeding stock), and that by itself would be enough to save us even from another dino-killer.
Humans collectively spend thousands of gigadollars per annum on education, research into alternative energy, new drugs, etc., which will all be worthless in the Post-Collision world. There would be enormous economic value in foreseeing the collison so we could forgo such waste and instead direct spending toward [del]hookers and blow[/del] drinking fine wines and watching reruns of Breaking Bad.
Comet ISON, which appears to have broken up as it passed the sun last week was about 2 kilometers.
Comet Shoemaker-Levey9 which was broken into about a dozen pieces or so by Jupiter’s gravity before the impacts in 1994. Many of those pieces were as big as the Dino Killer (10 kilometers), so the whole comet was pretty huge.
So far, assuming we leave out minor planets like Chiron, it looks like the size king is Hale-Bopp, with a nucleus anywhere from 35 to 60 km in diameter. The wiki helpfully states that a collision with the lower bound of the mass would yield somewhere in the ballpark of 2 X 10^25 Joules, or around 3 times the energy the Earth receives from the Sun each day. It would not be pleasant.
So who first finds about about the impending doom? Who do they inform? Would governments be willing to inform their citizens anyway? If it’s inevitable would it do any good to scare the population years in advance of certain death?