# Tag Info

53

“Bouncing off the atmosphere” is a misleading turn of phrase. When returning to the Earth from the Moon, a spacecraft is on an elliptical orbit with the high end somewhere around the moon’s altitude and the low end just grazing the top of Earth’s atmosphere. The concern around a too-shallow reentry angle is that it won’t slow the spacecraft enough for a ...

25

Short answer: Yes. Mars is not windy enough to properly wave most flags. Long answer: In storm conditions, a flag constructed out of a very light material would be able to properly wave. If we take a standard flag, say 3'x5' that's made of 200g Nylon $\ell= 1.5$ meters $h_f = 0.9$ meters $W = A * 0.2 * g_{Mars}$, $W = 1$ Newton Going off the calculations ...

25

No, unless your structure is located directly on the equator and your satellite follows a perfectly circular orbit, atmospheric "orbits" aren't possible, even in a vacuum tunnel. Because the Earth is on an axis of ~23 degrees and rotates every day, it is not possible to create an orbit which has no ground track precession except for equatorial ...

18

Escape from planetary atmospheres of the terrestrial planets in our solar system is dominated by ions in absolute numbers, as opposed to neutral particle species. Particles can be any type of molecule or atom here, mostly $\mathrm O^{+}$ and $\mathrm N^{+}$ for Earth. For the case of Earth, a particle, once ionised in the upper thermosphere, can couple to ...

16

In order to achieve "weightlessness", you don't need to achieve a certain speed, you need to achieve a certain acceleration. Earth pulls down at approximately 9.8 m/s^2 which means that any object falling gets faster by 9.8 m/s for every second that it falls. For example, a ball that falls from a tower (disregarding air resistance) and takes three seconds to ...

15

Such a tunnel is not plausible for a number of reasons. 1. Problems with orbits First of all, as other people have said it would only work for equatorial orbits which were either circular (very long tunnel) or had a period which is some rational multiple of the Earth's rotational period. And, again as other people have said, the real Earth is nothing like ...

11

Going straight down takes a lot more fuel for deceleration. In order for a spacecraft to stay in orbit, it needs a substantial velocity perpendicular to the direction of the vector of gravitational force. This velocity is referred to as your orbital velocity; for a near-Earth orbit about 200 miles up, you need an orbital velocity of 7.79 km/s. In order for ...

11

Your premise is incorrect. In no case does "skipping off the atmosphere" leave you going faster than you arrived, engines on or not.

11

Entering the atmosphere introduces drag, which could only reduce your energy. That is, reduce your speed relative to the planet. If you hit the atmosphere at 18,000 mph at too shallow an angle you could bounce off, but not with more energy than you had on approach. You'll fall back, but your landing point may then be outside of your control. You may be ...

9

The problem in making propellant on Venus, is surely in finding the fuel, rather than the oxidizer. Oxygen can be made by electrolyzing $\text{CO}_2$ if necessary and while $\text{S}_2\text{O}_8^{2-}$ is a very strong oxidizer, meaning that it will oxidize lots of things, the mass of the sulphur atoms means that pure oxygen is probably a better rocket fuel ...

9

We don't really know. A study from 2012 suggests that lichens and cyanobacteria could indeed survive the "obvious" perils of Mars, including radiation, low pressure, and temperatures dropping as low as $-50°\text{C}$. In 2012 the Planetary Society reports a two-stage experiment performed at the German Aerospace Center, in which (1) organisms were ...

9

It's not so much a matter of speed but one of altitude: where the atmospheric pressure is low enough that there's no air drag so one can longer be weightless without any air limitation. Basically the altitude where there's a low enough air drag so your parabola can be of any size and where you don't necessarily have to immediately fall onto the Earth. This ...

8

This would be a tunnel a couple thousand kilometers long, that extends from the surface of the Earth to approximately low earth orbit altitude at both ends, strong enough to keep vacuum inside and the atmosphere out at sea level, such that its openings are in place for a space station in a highly-elliptical geosynchronous orbit comes flying through at about ...

7

You're not overthinking it, but it's not a problem. The important concept is that of the 'homosphere`, which is the part of the atmosphere where it is well-mixed by turbulence. This is the part to the left of the diagram you show, where you can see that the proportions of the various constituent gases in the atmosphere don't change with height. (It's not ...

7

Update 2 The taper ratio formula seems to be correct. The crosscheck stress calculation error mentioned in the Update 1 below is found to be due to: a) incorrectly calculated tether volume (and therefore mass). I assumed a linear change of cross section area across the tether length. In fact, to achieve the uniform strength, a logarithmic increase in erea ...

7

Basically it is blown away by the solar wind, headed to interstellar space. A light particle in orbit around the Sun will tend to be pushed further out with time because of both solar wind and photonic pressure. Note there are a few pockets of dust around, but they are very hard to see. It takes being in a relatively stable point, usually the L4/ L5 points ...

7

In addition to the space weathering phenomenon described by ebv, optical surfaces can and do get dirtied by other mechanisms as well. In particular, outgassing from the rest of the spacecraft and produce various compounds that may recondense on lenses, especially if these surfaces are cooler than the rest of the spacecraft exterior, which they sometimes are ...

7

Peroxydisulfates and acid itself would be a bad choice for rocket oxidizers for several reasons: H2S2O8 is actually a solid with melting point of 65°C, therefore unsuitable for liquid fueled rockets, In pure form acid is even more aggressive and corrosive than sulfuric acid, it's like sulfuric acid on steroids. It can explode in contact with organic ...

7

The partial pressure of oxygen (ppO2) should be higher than about 0.16 bar and lower than about 0.4 to 0.5 bar for longer exposition of some days up to a week. So breathing pure oxygen for a week is possible when the total pressure is not above 0.4 bar. See also my answer to the question How long were the Apollo astronauts allowed to breathe 100% oxygen at ...

6

In addition to solar radiation/wind, cosmic radiation. Regarding surface conditions (soil) you will need consider: Potential nutients for plants as well as toxins for plants, humans and other animals – see perchlorates on Mars. Abrasiveness of soils and the fineness of soil particles – see the issues for the Apollo moon missions. Any abrasive particles in ...

6

No, it would float lower if anything. To see this think about the forces on the balloon: the acceleration due to gravity is $g$ and I assume this is constant (the planet is large, the balloon isn't getting very far up: this is a good assumption); the density of the gas inside the balloon is $\rho_H$, the density of the atmosphere is $\rho_A$. If the '...

6

High-altitude ballooning is kind of a gray area as far as space exploration goes, because they can't leave the atmosphere, but they do go high enough to experience space-like conditions (e.g. the pressure is blood-boiling low, and it's darn hot on the sunlit side and cold on the shaded side). Balloon experiments measure things that you might normally ...

5

Optically focusing sunlight does not work when you are deep inside the clouds, collecting your raw material. Inside the cloud, the light will be diffuse and seemingly coming from all directions, even reflecting up from below your ship. If you are able to alternate flying altitude, you could use the lower altitude for cloud mining, then rise higher to get ...

5

The areoid is simply the Mars analogue of the geoid. It's not clear whether the zero elevation level discussed by Zeitler et al. (referenced in the question) is actually a geoid. A more recent paper by Ardalan et al. (2010) defines the geoid using geodetic and gravitational measurements. It is the gravitational equipotential that best fits the shape of Mars ...

5

This is not a stand-alone answer but is an addition to tfb's answer; but since I provide multiple references, one with a rather lengthly URL, it wouldn't work well as a comment. The homopause altitude at Venus isn't a constant. It varies with latitude, "time of day" (longitude with respect to the subsolar point), and solar activity. This abstract shows an ...

5

The resistance is proportional to the density of the air. If you come in at a shallow angle you decelerate more gently as you spend more time at a higher altitude.

5

Tl, dr: With some care, high surface winds can be avoided. They seem to occur only once per fifteen years or so as Titan is in equinox. If we avoid the storms, which would likely be done anyway to simplify landing the craft and taking off again, surface winds will not be a problem at all. From Wikipedia: Surface winds are normally low (<1 meter per ...

5

It would move Down! By the definition of the Karman line on wikipedia, the lift force and the "centrifugal force" must be equal to the gravitational force and, therefore, each other. This gives the following equation: $\frac{1}{2}\rho v^2C_LS = \frac{v^2m}{R_e+h}$ Where $\rho$ is density, v is velocity, $C_L$ is lift coefficient, S is wing area, m is ...

5

The original spy satellites used film cameras. They were pressurized with a half atmosphere of nitrogen. What’s really interesting is that they didn’t need to be, according to the manufacturer. This predates the Mercury program, so I get the sense this was tacked on to the earliest space program to prepare for “Man in Space” The interior of the spacecraft ...

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