Slip-streaming in cycle races saves a lot of energy by shielding the follower from a head wind.

A sacrificial meteoroid precedes the re-entering spacecraft. It directs the hot plasma away from it. As it burns up it also slows. The spacecraft stays behind and deploys retro-rockets as and when necessary. The spacecraft shelters in the slipstream until velocity is low enough to survive on its own.

Would this work for re-entry, at least in theory?

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    $\begingroup$ If you had enough rocket fuel to slow down that way, you wouldn't have a problem -- just brake to a stop above the atmosphere and reenter slowly. The fiery reentry is needed to get rid of speed. I guess reentering along a channel of partial vacuum close behind a rock might enable a light but large spacecraft to decelerate more slowly than it otherwise would, which might be helpful sometimes.\ $\endgroup$ – Steve Linton Nov 29 '18 at 18:52
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    $\begingroup$ So the spacecraft and the asteroid are traveling at the same speed with the asteroid in the lead. The asteroid hits the atmosphere and slows down. The space craft, riding in the area behind it, doesn't. The spacecraft hits the asteroid. Ablative coatings are cheap and reliable. Far easier to use one than trying to find and asteroid and move it into the right position only to smack into it anyway. $\endgroup$ – zeta-band Nov 29 '18 at 18:54
  • $\begingroup$ Also an ablative heat shield is designed so that the ablation does negligible damage to the spacecraft as it heats up and disintegrates. A meteoroid does not have that design feature. Pieces of the rock are going to break off and potentially do severe damage to your spacecraft. $\endgroup$ – notovny Dec 1 '18 at 19:11

The problem with that is that the atmospheric drag that causes the heat is what allows you to slow down to the point that other methods of decelleration can convert a ballistic impact to a landing.

In reality, I suspect that the meteor would slow down and you would impact the meteor. If that didn't ruin your day completely, the meteor would then just act as a heat shield attached to the front of your ship.

Another problem is that it would probably take more reaction mass or energy (for the more scifi methods of acceleration) to move a rock big enough to provide a big enough pocket for your ship to ride in than it would take to decelerate the ship in the first place.

Furthermore, I don't think that whoever is on the ground is going to be happy you throwing big rocks at their planet.

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    $\begingroup$ Agreed, the rock will decelerate rapidly and the spacecraft will ram it. $\endgroup$ – Organic Marble Nov 29 '18 at 19:25
  • $\begingroup$ Heat is not caused by drag. Its mostly compression heating. $\endgroup$ – Mołot Nov 29 '18 at 23:57

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