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China had sent it's own rover Chang'e-4 called Jade Rabbit/Yu Tu 2 to explore the far side of the Moon, the news said it softlanded... I remembered there is no atmosphere to deploy parachute so what is this softlanding? Is it difficult compare to Cassini landing on Titan, Saturn's moon.

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  • $\begingroup$ Would Space Exploration be a better home for this question? $\endgroup$ – Qmechanic Jan 5 at 5:11
  • $\begingroup$ @Qmechanic: how do i migrate the question to space exploration? can u do it for me? $\endgroup$ – user6760 Jan 5 at 5:16
  • $\begingroup$ It's called softlanding because back in the ancient days most probes would splashdown and be destroyed as part of their plan because it was too hard to build rockets like you see today that can land themselves. $\endgroup$ – Mazura Jan 5 at 11:34
  • $\begingroup$ @Mazura read wikipedia about Moon landings. The first successful unmanned landing was nearly 53 years ago and the first manned nearly 50. There were a lot of unsuccessful landings in the first half of the sixties. A splashdown on Moon is impossible anyway without water but there were many unintended crash impacts $\endgroup$ – Uwe Jan 5 at 11:58
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As the Moon essentially has no atmosphere, the techniques used to land on Mars or Titan are not available for a Moon landing.

I'll assume a vehicle that is to land on the surface has already been placed in low lunar orbit. At some point, the vehicle needs to perform a deorbit burn. This is a fairly small burn (20 to 30 meters per second) that makes the vehicle's perilune be very close to the lunar surface. One of three things will happen if nothing else is done:

  • The vehicle will hit the lunar surface with a small vertical velocity and a large horizontal velocity (about 1.7 kilometer per second) just before reaching perilune because perilune is below the surface.
  • The vehicle will barely miss the lunar surface because perilune is just above the surface, but shortly later will plow into the 2 kilometer tall lunar mountain that's just ahead.
  • The vehicle will barely miss the lunar surface, barely miss the mountains ahead, and rise back to the altitude at which it performed the deorbit burn (and then start falling again).

Obviously something else needs to be done. That "something else" comes in multiple parts. The first thing that needs to be done is to negate that huge 1.7 kilometer per second horizontal velocity. This is what makes landing on the Moon so much more expensive delta V-wise than landing on Mars, Titan, or the Earth. This large of a maneuver cannot be done as an impulsive burn. What's needed instead is a so-called "gravity turn", a non-impulsive burn where the vehicle simultaneously fires opposite the velocity vector and rotates so as to keep the vehicle on the desired trajectory. Doing so minimizes gravity losses.

Ideally, this gravity turn will result in the burn ending just as the the vehicle reaches the lunar surface, with the vehicle oriented vertically and having zero horizontal velocity and near zero vertical velocity. Perfect landing! This is called a "suicide burn". It assumes perfect knowledge of the Moon's gravity field, perfect knowledge of the Moon's topology in the vicinity of the landing site, perfect knowledge and control of the vehicle's orientation, and perfect behavior of the vehicle's thrusters. Nothing is ever perfect; the end result of a suicide burn is not a perfect landing.

The braking burn instead targets a spot that is a short distance above the lunar surface, a few tens of meters to perhaps 100 meters above the intended landing site. The intent is to have the braking burn ends at or near this point with the vehicle having a near zero horizontal velocity and a small downward velocity. This slop allows for some imperfect of knowledge and control. At this stage, the vehicle starts its terminal descent. The vehicle gently brings the vehicle down to the surface. In order to do this, the vehicle needs to "see" the lunar surface. At a minimum, it needs an altimeter that detects the distance to the surface.

Another aspect of lunar landing is hazard detection and avoidance. What if the landing site turns out to be strewn with boulders? It would be a bad idea to land with one leg on top of a two meter tall boulder and another leg in the bottom of a small crater. One could hope for a landing site that is very flat and does not have any sizable boulders or craters. (Such sites do exist.) The Apollo program relied on extremely advanced navigation sensors and extremely adept guidance and control systems to solve this challenge: Human eyes and human brains. Hazard detection and avoidance remains an interesting challenge for automated landers.

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You are right that parachutes can't be used on the Moon because of the lack of any atmosphere. Moon landers have to use rockets to slow themselves down as they descend to the surface.

Luckily, the Moon is much smaller and has less gravity than the Earth, so less powerful rockets that use less fuel are needed to safely land.

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