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?

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]

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]

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

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.