Interesting, first soft landing on the Moon (or anywhere). Used rockets to slow down and then air bags for the final landing, bounced a few times. Luna 9

I can see that potentially working for a small spacecraft like a cubesat, sent to the Moon.

WHY LANDING ON THE MOON IS EASY WITH ROCKETS

The thing about the Moon is that with its low gravity you don't need much of an engine to land there. As you can tell by the tiny rocket engines of the lunar module compared with the size of the Saturn V that was needed to launch from Earth.

And with modern technology, easy to control the landing, and no atmosphere makes it much easier to land than, say, on Mars.

So the question is - what could airbags offer that might give them an advantage over rockets when rockets work so well on the Moon?

So would need to be a situation where for some reason you can't put a rocket engine into your spacecraft, or can't control it precisely, or can't do ranging estimates of the surface, or some such scenario.

So - gets me thinking first of a scenario where perhaps your main spacecraft landing on the Moon carries dozens of cubesats which it fires in all directions as it lands - and they then use airbags to cushion their landing on the lunar surface. Something like that perhaps?

That could be useful for instance if you don't have a rover, and want to explore a region around your lander. A bit like Penelope Boston's cave bots idea for exploring Mars caves

HOPPING MICROBOT SWARMS TO EXPLORE SURFACE AND CAVES

Small, spherical microbots filled with minature fuel cells, instruments and an artificial muscle for hopping

Microbot Madness: Hopping Toward Planetary Exploration

They could fit a thousand of these into 174 Kgs.

Penelope Boston also talks about them here - for exploring caves on Mars.

(this bit is extracted from my Soaring, Buzzing, Floating, Hopping, Crawling And Inflatable Mars Rovers - Suggestions For UAE Mars Lander )

The Moon has caves too, so her idea may be useful there as well.

Down the Lunar Rabbit-hole

Though she doesn't talk about airbags, I can imagine they might be useful especially when you don't know the depth of the cave in advance. A few tiny air bags attached to each microbot, inflates during the landing, and then detaches on landing. Either eject them during the landing of the main craft (which communicates back to Earth), or fire them into the cave after the landing.

Or a similar idea, to explore the ice deposits of the craters of eternal night at the poles. Can't see the surface visually, just fire lots of cubesats or microbots and cushion them with air bags. Even if you can see the surface, it lets you explore a large area with relatively simple technology without needing a rover. Might be useful in some scenarios.

SIMPLIFYING THE LANDING PROCESS

Another possible scenario - if your spacecraft doesn't have any video feed or radar ranging - perhaps a tiny cubesat type spacecraft with a small retro rocket to land - but no way to tell the distance from the surface. Program it to land approximately on the surface and then use air bags to deal with the last few meters of fall.

Or generally, it could be useful as an extra precaution to save your lander in event of a hard landing, a bit like airbags in a car. If you hit the surface at a few meters per second, maybe some computer glitch during the final approach - well an air bag could save your spacecraft.

Seems like a technology for lunar landers to keep in mind in case it is useful.

And just looking it up, one of the lunar Xprize  contenders, Plan B say

"Main weight target on low-earth orbit for a probe and vehicle total is 100-150 kg. Flight schema will include two orbit correction impulses, one main and one brake impulse with direct arrival to the moon surface and soft landing with air-bags assistance. "

So yes, it's a good enough solution so that one of the X Prize contenders considered using it.

It makes the landing simpler to just have a preset landing sequence and use air bags for correction of errors.

"Brake impulse can be preset and constant at design stage, plus-minus variation verified at testing stage. “Earth-to-moon” orbit correction together with intensive calculations on mission control system, and on board computers has to deliver probe in specific point, with specific velocity in sun-earth-moon celestial point. Astro-orientation system must set impulse’s vector based on desired landing place. Brake impulse should slow probe enough to be allow soft-landing (air-bag based) system to adsorb impact at lunar surface. Before soft landing all unused parts and engines frame needs to be disconnected / ejected. Probe's antenna needs to be placed into a landing position."

...
" Moment of engine stop, detected by acceleration sensors will signal to deploy airbags. Deploying airbags will remove engine frame mounted on a lunar vehicle and the frame itself will be ejected by the rotation’s momentum from brake stage. Air bag will be inflated by evaporated ethanol require for low-thrust engine burning."
...
"Airbags should be capable to reduce maximum speed at surface impact from 80m/s to 0 m/s with landing on lunar powder 20 cm deep. On impact airbags will be ruptured. Orientation to surface at landing will be achieved by rotation of probe. All landing will be performed automatically by on-board computer with telemetry recorded. It is not practical to make video after airbags will be deployed, but some video recording is possible at brake-engine firing."

But later they dropped the airbag idea and plan instead to use a method a bit like the crumple zone in a car:

"To survive impact, air bags idea was abandoned because of a weight restriction, complexity of inflation process, and unidirectional capability to adsorb impact. Instead of this we adapted idea of making impact adsorption shield made of composites, each layer of a shield crushes into each other with energy of impact transforming to a process of destruction. Shield will be on one side of a rover only and has to be orient before impact to face lunar surface. This is a very, very weak point in design – after separation from a break engine, rover has to align itself to brace impact."

Adobri Solutions Ltd

G FORCE FOR AIR BAGS LANDING

If I understand it right, the original Plan B idea was for airbags that puncture and deflate immediately on impact.

But the landings on Mars used air bags that were more robust and let the spacecraft bounce several times. Even if you approach at just 80 meters per second, or 178 mph, if you need to get rid of all that delta v in say one centimeter, so an 800th of a second, that's 6400 gs.

So it's not just the delta v, but how quickly you need to stop it. If your bags can compress a lot, say in one meter, that's 1/80 of a second so it's 80/(1/80) or 6400 meters per second per second deceleration, so around 640 g. Still a very rough landing.

Bouncing doesn't help with the g force of the initial impact if you hit the surface at a steep angle, as you still have the deceleration to a stop for the first bounce, and then bounce back up again.

If your spacecraft can approach at a more shallow angle, then it can slow down over a much longer period of time. This has the disadvantage that it will set the lander spinning.

See discussion here Hard landing for a lunar rover

The Spirit rover on Mars hit the surface at around 50 km / hour or about 14 meters per second.

NASA - NSSDCA - Spacecraft - Details.
(image from Mars Exploration Rover Mission: The Mission )

So that much at least is survivable by a spacecraft with airbags.  (I doubt if the near vacuum Mars atmosphere makes much difference to an airbag impact at those slow speeds?)