Near-Earth asteroid twice as big as previously thought

By Phil Plait | June 22, 2012 11:00 am

On June 14, 2012, the asteroid 2012 LZ1 passed the Earth. It missed us by a wide margin, over 5 million kilometers (3 million miles), so there was no danger of impact. While it does get near us every now and again, using current orbital measurements we know we’re safe from an impact by this particular rock for at least 750 years. Phew.

Good thing, too. New observations using the Arecibo radio telescope in Puerto Rico indicate LZ1 is bigger than we first thought. Much bigger: it’s about a kilometer across, when it was thought to be half that size before these observations.

That’s a big difference. The problem is that the size of an asteroid is hard to determine. Even a big one may only appear as a dot in a telescope, so even though we may know its distance and trajectory very accurately, directly measuring its size isn’t possible. Usually, the size is estimated by knowing its distance and how bright it appears. In general, a bigger rock will look brighter than a smaller one at a given distance.

But that assumes they both reflect the same amount of light. Most asteroids reflect about 4% of the sunlight they receive (this property is called the albedo), but that depends on their surface. Some have darker surfaces, some brighter. If you don’t know how reflective it is, the size can only be estimated.

But the Arecibo telescope can actually directly measure the size of a nearby asteroid. It can send pulses of radio waves at an asteroid and then receive the reflected waves, much like a cop on the side of the road uses radar to measure a car’s speed. The method is technical (Emily Lakdawalla has a great explanation on her blog), but it was used for LZ1 to get the new size measurement. The picture above is the actual image generated using Arecibo when the rock was still 10 million km (6 million miles) from Earth. Apparently, LZ1 is much less reflective than assumed earlier, which is why the size was underestimated by a factor of two.

An asteroid this size hitting the Earth would be, um, bad. That’s big enough to be considered a global hazard, causing immense devastation. It might not be an extinction event — the dinosaur-killing asteroid was 10 km across, so it had 1000 times the mass of LZ1 — but it wouldn’t be fun. So I’m glad we’re safe from this guy for some time!

But I’ll be honest: LZ1 was only discovered a few weeks before it passed us. Asteroids this size passing near us are pretty rare (we haven’t had an impact from something this big for many, many millennia) so as usual I’m not panicking about this. But it just shows once again that we need more eyes on the sky, more people looking. And we need a plan in place in case we do see one with our name on it.

Related Posts:

Asteroid 2011 AG5: a football-stadium-sized rock to watch carefully
My asteroid impact talk is now on TED
Another tiny rock will pass Earth tomorrow
Updated movie of asteroid YU55, plus bonus SCIENCE
Just to be clear: asteroid YU55 is no danger to Earth
Armageddon delayed by at least a century… this time


Comments (23)

  1. I assume that the NRAO’s GBT in Green Bank acted as the receiver for the radar pings, though the Alan Boyle “Cosmic Log” article I read on MSNBC didn’t mention it. This rock was probably too close (around 30 seconds at lightspeed) for the Arecibo instruments to shift over from transmitter to receiver in time to catch the return pulse.

  2. BJN

    Given that we only found this object two weeks before it got so close, we need to get a lot better at finding these objects a lot further away. No probable technology I’ve heard of to deflect a large asteroid would have time to work in such a short time window. It’s obviously not easy to identify large, dark objects at “early warning” distances but the consequences of a rare impact are enormous compared to a relatively tiny investment in sky scanning technology.

  3. David C.

    ok, not as bad as a Dinosaur Extinction Event, but it would make Global Warming seem like a picnic in comparison; the Global Winter would be devastating enough for millions to die, from famine and climate change; so, while it didn’t hit us this time, in 750 years time it could, and that would be a bad day for our descendants; as well, if this was found only haphazardly, as it passed by, why do I feel like a duck in a shooting gallery, with x number of shooters aiming at me; sorry Phil, I don’t feel any more safer ;( I am just glad that Planetary Resources are going to add their weight to the number of people looking at these things; I have more faith in there “self interest” than our Gov’ts ponying up the money to fund more research; just my 2 cents;

  4. Gary Ansorge

    Man, if only my salt water nuclear thruster was up an running. I’d love to mount a mining expedition to that asteroid. Wonder what it’s made of…

    Gary 7

  5. Matt B.

    The basic way of finding the radius of the asteroid isn’t all that complicated. You just hit it with some radar and time the difference between the first and last echoes. Then you use r = cΔt to get the radius. The hard part would be adjusting for relative motion of the emitter-receiver and the asteroid.

  6. Phil,

    When you twice the size, do you mean twice the mass, or twice the diameter (and thus 8 times the mass)?


  7. Matt B.

    ^Sean – I’d go with diameter. Referring to mass as size doesn’t normally make sense, and in the case of an asteroid, it’s very rare to determine mass (you’d need to know the composition and volume pretty well, but the composition’s always a guess and the lumpiness of small asteroids can mess with volume estimates, so you’d pretty much need to have something orbit it).

  8. Mike M.

    What if this is just the beginning of a meteor storm, and there are many more on the way? What if the Mayans were right?

  9. I Imaged this asteroid on 15 June using the Rigel (MPC 857) 37-cm Cassegrain. Imagery can be seen here:

    The apparently low albedo could perhaps suggest a carbonaceous composition.

  10. Isaac

    Gee, I dunno, Mike (#8). What if the whole “The Mayans predicted the end of the world” thing is just a load of crap? Oh, wait. It is. Even the Mayans say it is.

  11. Peter B

    Mike M @ #8 asked: “What if this is just the beginning of a meteor storm, and there are many more on the way?”

    Why should it be? As the expression goes, one swallow doesn’t mean it’s summer. Likwise, one rock doesn’t make a meteor storm.

    Another way of looking at it is if there were lots of rocks out there, it’d be easier to spot them.

    Still, looking is a good thing to do, and that takes money. Does that mean we can count on you to lobby your local politician for more funding for this research?

    “What if the Mayans were right?”

    Have you read *anything* the Bad Astronomer has written about Mayan 2012 predictions? If you haven’t, please do so. If you have, why are you not convinced the 2012 disaster industry is a load of rubbish?

  12. @8. Mike M. :

    What if this is just the beginning of a meteor storm, ..

    Very unlikely given that meteor showers – and storms – usually derived from materila shed by comets and not asteroids – albeit there are occassional asteroids that are responsible for meteor showers such as 3200 Phaeton (click my name for wiki-link)which almost certainly used to be a comet and simply ran out of volatiles to fuel its coma and tail.

    I’d love to see a meteor storm in my lifetime along with a supernovae but these events are very rare and unpredictable.

    For 2012 LZ1 to produce such a storm it would have to be shedding material and spreading that along its orbital path. There doesn’t seem to be any reason to think this will be the case here or that 2012 LZ1 will produce any meteor shower at all as it seems to be a single solid body.

    ..and there are many more on the way?

    Well, if there were more on the way, we’d expect astronomers to detect them like they detected this one and the many other space mountains that have drifted by us. However, there’s no reason to expect that 2012 LZ1 indicates any unusual increase in passing asteroids.

    What if the Mayans were right?

    About what? The Sun being a god that requires human sacrifices in order to rise each day? 😉

    I take it you are referring to the whole 2012 = Eennnd oooof the woooorld silliness because of some supposed mark in the Mayan clandar nonsense? :roll:

    As (#11) Peter B. has noted if you think this has any credibility – and it doesn’t – you should read what the BA has to say about it on this very blog. When its all distilled down the 2012 Mayan Armageddon is simply rubbish and the world has been predicted to end an almost endless number of times – by failed Rapture-predictor Harold Camping most famously most lately and hasn’t.

    Why give this Mayan 2012 tripe any more credibility than any other such “Teh End is nigh! Niiighh!” prediction deserves? I.e. None whatsoever! 😉

    If it happens, well, that’s a very big “*if*” indeed. It won’t. Hope you didn’t waste any money on it.

  13. @4. Gary Ansorge :

    Man, if only my salt water nuclear thruster was up an running. I’d love to mount a mining expedition to that asteroid. Wonder what it’s made of…

    We should be able to find out via spectroscopy and its asteroid type. Given its low albedo, I suspect it could well be carbon rich, perhaps a carbonaceous chondrite? The darkest asteroids are C and D types with darkest D types being :

    .. suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors

    Mnd you theD-types are usually found furtherest out; so perhaps more likely its a carbonaceous C type asteroid which are rather amusingly described as :

    .. very similar spectra to carbonaceous chondrite meteorites (types CI and CM), whose chemical composition is approximately the same as the Sun and the primitive solar nebula, except that they do not contain hydrogen, helium and other volatiles. Hydrated (water-containing) minerals are present.

    Source : Wikipedia for both quotes click on my name for link to C-type asteroid

    My preliminary web search, alas, has turned up no specifics for 2012 LZ1 – amazingly there’s no wikipedia entry for it yet. 😮

  14. RossMelb

    You would probably enjoy a new bk BEFORE THE DELUSION by Wm Gleeson, which contains impressive new research on an ancient impact. Its well documented and a page-turning read – worth a look


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