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The Hayabusa 2 probe is firing tantalum bullets at the asteroid Ryugu to knock off material that can be sampled. Why tantalum? That’s a pretty obscure element and I would assume rather rare. What properties made it the best?

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The main reason for using tantalum is the fact that it is a rare element. When they capture the debris thrown up by the bullet, the fact that the bullet itself is tantalum makes it easy to distinguish it from the asteroid's material. If they used a more common element like iron, scientists could not be sure that any iron was from the asteroid, instead of from the bullet.

The second sample attempt will use a heavy (2.5 kg) copper slug, again because copper is expected to be relatively rare on the asteroid.

From AMAZING Video: Hayabusa-2 Shot A Bullet At An Asteroid And Collected The Shrapnel:

Why tantalum for the bullet? Because it’s so rare that if a piece of it gets into the sample scientists will know right away it was from the bullet and not the rock. If they used something common, like iron, there wouldn’t be a way to distinguish it from the native materials.

And Hayabusa2 is not done yet. There’s another surface sample event planned, as well as an ambitious subsurface sample mission: It’ll deploy what is essentially a floating cannon that will fire a much more energetic 2.5 kilogram copper projectile into Ryugu, overturning the material sitting on top and digging up stuff from below (the spacecraft itself will maneuver to the other side of the asteroid for safety... and again, copper is used to distinguish it from the sample material). Once things settle down, Hayabusa2 will come back to that site and gather a sample from the crater left by the impactor. This is planned for April.

EDIT

Phil Plait, the Bad Astronomer posted a comment (and video) about the second bullet. He also commented on the first, including the reason for using tantalum, in an earlier post.

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    $\begingroup$ Why didn’t they use certain isotopes of an element which don’t/can’t occur naturally in the asteroid? $\endgroup$ – Michael Jul 30 at 19:18
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    $\begingroup$ @Michael While tantalum is expensive, they only needed 5 grams, and they can buy it on the open market. Compared to tainting the results of a $150 million six year project, that's peanuts. I don't know what 5 grams of ultra-pure tantalum costs, but to give an idea there are coins with 20 grams of tantalum selling for less than €200. Modern electronics have a few milligrams of tantalum. $\endgroup$ – Schwern Jul 30 at 21:53
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    $\begingroup$ @Michael, isotopically-pure samples of elements are incredibly expensive. You might be familiar with the project to produce a sample of isotopically-pure plutonium-239. $\endgroup$ – Mark Jul 30 at 22:41
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    $\begingroup$ @Mark you wouldn't need isotopically pure samples. If, say, you make bullets of 10% rare isotope and 90% common, then if you see 0.1g of the rare one in the sample, you may subtract 0.9 (adjusted for the mass difference?) from the common one. Assuming that the rare isotope isn't in fact more common in the asteroid than we thought. However, even if this assumption is justified, an iron bullet of non-standard isotope distribution might prove more expensive than a tantalum one. $\endgroup$ – IMil Jul 31 at 0:00
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    $\begingroup$ @Mark Even today, isotopic enrichment is probably more expensive than just using something like tantalum. Tantalum is about \$ 125/kg; heavy water (D2O) is over \$ 400/kg. $\endgroup$ – hdhondt Jul 31 at 9:50
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Tantalum is an extremely dense, readily obtainable, machinable and hard metal.

Lead and gold are fairly soft an malleable and wouldn't retain their shape upon impact.

While tantalum is relatively safe to machine and be around, even depleted uranium has chemical toxicity. As @Uwe and Wikipedia point out, while Tungsten is slightly more dense it is also more difficult to machine.

For ultra-dense radiation shielding (different application than this) tantalum is in widespread use because of its extremely high density, machinability, and comparative safety around humans.

The Juno spacecraft around Jupiter also uses a tantalum radiation vault for it's electronics, likely for similar reasons; high density, machinability, obtainability, and no major health/safety risks. (see also If Juno's long orbit means a lower rate of radiation damage, why the planned short orbit?)

metal     density (g/cm^3)
Cu            9.0
Pb           11.3     (for reference)
Ta           16.7
U            19.1
W            19.3
Au           19.3     (for reference)
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    $\begingroup$ Tungsten is more dense, 19.3 but it is too hard and brittle to be machinable. $\endgroup$ – Uwe Jul 30 at 9:45
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    $\begingroup$ @Uwe Thanks, I've now included tungsten that in the text and table. $\endgroup$ – uhoh Jul 30 at 10:07
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    $\begingroup$ Why is retaining the slug's shape on impact important? Copper is a fairly soft metal yet they're using it for the second slug. $\endgroup$ – 8bittree Jul 31 at 20:14
  • $\begingroup$ @8bittree That's a good point, if it is elementally pure copper and not some alloy the copper will be softer than tantalum, but certainly not as soft as lead, but I don't know for sure how relevant that is. This will take some further reading, thanks! $\endgroup$ – uhoh Jul 31 at 22:32

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