# What precision is needed for aerocapture?

In his excellent answer to the question of how we make a round trip journey to Mars, @MarkAdler mentions that aerocapture would be used to get into orbit around Mars, saving fuel and therefore weight. This sounds like a nice idea in principal.

However, if the spacecraft comes in too near the planet there is a risk of slowing too much and crashing into the surface. On the flip side, coming in too far away from the planet risks not being captured and flying off into deep space. My question is: Given a normal amount of fuel for such a mission; how narrow is this altitude window in which the spacecraft is captured but doesn't crash or get lost in space?

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## 1 Answer

A typical metric is that you want to exit with no more than 100 m/s of correction $\Delta V$ to get to your targeted conditions. (That does not include the deterministic $\Delta V$ required to raise periapsis assuming a perfect exit state.)

That generally requires that the flight through the atmosphere be guided or drag-modulated to account for both uncertainty in the entry flight path angle and uncertainty in the atmospheric density. With such guidance, the entry flight path angle accuracy required isn't any tighter than what is needed for targeting a landing site, around a quarter of a degree.

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