Asteroids

[KvickFlygarn87]

So, I'm writing a story. More specifically, a hard (err.. hard-ish) sci-fi short story. But I need to know, how much would an asteroid weigh if it was around 8km in size? I'm basing it around the Munroe asteroid (because it's named after the creator of xkcd) and Randall Munroe has stated that it it is "6-10km" in diameter. However, I can't seem to find any info on how high density asteroids have. the Small-body Database Browser has no answers.

I'm fumbling in the dark here, can someone pass me the flashlight?

[VirtualCLD]

Well, the Chelyabinsk meteor was estimated to be approximately 12000-13000 metric tons with a diameter of 20m. So, taking a cue from an old physics joke, "assume the asteroid is a perfect sphere" and since mass increases withe the cube of the radius, extrapolate from there. Basically: density = mass/volume, volume = 4/3*pi*r^3. Find density of Chelyabinsk meteor, use density to find mass of gigantic asteroid. Too lazy to do math now (in bed), but realize that density can vary widely in real life.

OK, I get a density of about 0.373 metricT/m^3 if we assume Chelyabinsk was 12.5k metricT, so an 8000m asteroid would be about 8E11 metrics or 800,000,000,000 metricTons. However I just woke up and one should never mix grogginess with logic and math, so someone better double check that.

Edited April 24, 2014 by VirtualCLD

[PakledHostage]

OK, I get a density of about 0.373 metricT/m^3

That density seems a bit low. After all, water has a density of 1000 kg per cubic meter, near as makes no difference.

Maybe have a look at Purdue University's Impact Earth website for some representative numbers?

[KerikBalm]

http://en.wikipedia.org/wiki/10_Hygiea

Well, this one's pretty big, so its density is probably a bit higher, but its basically twice as dense as water... 2 grams per CC instead of 1

http://en.wikipedia.org/wiki/16_Psyche

This one should be denser than normal

Its estimates are between 3.3 -> 6.5 gm/cm^3

http://en.wikipedia.org/wiki/243_Ida

This one is much closer in size to what you're talking about, and it seems to be 2.6 gm/cm^3

So.... it depends on the composition of course, but 2 gm/cm^3 should be in the ballpark for most of them

[VirtualCLD]

That density seems a bit low. After all, water has a density of 1000 kg per cubic meter, near as makes no difference.

Maybe have a look at Purdue University's Impact Earth website for some representative numbers?

You're right, I used diameter instead of radius, so the volume of Chelyabinsk was ~4200m^3 not 8400m^2. So that gives a density of 2.98metricTons/m^3. Since I made the same error when calculating the volume of Chelyabinsk as with an asteroid with a diameter of 8km, the mass works out to be the same: 8E11 metricTons.

According to that Impact Earth website, that means Chelyabinsk was probably dense rock.

[KvickFlygarn87]

Wait. That's bloody insane! That'll take heaps to deflect! Say we only need 10m/s, that still makes for a metric crapton of fuel! When I get home I'll do the math as to just how much is needed.

[Pds314]

I've heard the densities range pretty broadly, from 2.6-3.6 Tonnes/m^3 for rocky asteroids, 7.8 for Iron-nickel ones, and as low as 0.9-1.2 for mostly icy ones. An 8km asteroid with a density of 3.1 would be 831,055,976,630 tonnes. Roughly speaking, that's equivalent to all of the carbon in Earth's atmosphere.

[KvickFlygarn87]

Hmm. Thinking done, an asteroid that big is basically a crappy story. Instead, I'll go for a 10000 tonne thingy. Can still be a neat story, but not too easy.