# Ways to mitigate the forever violent re-entry? (Moon landings vs Earth landings)

The frequently used reasoning for why the re-entry should always be performed at neck-breaking speeds is that it would take almost as much fuel to slow the craft down as it takes to launch it into orbit, and that's a whole lot of extra weight to carry.

Now correct me if I'm wrong, but the Apollo crafts were, in fact, extremely fast; they reached the Moon and inserted themselves into orbit in mere days; after which the lunar module would detach itself from the mothership and begin a hovering descent to the surface (in the final stage, the descent was vertical). They slowed down sufficiently to make their landings smooth (i.e. from orbital speed to near-zero) and had enough fuel left to get back into orbit afterwards. And this was nearly half-a-century ago.

And yet each time someone returns from orbit these days, they run the risk of getting bounced back into space and getting lost there forever, or burning up before they reach the surface, due to their great speed. And then they use parachutes to perform an uncontrolled splashdown or touchdown wherever, and have to be "rescued" every time. Why?

• There's no such risk of "bouncing back into space" from Earth's orbit, why would there be? Orbital speed is $1/\sqrt{2}$ the escape velocity, and you're not getting any additional speed from reentry. That risk you describe is only there when you're approaching primary with some significant hyperbolic excess velocity ($v_\infty$) that might not be removed during skip reentry. We hadn't had anyone returning to Earth at such speed since Apollo 17, a bit over 43 years ago. Also, what is the question? Please edit to make the actual question stand out. – TildalWave Dec 28 '15 at 20:24
• @TildalWave: My bad. You're right. – Ricky Dec 28 '15 at 20:26

The Apollo craft started its trip to the Moon at a high speed. But as it got further away from Earth (climbing out of Earth's gravity well), its speed dropped steadily, and it orbited the Moon at a speed of about 1.5 km/s. That's much lower than Earth orbit (around 8 km/s), which makes a propulsive landing much easier.

In the lunar landing case, there's simply no choice; there's no atmosphere available to decelerate the landing craft, so powered landing is the only option. Fortunately, with 1/6 of Earth's gravity, and starting from about 1/5 the speed of Earth orbit, decelerating, hovering, and landing on the moon is much cheaper in fuel -- but we still had to use the largest successful rocket launcher ever built, the Saturn V, to send that fuel mass to the moon.

In the Earth return case, the deceleration is achieved almost for free using atmospheric drag; the mass of parachutes is tiny compared to the mass of fuel that would be required, and even tinier compared with the launch fuel that would be needed to boost the landing fuel into orbit.

In 54 years of manned space flight, no ship has ever gotten "bounced back into space and lost there forever"; only one has "burned up" (Columbia), though other failures during reentry killed Soyuz crews in the early days (1967 and 1971).

For the last 44 years, blunt-capsule reentry and parachute landing has been 100% safe.

Between 1981 and 2011, 99.25% of manned spaceplane reentries from orbit were successful.

It's unlikely that powered rocket landings would prove to be any safer.

• +1, but ... and starting from about 1/5 the speed of Earth orbit Yeah, but didn't they start from a low Earth orbit every time? Which would make it necessary for them to decelerate anyway just to get into orbit around the Moon? Decelerating in stages may be even more expensive than in one fell swoop, no? the mass of parachutes is tiny compared to the mass of fuel that would be required Yes, but there's all that heat insulation. – Ricky Dec 28 '15 at 20:24
• See Hobbes' answer, but be aware that an orbiting object will have different orbital speeds if you're measuring from Earth's frame of reference or the moon's. – Russell Borogove Dec 28 '15 at 20:39
• About 20% of the mass of the Apollo CM is heat shielding (850kg) and parachute and recovery equipment (240kg). Replacing them with equivalent mass in engine and fuel, and applying the rocket equation shows that it could cancel out about 600 m/s of orbital speed. Orbital speed minus surface rotational speed is about 7300m/s, so atmospheric drag would still have to do 92% of the work, and now you don't have a heat shield to protect yourself from that. – Russell Borogove Dec 28 '15 at 20:51
• Well, yeah, you can decelerate relativistically by chasing the Moon along its orbit (not much, though, since the Apollo's speed was 10 times greater, I think? - unless they used the Earth-Moon system's orbital speed around the Sun? Couldn't the same trick be applied to the Earth re-entry, then?) – Ricky Dec 28 '15 at 20:55
• I don't know what you mean by a trick; I'm just pointing out that an object's speed relative to the moon is different from its speed relative to the Earth. – Russell Borogove Dec 28 '15 at 21:00

Now correct me if I'm wrong, but the Apollo crafts were, in fact, extremely fast; they reached the Moon and inserted themselves into orbit in mere days; after which the lunar module would detach itself from the mothership and begin a hovering descent to the surface (in the final stage, the descent was vertical).

The flight from lunar orbit to the surface was mostly ballistic. The Apollo lander made a small burn (33 meters/second) to depart from that lunar orbit, placing the vehicle on an elliptical orbit whose perilune was slightly inside the Moon. Then it simply fell for about an hour. It was only in the final twelve minutes of descent where the lander expelled a sizable amount of fuel to cancel that orbital velocity.

Even then, the total delta V from lunar orbit to landing was only 1750 meters/second. That's a fraction of the delta V that would be needed to have a vehicle enter the Earth's atmosphere at sub-orbital speeds -- and then the vehicle would still have about 100 km to fall before landing on the Earth.

The only technology currently available to dump the 6750 meters/second (or more) of orbital velocity is to use the Earth's atmosphere to perform most of the braking.