# Can a blimp dead drop on Mars or Earth this way?

I saw a space jump where someone had exceeded the speed of sound before the air slowed him down enough to deploy a parachute, because the air was not dense enough to tear him apart in the upper atmosphere.

On Mars: Can a blimp also enter the atmosphere at an angle with a skip entry and will the gradual increase of air pressure be enough to lessen the need for retrorockets or heat shield?

It would be inflated outside the atmosphere and inflating during entry with a stiff under body.

If an entry is only possible at slower speeds for this, could a blimp skip all the way around the planet like a skipping stone to water or skis to water to slow down if the surface on the bottom is large enough and the blimp light enough?

other related questions: https://physics.stackexchange.com/questions/293671/can-a-blimp-skip-on-the-atmosphere

Blimp on Earth, but a glider on Mars?

• @DanPichelman The question is about Mars, so I assume you'll be in orbit. – ventsyv Nov 17 '16 at 21:30
• @ventsyv - my bad, I missed reading the tag. Thanks – Dan Pichelman Nov 17 '16 at 21:47
• There is an important difference between a high parachute jump and being in orbit or arrival at Mars. Felix Baumgartner started off stationary at 39 km above ground; orbital speed is 7 km/s at around 400 km, and interplanetary arrival speeds are similar, i.e. something like Mach 20. If you want your blimp to enter the atmosphere at the speed of sound, first you have to work out how to slow it down to this relatively slow speed. – djr Nov 19 '16 at 20:48
• I don't think that's an achievable trajectory... Basically when the orbit degrades you've got to go from orbital velocities to landing in a matter of minutes. But that's a solved problem, that's what heatshields are for (including the inflatable heatshield that @Hobbes mentioned). – djr Nov 20 '16 at 0:23
• But I think you don't understand the problem with retros. Using retros to cancel the 7 km/s orbital velocity is very expensive, which is why everyone uses atmospheric drag to do most of the work. On Mars or Earth, retros are used to slow from 10's of m/s under parachutes to near-0. Doing this with retros requires less mass than making the parachute bigger. But Beagle shows that non-retro landings are possible. – djr Nov 20 '16 at 0:30

That should prove to be very difficult, but I think it depends how long the blimp would have the form of a blimp. Let me say why:

First of all:
The jump you are referring to, was a PR-event so I'm not gonna mention names. However the jump was not a re-entry from space, it started in the Stratosphere, some 25km above the ground, while low-earth-orbit (LEO) is at about 300-500 km.
Reaching the speed of sound up there (or Mach 1) is easier, because the speed of sound changes with height, along with atmospheric temperature, as seen in this plot from wikipedia (follow the blue curve):

So the alleged record is actually a cheat.

Your blimp will come in at way higher speeds than Mach 1. The temperature on Mars is roughly a factor of $$\sim 2$$ lower than on Earth (measured in Kelvin), so the speed of sound there is also a factor of 2 lower. But then an object coming in with the same velocity as on Earth will have Mach 2 instead of Mach 1. Or 60 instead of 30.
This is important, because objects coming even from LEO on Earth reach Mach numbers of 30 (like the ex-space shuttle) and then have trouble getting rid of the heat.
And the higher the Mach number and the bigger the surface area of your object is, the more heat you'll have produced during re-entry. So you will want to make your blimp very compact for Mars atmospheric entry, and inflate it later. Then you need boosters to slow down, as well as tanks for the blimp becoming a blimp. I have no engineering perspective on this, how feasible that would be.
Additionally you'd have to be a VERY big blimp, because buoyancy forces depend linearly on the atmospheric density surrounding the blimp, which is ~100 times lower than on Earth.
Thus, your blimp would need to have a $$\sim 4.6$$ times bigger on all axes to reach the same stability as on Earth.

So to summarize:
The blimp will not enter the atmosphere of Mars with Mach 1, if coming from space. You need to slow it down beforehands, but for it to survive this, the blimp should be packed. And then you might want to think if a blimp on Mars makes sense at all.

• The amount of heat you need to remove relates to atmospheric pressure and speed, not directly to Mach number. "But then an object coming in with the same velocity as on Earth will have Mach 2 instead of Mach 1 " is just wrong. – Hobbes Nov 18 '16 at 14:58
• @Hobbes: No it is not. M=v/c. Now change c while keeping v constant. Also the denstiy, pressure and temperature jump in a shock is a function of mach number only. Basic hydro. – AtmosphericPrisonEscape Nov 18 '16 at 17:05
• Speed of sound goes as sqrt(T), not T, so the value on Mars is around 240 m/s. – djr Nov 19 '16 at 0:03
• @djr: Fair enough. It's more 200m/s however, when taking the variation of the mean molecular weight into account. And this is ofc true for the surface only, where you are not entering the atmosphere, obviously. – AtmosphericPrisonEscape Nov 21 '16 at 13:54

NASA is working on an inflatable heat shield. Not quite a blimp, but a heat shield that's inflatable to a size much larger than the probe it's attached to. The extra area helps decelerate the spacecraft. So inflatable structures can be made to withstand the heat of reentry.

Because Mars' atmospheric density is only 0.6% of Earth's, a blimp's lift will only be 0.6% of its lift on Earth.
The Hindenburg could carry 90 passengers on Earth. It weighed about 200 t. It'd be able to carry about 1 passenger on Mars. A blimp would be lighter (Hindenburg was a rigid airship), but you can see you'd need a ridiculously large structure to ensure a soft landing. To compare with curent landing methods: one passenger is well within the load capacity of Curiosity's sky crane.

• The question is about entry, not uplift. – AtmosphericPrisonEscape Nov 18 '16 at 17:09
• Yes entry and that would be enough to safely enter at a controlled speed? It could plant the rocket upright? – Muze the good Troll. Nov 18 '16 at 20:19
• The question is about enabling a soft landing without retrorockets, i.e. vertical speed close to 0. For that you need lift to be about equal to weight. – Hobbes Nov 19 '16 at 9:43

Regarding your recent edit: No you can't just bounce around the atmosphere to slow down with a blimp.

Unlike water, there is no clear limit between space and atmosphere, which leaves you with two options:

## Phase 1:

You can decelerate at very high altitude where there is only faint trace of atmosphere

=> This will eventually decrease some (i.e. not nearly enough) of your speed, until you will enter phase 2.

This has been used multiple times by satellites, and was first used by the Hiten mission

## Phase 2:

To generate enough lift to ascent and skip the atmosphere like you describe, you need too have substantial atmosphere. By the time you reach the point where you can do this, this is what you looks like:

A giant ball of fire. A blimp cannot survive this since it cannot afford to carry any significant weight (shield) that would allow it to reenter safely.

There is no in-between: you cannot maintain altitude and slow down gradually until you reach survivable speed.

As a side note: Orbital altitude is a function of speed. If you decrease speed you decrease altitude. That's why you can't do that.

• It wouldn't work neither: There is really a gap between Height where you can have significant lift and Height where you start burning up. The latter being higher, we are stuck using conventional designs. The only blimp style design that could be considered is one that you would deploy from the ground after landing. – Antzi Dec 9 '16 at 1:11
• @Muze The comment space is not the right place to ask new questions. – called2voyage Jan 17 '17 at 13:37

Yes, at least in theory, but in practice there is very little atmosphere on Mars, so your blimp will have to be huge, or the payload very small.

• The payload is the blimp/bio-dome. – Muze the good Troll. Dec 30 '16 at 23:16

No one used XKCD? the interplanetary Cessna should give a good indication:

https://what-if.xkcd.com/30/

The problem is how airships and especially blimps float. They are on average less dense than the air around them, and then float up to a region where the air is equally dense in average as the blimp.

Unfortunately Mars has about 1% of the earths atmosphere in density. Even if the blimp was inflated gently on the surface it might still be denser than the air around it and not float! Now imagine trying to slow down a fall at the speed of sound on an atmosphere barely dense enough to float your blimp...

I'd give the Cessna a better chance of survival.

• There are baloons on Earth which can go to 40km high. But these are really high-tech. These would work also on the Mars, although it is still probably not the best option. – peterh - Reinstate Monica Aug 11 '19 at 0:29