One topic that has come up in at least two science fiction stories is the possibility of someone in a protective suit dropped off above the Earth’s atmosphere, surviving a free fall to the surface.

I don’t believe that to be possible for Earth, Venus, Mars. But it would be possible on a small enough asteroid. How small?

Now if the atmospheric density profile was right, it could be possible to survive a skydive without parachute on a much larger world. Perhaps Titan. Perhaps a Ceres-sized world with a thick atmosphere?

Consider a protective suit like the Newtsuit, but with better thermal protection.

mollwollfumble said:

I don’t believe that to be possible for Earth, Venus, Mars.

OK, why not?

If the person could be dropped off with no horizontal speed and just drops completely vertically over one fixed spot it should be quite doable (provided the spaceship does not fall on his head)

I think the atmosphere is used as a retrorocket to slow down a returning vehicle, if you have enough fuel you can come down the way you left and not use the atmosphere as a brake at all.

Earths Moon?

mollwollfumble said:

I don’t believe that to be possible for Earth, Venus, Mars. But it would be possible on a small enough asteroid. How small?

You could only slow for a soft touch-down on an asteroid by using a rocket, though being an asteroid the gravity would be so low that it wouldn’t need much braking – It reminds me of the Arthur Clarke story, Hide and Seek

The problem with doing a vertical drop into Earth’s atmosphere is that by the time you got down into the area where the air is thick enough to really start slowing you (about 15 – 20 km high), that doesn’t leave a lot of distance to slow from the very high speed you’d be doing. The temperature rise from air friction would be quite a substantial problem, as would living through the deceleration forces.
The only way you could really do it would be to have some sort of braking system before you got down into the thicker air. I remember a few years back I started a thread on this kind of thing, on the SSSF site, titled “Skydiving from orbit”.

mollwollfumble said:

Now if the atmospheric density profile was right, it could be possible to survive a skydive without parachute on a much larger world. Perhaps Titan. Perhaps a Ceres-sized world with a thick atmosphere?

WANDERERS -Short Film – Erik Wernquist

http://www.erikwernquist.com/wanderers/

See 2 min 45 sec into the film.

I know it’s all CGI, but i reckon he’s not fudging the physics/gravity stuff.

Can one fall to their death on the moon?
https://www.quora.com/Can-one-fall-to-their-death-on-the-moon

yes.

FWIW here’s Arthur Clarke’s Hide and Seek story

Spiny Norman said:

FWIW here’s Arthur Clarke’s Hide and Seek story

As soon as I started to read it I remembered it. Probably read it well over 40 years ago :)

Thanks folks.

Let’s set some ground rules – pun intended.

Two things can be deadly:

• The ground
• Burning up in the atmosphere.

Consider three types of ground – hard ground, water-like liquid, and soft ground.

By soft ground I mean deep snow or panes of glass (unlaminated). Why glass, because it’s about the limit for a sheet material that will slow gown a human without killing him/her and because the deceleration is independent of initial speed. The protective suit will protect against cuts.

By soft ground I do not mean: bubble wrap, a net, any system containing springs, vegetation, rubber. They are all deadly at high speed.

Let’s say 24 g is survivable. (People have survived brief accelerations of > 100 g in a crash).
The current high diving record is 52 metres.

I’m not sure what the drop height should be, above 99.99% of the atmosphere for sure, but what it there’s no atmosphere?

Start with the worst case – escape velocity.
Mars, 5,030 m/s
Titan, 2,639 m/s
Moon, 2,380 m/s
Pluto, 1,230 m/s
Ceres, 510 m/s
No chance of survival without an atmosphere there.

The escape velocity is sqrt(2*g*r) where g is the local acceleration due to gravity and r is the radius.

Where g is not known, the escape velocity is proportional to the radius times the square root of the density. So using data from Ceres, for a back-of-envelope approximation, the impact velocity in m/s on an asteroid or comet is equal to its radius in km.

Ryugu has a radius of 400 metres, so the impact velocity is 0.4 m/s. Definitely survivable.
Asteroid Mathilde has a radius of 25 km, so the impact velocity is 25 m/s. Hmm.

Atmosphere, consider only terminal velocity. The drag coefficient is about 0.5.

Heating is almost solely adiabatic (ram) heating. That’s calculable from atmospheric temperature, density, and fall speed. Or, for subsonic speeds it is. I’m not sure how to calculate heat at supersonic speeds where “A hot shock wave was created in front of the vehicle, which deflected most of the heat and prevented the orbiter’s surface from directly contacting the peak heat. Therefore reentry heating was largely convective heat transfer between the shock wave and the orbiter’s skin through superheated plasma.”

At low densities, the temperature is greatest (because terminal velocity is huge) but because of low air density this is more survivable than at slightly higher air densities.

Then there’s thermal insulation of the protective suit. see https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system

FWIW I’ve been curious for years as to how I’d build a small rocket to get me up to 100 km, and then down safely, so I’d qualify for astronaut wings. Turns out going up isn’t too difficult, all you need is 1 G acceleration for about two minutes and that’ll get you about the official lower limit.
I made an Excel spreadsheet of the speed Vs altitude going up and then down again. You can see how fast you’d be going down by the time you start getting to the thicker parts of the atmosphere, say, 30 km up. You’re going 1,200 m/s. And that’s only falling from about 100 km, so from further up your speed would be considerably higher.

Another FWIW – Racing driver Kenny Brack lived through a crash that had a stop of up around 200 G’s, quite amazing. Link

And you can occasionally get lucky. This stewardess managed to fall from an airliner that had a bomb go off from about 10,000 metres up, and she survived.
Amazing

Spiny Norman said:

And you can occasionally get lucky. This stewardess managed to fall from an airliner that had a bomb go off from about 10,000 metres up, and she survived.
Amazing

Wow, that is amazing.

I still haven’t got a handle on how hot a supersonic object gets.

Probably you’ve all heard of Project MOOSE, which was a bailout system designed by GE in the 1960s. Seems as though they thought it was viable, but the government wasn’t interested. It was a bare bones system for getting a person from orbit to surface in the event of an emergency.

https://en.wikipedia.org/wiki/MOOSE

The design was proposed by General Electric in the early 1960s. The system was quite compact, weighing 200 lb (91 kg) and fitting inside a suitcase-sized container. It consisted of a small twin-nozzle rocket motor sufficient to deorbit the astronaut, a PET film bag 6 ft (1.8 m) long with a flexible 0.25 in (6.4 mm) ablative heat shield on the back, two pressurized canisters to fill it with polyurethane foam, a parachute, radio equipment and a survival kit.
The astronaut would leave the vehicle in a space suit, climb inside the plastic bag, and then fill it with foam. The bag had the shape of a blunt cone, with the astronaut embedded in its base facing outward. The rocket pack would protrude from the bag and be used to slow the astronaut’s orbital speed enough so that they would reenter Earth’s atmosphere, and the foam-filled bag would act as insulation during the subsequent aerobraking. Finally, once the astronaut had descended to 30,000 ft (9.1 km) where the air was sufficiently dense, the parachute would automatically deploy and slow the astronaut’s fall to 17 mph (7.6 m/s). The foam heat shield would serve a final role as cushioning when the astronaut touched down and as a flotation device should they land on water. The radio beacon would guide rescuers.