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I have seen videos of simulated lower gravity (possibly for training astronauts). I am curious what methods/techniques can be used to simulate lower gravity like environments without leaving the planet itself (that of course exculudes free-fall by definition). Or is it impossible?

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  • $\begingroup$ You may use a tall evacuated tube like the Fallturm of Bremen to simulate zero gravity for a short time. If the speed of free fall is lowered precisely, nonzero gravity may be simulated. Any value of gravity between the planet's gravity and zero could be simulated. Being within the tower is no leaving the planet itself. $\endgroup$ – Uwe Sep 8 at 10:21
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    $\begingroup$ Is using a plane "leaving the planet" or not, in your definition? $\endgroup$ – Polygnome Sep 8 at 10:29
  • $\begingroup$ @Polygnome a plane needs an atmosphere of the planet. Staying in the atmosphere is no leave of the planet. But you need more pressure than the martian atmosphere for a plane to fly. $\endgroup$ – Uwe Sep 8 at 10:35
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    $\begingroup$ @Uwe Yeah but the title says "leaving the surface" and the question body says "leaving the planet", so it would be nice if OP clarified what is meant. $\endgroup$ – Polygnome Sep 8 at 10:36
  • $\begingroup$ Did they ever try wearing harnesses and a long rope with a counterweight or spring? In terms of walking, jumping, etc. the physics would be very similar to lower gravity. $\endgroup$ – Ray Butterworth Sep 9 at 14:35
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The two most commonly used techniques for humans are neutral buoyancy and parabolic flights.

Neutral Buyoancy

Neutral buoyancy simulates the weightless environment of space. First equipment is lowered into the pool using an overhead crane. Suited astronauts then get in the tank and support divers add weight to the astronauts so that they experience no buoyant force and no rotational moment about their center of mass.

One downside of using neutral buoyancy to simulate microgravity is the significant amount of drag presented by water. Generally, drag effects are minimized by doing tasks slowly in the water. Another downside of neutral buoyancy simulation is that astronauts are not weightless within their suits, thus, precise suit sizing is critical. [2]

Astronauts train in the Neutral Buoyancy Facility at the Johnson Space Center in Houston, Texas

Parabolic Flights

The sensation of weightlessness is achieved by reducing thrust and lowering the nose to maintain a neutral, or "zero lift", configuration such that the aircraft follows a ballistic trajectory, with engine thrust exactly compensating for drag. Weightlessness begins while ascending and lasts all the way "up-and-over the hump", until the craft reaches a downward pitch angle of around 30 degrees. At this point, the craft is pointing downward at high speed and must begin to pull back into the nose-up attitude to repeat the maneuver. [1, 3]

Parabolic Flight Trajectory

Drop tube / Drop tower

For non-human payloads, a drop tower or drop tube can be used (e.g. Fallturm Bremen (de, en)).

In physics and materials science, a drop tower or drop tube is a structure used to produce a controlled period of weightlessness for an object under study. Air bags, polystyrene pellets, and magnetic or mechanical brakes are sometimes used to arrest the fall of the experimental payload. In other cases, high-speed impact with a substrate at the bottom of the tower is an intentional part of the experimental protocol.

Not all such facilities are towers - NASA Glenn's Zero Gravity Research Facility is based on a vertical shaft, extending to 510 feet (155 m) below ground level.

The duration of free-fall produced in a drop tube depends on the length of the tube and its degree of internal evacuation. The 105-meter drop tube at Marshall Space Flight Center produces 4.6 seconds of weightlessness when it is fully evacuated. In the drop facility Fallturm Bremen at University of Bremen a catapult can be used to throw the experiment upwards to prolong the weightlessness from 4.74 to nearly 9.3 seconds. Negating the physical space needed for the initial acceleration, this technique doubles the effective period of weightlessness. The NASA Glenn Research Center has a 5 second drop tower (The Zero Gravity Facility) and a 2.2 second drop tower (The 2.2 Second Drop Tower). [4]

References:

  1. https://en.wikipedia.org/wiki/Reduced-gravity_aircraft
  2. https://en.wikipedia.org/wiki/Neutral_Buoyancy_Simulator
  3. https://en.wikipedia.org/wiki/Astronaut_training
  4. https://en.wikipedia.org/wiki/Drop_tube
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  • $\begingroup$ These techniques may be modified to simulate lower nonzero gravity. $\endgroup$ – Uwe Sep 8 at 11:12
  • $\begingroup$ @Uwe Modifying the drop tube is the hardest; the tricky part making it simulate a stable nonzero gravity. Especially the up-and-down version! You'd need to accelerate upwards constantly; on the down, this is just controlled limited breaking; on the way up, it is trickier (if possible) $\endgroup$ – Yakk Sep 9 at 14:32
  • $\begingroup$ @Yakk couldn't you make the drop tube into a slide rather than dropping straight vertically down? $\endgroup$ – Skyler Sep 9 at 15:03
  • $\begingroup$ @Skyler Friction does interesting things when you touch things. Frictionless surfaces are not a thing. I'd expect some kind of dynamic control over friction is easier than trying for frictionless. $\endgroup$ – Yakk Sep 9 at 15:42
  • $\begingroup$ @Skyler: Even if we could create a frictionless slide, it's also worth noting that the direction of the "gravitational acceleration" relative to an experiment on a slide is neither straight down nor along the slide, but at some angle in between. It's not an insurmountable obstacle, but it's not simple either. $\endgroup$ – Michael Seifert Sep 9 at 17:15
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NASA used several methods to simulate the effects of lunar gravity on astronauts, in preparation to the lunar landing. Neil Armstrong mentions "various simulations" when walking on the Moon for the first time:

"There seems to be no difficulty in moving around as we suspected. It's even perhaps easier than the simulations at one sixth g that we performed in the various simulations on the ground."

According to Lunar Gravity Simulation and its Effect on Human Performance, R.J. Shavelson, 1968, the main methods of lunar gravity simulation at the time were:

  • parabolic aircraft flight
  • water immersion
  • vertical suspension, with counterbalances or springs to reduce weight
  • inclined plane suspension

The inclined plane method, developed and patented by NASA in 1960s, involved sideways suspension of the test subject, allowing locomotion in a straight line (see short film on youtube). NASA has also more recently (1996) developed a new type of suspension system, called the "enhanced Zero-gravity Locomotion Simulator" or eZLS, which can be adjusted to simulate different levels of reduced gravity.

Long-term effects of reduced gravity on the body are also very important to study on the ground. Long-term reduced gravity is usually simulated either with bed rest, or dry immersion rest: floating partially under thermoneutral water in a waterproof loose "hammock" that keeps the skin dry.

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  • $\begingroup$ there's a fifth method: the lunar lander simulators had a jet engine that produced enough thrust to counteract 5/6 of the simulator's weight. $\endgroup$ – Hobbes Sep 8 at 14:13
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    $\begingroup$ Note Hobbes's method would work for simulating the craft's trajectory, but probably not for training people, since the people inside the craft would still feel their normal weight. $\endgroup$ – immibis Sep 9 at 5:53
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In addition to the topic. About a planetrovers. For testing a planetrovers, the method with parabolic flight is used. There are also special crane systems to which the device under test is suspended (using the same method, the moving parts of satellites are tested). As well as inclined ramps.

All this can be seen in the old documentary newsreel - "Self-propelled chassis of Lunokhod (Луноход)": from 1:06 to 3:36

enter image description here

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In theory one could induce a reduced gravitational field over a surface region on the ground by placing an extremely large mass over that region, counter-attracting whatever is beneath it*. In practice such a mass would almost certainly be so large as to collapse whatever was supporting it and crush the region.

(* - A technique used to trap a collection of 'space gnats' in the short story 'The Singing Diamond' by Robert L Forward)

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  • $\begingroup$ This is possible, consider for example a binary planet, the bodies almost filling their roche lobe and almost touching. There will be a neutral surface-near Lagrange point between them. I do believe the question is looking for more practical approaches though. $\endgroup$ – Hohmannfan Sep 10 at 18:54

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