How should I introduce heavier elements on this world?

Question

Based on this question I asked a while ago, I would like to know how to safely introduce new heavier elements to the world by asteroids without wiping the entire life and even keeps handy quantity of civilization to stay alive and prosper.

Assuming the quantity that would be delivered is roughly similar to our current available (refers to ground ores and processed ores we had by now), how could I safely deliver them to the surface via asteroid or asteroids (either a huge one or many smaller chunks)?

Requirements:

1. Maximum half the globe would be affected heavily by the bombardments
2. Its effects should be roughly similar to the meteor that hit earth 65 million years ago (though smaller effects would be preferred, consider it as the upper allowed limit)
3. The materials must be readily harvested, any that land on the sea might not be attainable given current technology
4. Please provide rough estimate of the asteroid(s) and its approximate intervals, given the event must be sufficiently alarming enough to the natives

Forethought:

• I thought of the possibility to have some smaller (considerably big though) meteors over short periods of time spreads over large land area.
• Also the possibility to have much more smaller meteors spreads over larger areas and larger timespan (days, weeks?), though I don't know whether it would be convenient.

Edit 1: narrowing the question's scopes.

Answer 1

The smaller and lighter your asteroids are, and the slower they're moving, the less damage they'll do.

The Chicxulub impact was huge. It killed lots of stuff, not just the dinosaurs - I'd suggest you just consider that to be out of range because it would likely kill far too many of the natives.

So, instead you should have a region of space where there are a lot of asteroids, put them in the planet's path, and set them on an orbit around the Sun. Give them a velocity that is maybe 15 m/s less than Earth's, because then the relative speed between the two is low and you don't have a massive impact. True, they'll accelerate as they come through the atmosphere, but the atmosphere will also break them up a bit, and the resulting dust containing the minerals you want will get spread across the planet.

The length of the meteor shower is essentially a function of inter-meteor density, meteor mineral density, and how much stuff you want to end up with. Essentially, this:

$$ \text{time} = \frac{\text{required}}{\text{mineral density}} \div \text{meteors per second} $$ $$ T = \frac{R}{\rho_m} \div \rho_{im} $$

This is going to be pretty dramatic - if $\rho_{im}$ is anything above about $ 1 \times 10^{\text{big number}} $, you've got a lot of meteors streaking through the atmosphere at high speed leaving big trails behind them. The videos from the Chelyabinsk meteor event should show this: think this...

...multiplied to ten times that every minute.

Answer 2

We can use one of the many asteroid impact simulators around to estimate the damage for single impactors. I've used the one from Imperial Collage London

We can enter different values here for the size of the incoming asteroids. As an example I picked a 500m object with a density of 10000 kg/m3 (higher than iron as you said they are heavier elements) impacting at 11.00 km/s (slow) at 45 Degrees to the ground (the average impact angle) and had it hit crystalline rock.

The full result can be seen here: http://impact.ese.ic.ac.uk/cgi-bin/crater.cgi?dist=100&diam=500&pdens=10000&pdens_select=0&vel=11&theta=45&tdens=3000&tdens_select=3000

The headline is a final crater diameter of 9.27 km from an impact energy of 3.93 x 10^19 Joules. 9.40 x 10^3MegaTons

I would suggest playing with the numbers and seeing what kind of effects you get. It gives you different things at different distances from the target so have a play around with that setting.

Multiple impacts will be required to deliver the amount of material you are suggesting unfortunately I couldn't find any thing on the dissipation rates of dust from the atmosphere which would be the limiting factor between impact events if you don't want to kill every one.

Have fun with Armageddon

Answer 3

The problem with a single large asteroid is dust.

Any meteorite that large is going to throw up so much dust into the atmosphere that it's going to drastically change the climate worldwide.

That asteroid that impacted 65 million years ago and created the Chicxulub crater is believed to have caused the K-T Extinction Event (http://www.wikipedia.org/wiki/Cretaceous–Paleogene_extinction_event), which saw the extinction of three quarters of all plant and animal species alive at the time. It threw so much material into the atmosphere that it basically caused an extended, worldwide "nuclear winter" because all that dust reflected sunlight away from the earth, drastically reducing global temperatures. It blocked so much sunlight that many plants could no longer sustain themselves via photosynthesis.

So if you are looking at preventing global impact, you'll need a much, much smaller upper bound. You'll probably want to do many tiny asteroids with scattered impact points at spaced intervals instead of a few big ones.

The Chicxulub crater is 110 miles wide and 12 miles deep, but the asteroid that created it was only 6 miles in diameter. So if you want to avoid global impact, your asteroids will need to be pretty darn small.

Tunguska Event-sized asteroids, estimated to be between 200 and 620 feet in diameter, should be fine. That meteorite is the largest to impact during recorded history, so it's the largest we've seen hit the earth with no noticeable global effects.

I would regard an impact that releases energy/debris equivalent to the Krakatoa eruption (which reduced global temperatures by about 1.2 degrees C for several years following) to be an upper bound. I can't find good numbers on the energy released by that eruption though.

Honestly, I would just make sure that you have some serious retrorockets or other braking slowing the asteroid's descent, though. If you're willing to spend the energy and your civilization already has the tech and energy to burn maneuvering asteroids into orbit, it could probably manage that.

Best of all would be a space elevator with an orbital asteroid mining facility at the end.