Are there any techniques of heat shielding an irregularly shaped object?

Question

NASA is planning to bring an asteroid into a lunar orbit. As I understand, it's a tremendous task, even when using high specific impulse ion drives. It seems nearly impossible to safely land one on Earth without using aerobraking: the ion thrusters cannot be used anymore because of fast action required and therefor additional rocket engines with super reliability, high maneuverability, vast thrust power variability and huge amounts of fuel have to take over (seems pretty expensive and risky). Aerobraking would greatly reduce the total delta-V requirements, risk, price and landing complexity. But all the heat shields I've seen are robust, solid and far away from "flexible".

Are there any atmosphere entry protection capable cloths available? Would they tightly surround an asteroid without any holes and leaks under atmospheric pressure alone or some junctions before entry would be required? Could these cloths be strong enough to survive tearing force from the parachutes above or the parachutes should be anchored to the asteroid itself?

By the way, I'm thinking about commercial (not scientific) purpose landings of super-valuable space rocks, therefor a "safe landing" can be declared when the asteroid touches the ground (not burns up or shatters above ground) and it's parts remain within a small radius (1km or mile). I mean, it can reach the ground at 100 m/s and leave a fair crater, but as long as we can collect all the valuable material (platinum, may it be) scattered in a reasonably sized area, it's okay. Maybe even the parachutes aren't a necessity then, if terminal velocity of an asteroid (now meteorite) is small enough for this task (is it?).

Answer 1

TL;DR: You can apply a heat shield, but the shape of anything you want to re-enter is important.

While something that exactly follows the contours of your object may be hard to make, it's not that hard to cover your asteroid with tiles, which are small compared to your asteroid.

But there's a larger problem: You need to design some shape that has "good" aerodynamic properties, otherwise you'll need to absorb way too much heat.

Basically, re-entry means getting rid of your kinetic energy, and the most useful to do that is slamming into air, where your kinetic energy gets converted into thermal energgy, which we call heat.
And the simple explanaition for a heat shield is that it's used to absorb "the heat".

But that's not entirely true: the thermal energy is divided between heating the air (turning it into plasma) and heating your spacecraft. Generally, we don't really care about heating some air, but we do care about the amount of heat that needs to be absorbed by the spacecraft.

Most heat is generated at/by the shockwave around the spacecraft. So we want that shockwave to be seperate from our craft, and it turns out a flat surface in the front best accomplishes this, if you look at the apollo reentry capsules you see something close to the best shape (the flat part is the front), even though intuition would say turning it around would generate less heat.

This is the same reason that during reentry, the space shuttle flew "belly-first". I don't know the exact details of the aerodynamics during the Columbia disaster, but things started to go really wrong once air started to enter the hole in the heat shield. I bet the changed airflow and changed shape of the shockwave made things worse.

Same for your asteroid: you want it to re-enter with a flat surface in front, and if you build that, what you will get is something that basically looks like one big, sturdy heat shield attached to the front.

Answer 2

Ablative shielding works on irregularly shaped objects, and technically pretty much anything works as an ablator once. As noted in Emil Bodes answer what matters more is the aerodynamic properties of your object as it re-enters. If it is asymetric it will generate lift in some direction or another taking it off target and possibly lifting 'downwards' reducing the time/distance available to decelerate to terminal velocity (source - too much Kerbal space program).

Just building a dome heatsheild on the front may not help since it is not only the aerodynamics that matters but the mass distribution. If your carefully fitted heat shield leaves you with a center of drag ahead of the center of mass then your body will tend to swap ends in an exciting fashion. RCS and spin stabilisation will help but ideally you want a passively stable shape with the mass towards the front and drag being developed further aft, see for example the tail of the X-15.

A further complication is drag Vs mass as size goes up, with frontal area being squared but mass being cubed. Eventually you get something that cannot decelerate to terminal velocity before reaching the surface even if shaped to generate lift and 'flown' through re-entry, google is not helping with what that mass would be but the glide slope of the space shuttle at around 85 tons for something mostly hollow suggests it will be less than 100 tonnes even with lithobreaking

If the aim is to return bulk minerals to earth a plan may be to take waste products from your refining process and pack them around your payload metal into a suitably stable shape, and just let the waste ablate during re-entry to remove heat from your payload. There are some ecological side effects to dumping refinery waste into the atmosphere of course which may not be popular.