Hot answers tagged

102

(*) Jupiter, for all intents and purposes, doesn't have a solid surface to stand on. Not any more than you could say that Earth's atmosphere has it, before you hit Terra Firma. It's an enormous ball composed of mostly Hydrogen and Helium, but also other heavier elements in smaller parts, and it's so massive that its own gravity compresses these gases into ...


76

Standard atmospheric pressure at sea-level Earth is just 14.696 psi. Compare that to 340 or 300 psi (23.14 and 20.42 amt, respectively). The difference in internal tire pressure in Earth's atmosphere and absence of atmospheric pressure in vacuum of space is only 4.3 - 4.9%. Tires would experience far more dynamic pressure environment due to friction heating ...


58

Did it really happen? Yes. The investigation of Japanese Hitomi spacecraft's failure found that it was spinning too fast due to attitude control error. As a result, the spacecraft spun so fast that several pieces of debris were registered. But it was caused by thrusters, not reaction wheels.


52

To parallel @Heopps answer: Did it really happen? Yes. In spectacular fashion! In 1965 NASA launched a boilerplate Apollo command module on a Little Joe II rocket to test the Launch Escape System (LES), and got more of a test than they'd bargained for. Due to an erroneous installation of gyros the control vanes on the fins went to full deflection upon ...


52

Interesting but no, it wouldn't work for the same reason that astronauts in the International Space Station, other space stations, or orbiting shuttles or capsules do not "feel" gravity with respect to their station or capsule. When you are inside an object which is in orbit, you are in orbit too! The Earth pulls on the station with nearly 1 g and it pulls ...


41

Throwing it down at 5 m/s will do basically nothing. That will simply cause it to advance in its orbit a bit. To deorbit, you need to throw it backwards, not down. However in this case, since the feather has a such a low ballistic coefficient, it will promptly deorbit from ISS altitude on its own, without you having to do anything at all. Just wait a bit....


33

The main engineering challenge in implementing your proposal is that in order to be competitive with a chemical rocket engine, the grinding wheel must rotate at an extremely high velocity. A typical chemical rocket might have a specific impulse between about 250 and 450 seconds; therefore, the exhaust velocity is about 2500-4500 m/s. In a competitive ...


28

It turns out that yes, there are things you can do, but they depend on things other than the astronaut's body, and they will take a long time. Physics tells us that an object's translational momentum is constant unless acted upon by an external force. If the astronaut's net momentum with respect to the room is zero, there is nothing they can do to start ...


27

The Chelyabinsk meteor was travelling at over 65,000 km/h when it hit the brunt of the atmosphere 23 km high in the air. This is 60 times the speed on sound! NASA estimates that the meteor's mass at this point was 10,000 tonnes, and it had a diameter of 20 meters. At these incredible speeds, the body is placed under immense stress. Colossal pressure is ...


26

Was standard Newtonian mechanics sufficient or were relativistic effects included? Relativistic effects didn't have to be modeled; other sources of error would have swamped the effects of relativity, and midcourse corrections were made. Were the Earth, Moon, and spacecraft modelled as point masses or more complicated bodies? The moon's gravity was ...


26

I'm afraid you are incorrect. An object on the equator of Earth has a velocity of ~460 m/s. A satellite in geosynchronous orbit has a velocity of ~3000 m/s. You may be confused by the fact that both objects complete an "orbit" in 24 hours. But consider the fact that the satellite travels a significantly greater distance in that time.


25

I am referring to rockets capable of taking supplies and humans to other planets. For an interplanetary single-stage rocket with tens to hundreds of tons of payload capability, no existing propulsion system can do the job in a practical way. Chemical rockets lack the fuel efficiency; electric rockets don't have the thrust required to leave Earth's surface. ...


25

Fortunately, it turns out humans come with a nitrogen/CO₂ thruster built in... Assuming the room is filled with air, I reckon the best method is to use your breath. What you should do is, point your feet in the direction you want to go (there are quick standard techniques for this, like what cats do to land feet-down). Then breathe in deeply using your nose ...


24

I'm leaning toward option 2, that you "have something terribly wrong in [your] thought process". Here's at least some arguments why; I by no means claim this to be an exhaustive list: Mach 3 (which is pretty fast for a fighter aircraft, toward the upper end of the currently attainable range) is right about 1 km/s (Google calls it 1020.87 m/s), and you don't ...


21

We can launch from space and in a sense, we already are. If you consider upper stages of orbital launch vehicles that might send spacecraft into higher Earth orbit or even escape Earth's gravity well altogether, we really ignite those when they're already in the vacuum of space. And depending on the launch vehicle's capability and intended trajectory, they ...


21

I want to focus on aerodynamic stress however, like when a rocket deviates from its path or has a wrong angle of attack, what causes it to be destroyed? In many cases, it's not aerodynamic stress. Many launch vehicle explosions result because they are commanded to do so. Every launch vehicle launched from the U.S., including the solid rocket boosters on ...


21

tl;dr: No chance, not even close! The escape velocity from the surface of a round (spherically symmetric) body is given by $$v_{esc} = \sqrt{\left(\frac{2 GM}{r_0} \right)}, $$ showing that it is the $\frac{mass}{radius}$ ratio that's key here, not just the surface gravity given by $$a_{g} = -\frac{GM}{r_0^2}. $$ So since $$v_{esc} = \sqrt{a_g r_0}, $...


21

How small do you want to get? $F=G{Mm \over r^2}$ applies regardless of size. If you remove enough disturbances from other bodies you can get two neutrons to orbit a common barycenter on gravity alone - or send them against each other on a near-miss trajectory and they'll pass influencing each other gravitationally in essence performing a slingshot against ...


19

That is an excellent thought experiment to consider for a spinning vehicle. You are correct that if you simply enter the open space inside the rotating cylinder, somehow not following the rotation yourself, you will not experience a force to pull you to the outside. You could just hang there and watch the "floor" rotate below you. However you probably won'...


19

I don't know if it has ever been considered by anyone. In my view, this is not a good idea for at least the following reasons: It is equivalent to mechanically throwing things retrograde. See this video for an overly simple example. This is obviously not a good way for propulsion, as the specific impulse is very low. Let's talk just about the impulse $$p=...


18

Kirchhoff's law is only valid for objects in radiative equilibrium. The emissivity and absorptivity of a material are the same for a given wavelength, but can vary dramatically for different wavelengths. The radiators on a spacecraft are not in radiative equilibrium, since they lose heat to radiation. They emit heat in the longwave infrared spectrum, but ...


18

The frame of the spacecraft is used as a common ground. See this diagram from this handbook.


18

Rockets are basically devices which exploit Newton's Third Law, for every force there is an equal and opposite force. By throwing mass out the back as fast as possible this imparts an equal force that lifts the rocket, engines, payload, and all its own fuel. Single-Stage-to-Orbit can be done, but it's horribly inefficient. This is because of the Tyranny Of ...


17

It's fundamentally impossible to measure gravity from an object that is in freefall. This is the first principle of general relativity. What accelerometers will give you is accelerations (linear or rotational) induced by thrusters, atmosperic drag, reaction wheels, etc. vibrations from rotating solar panels, crew-induced forces, etc. The only thing you ...


16

Yes, it would explode. Most hand grenades are nowadays triggered chemically, electrically or contain a fuze enclosed within the assembly, so they don't require atmospheric oxygen to ignite, are watertight and otherwise more reliably go off at a preset time since activation. You would however create a large number of dangerous debris that would float forever,...


16

Yes, a properly functioning accelerometer that is stationary relative to the surface of the Earth will read the acceleration due to gravity. If it's a very good accelerometer, you could also see the factor of a few hundred smaller decrement in that acceleration due to the centripetal acceleration from the rotation of the Earth. If it's a really really good ...


16

Have the object tidally locked into its orbit around the sun This way we would get the 1g gravity from the sun on the opposite side of the asteroid Interesting idea, but you missed something in your math. You'd only get the tidal difference between the sun's gravity at the centre of mass vs. the sun's gravity 1 object-radius farther away. This varies ...


15

One of your instincts was correct; it is indeed the influence of the Moon. Wikipedia notes: Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon; both rotational energy and angular momentum are being slowly transferred to the Moon: see tidal acceleration. And here is the general case of a satellite ...


14

The term "friction" is a misnomer. The source of heat is adiabatic compression - gas on trajectory of the reentering object is compressed against its leading surface, and as result heats up. On molecular level you can think of it as number of molecules rising in given volume (compressed) and additionally speeding up (by elastic collisions against the fast-...


14

Reaction wheel? No, these are way overpowered in KSP, and don't exert nearly as high torque in real spacecrafts, never mind the issue of saturation (maximum speed of reaction wheel motor). But asymmetric thrust is a definite danger. If some engines of a rocket fail to fire, or stop firing early, the remaining ones will send it into a spin. And that may end ...


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