### Could the dinosaurs have seen the asteroid that killed them?

• Wikipedia says the Chicxulub impactor is thought to have been a 10-15 km diameter object. Would it have been visible to a (human*) naked eye before impact? And if so, would it have appeared like a star that grew brighter and brighter each night?

* I know, there were no humans at the time.

I'm assuming if you were *right under it*, you'd definitely be able to see it, y'know, before it hit you.

Chances are if they did not see it, they did feel it.

A lot of dinosaurs seem to have had ridges over their eyes and inflexible necks so whether they could look up at all is in question.

No, they only looked down at the food they ate. That's what killed them.

That and an asteroid :-)

if they could have looked up and had seen the asteroid they could have built shelters or something.

Yeah, if they hadn't had such tiny hands.

Why has this particular QA attracted so many (lame) jokes? :)

• ProfRob Correct answer

4 years ago

The answer is yes; for a few nights prior to the impact (assuming they had eyes with a similar sensitivity to our own and could look up!). It could also be much longer than this if the object was a cometary body.

Details:

Impacting solar system objects would have relative closing speeds from around 11 to 72 km/s.

We could take the optimal case that the asteroid approaches whilst fully lit by the Sun (which probably precludes the minimum and maximum speed in the range quoted above) and then scale from another similar body - say the asteroid Vesta. This has a diameter of around $$a=520$$ km, gets as close as $$d=1.14$$ au from the Earth and has a maximum brightness of about $$m=5.2$$ apparent magnitude (and is hence just visible to the naked eye) and an observed flux $$f = f_0 10^{-0.4m}$$, where $$f_0$$ is a zeropoint for the magnitude scale.

Thus the flux $$f_n$$ received by a near-Earth asteroid of diameter $$a_n$$, at a distance $$d_n$$ from Earth (in au) and with the same reflectivity would be
$$f_n = f\left(\frac{a_n}{a}\right)^2 \left(\frac{1+d}{1+d_n}\right)^2\left(\frac{d}{d_n}\right)^2$$

The magnitude of the dinosaur killer would then be
$$m_n = m -2.5\log (f/f_n)$$

To be an at all conspicuous naked eye object, $$f_n \geq f$$. If we assume the dinosaur-killer had $$a_n=10$$ km, then
$$d_n^2(1+d_n)^2 \leq 0.0022$$

An approximate solution is obtained by assuming $$d_n \ll 1$$ and thus we find
$$d_n \leq 0.047$$ au or 7 million km.

Moving at say 30 km/s, then it gets closer by 2.6 million km per day, thus hitting the Earth about 3 days after becoming a naked eye object. Obviously this would be longer for a slower approach speed or for a larger or more reflective asteroid. But shorter for a smaller, faster asteroid or if the asteroid approached from a direction not fully illuminated by the Sun or had a smaller albedo than Vesta.

Another possibility is that the object is of a cometary origin with an icy composition. If that were so then it could be much brighter as a result of sublimation, outgassing and having a bright cometary nucleus and tail. he answer would still be yes, but the visibility period could be weeks (comets are rather unpredictably bright).

It thus seems to me that there is a plausible range of parameters and trajectories where a dinosaur-killing asteroid could be observed and then observed to grow brighter over a few nights, but probably not much longer than that unless it was a comet.

Interesting to calculate how long it would have been visible during the day.

An unstated assumption is that dinosaur eyes are as good at night vision as human eyes, no better or worse :)

@gerrit Question says "human" naked eye. No unstated assumptions there.

it would be brighter than venus for 20 minutes and brighter than the moon for about 3 minutes if you were on the same 2000 miles.

@gerrit Most extant dinosaur species have better eyesight than your average mammal, so the same may well be true of extinct species.

3 days? 72 hours remain

@MikeScott While it's not an entirely unreasonable suggestion, it does bear mentioning that extant dinosaurs are a) in aggregate smaller, so more dependent on fine vision, b) almost universally fly, so more dependent on gross vision, and more dependent on discerning detail at range, and c) have 65 million years of dinosauring it up on their extinct brethren.

Days were shorter back then, maybe 23 hours? Still about 3 days.