Hot answers tagged

48

In space you don't just "go somewhere". You have to match orbits, while not wasting too much fuel. If you're in a low circular orbit, and you want to get to a high circular orbit, it takes two tangential burns, one to elongate your orbit into an ellipse, and another at the high point of the ellipse to make it circular again. This is called a Hohman transfer....


44

It's not a mechanism so much as a misconception of the nature of space (and its relationship to time): at low velocities, everything looks linear and Euclidean so we assume it is, but in reality it is not (as can be determined by appropriate experiments). It's kind of like asking by what mechanism you can reach something to your west by traveling east: if ...


40

Ok David asked me to bring the rain. Here we go. Indeed it is very feesible and very efficient to use an electromagnetic accelerator to launch something into orbit, but first a look at our alternative: Space Elevator: we don't have the tech Rockets: You spend most of the energy carrying the fuel, and the machinery is complicated, dangerous, and it cannot ...


34

The Sun isn't "made of fire". It's made mostly of hydrogen and helium. Its heat and light come from nuclear fusion, a very different process that doesn't require oxygen. Ordinary fire is a chemical reaction; fusion merges hydrogen nuclei into helium, and produces much more energy. (Other nuclear reactions are possible.) As for rockets, they carry both ...


33

Ignoring the "no gravity" part of your question - there is, it's 1/6th that of the Earth - and assuming that you mean "no atmosphere", the flag looks like it's "waving" because the horizontal rod that runs along the top of the flag got stuck part way out. This meant that the flag didn't "unfurl" fully and is hanging like a curtain rather than being stretched ...


31

There have actually been cases of (accidental!) exposure to near-vacuum conditions. Real life does not conform to what you see in the movies. (Well, it depends on the movie; Dave Bowman's exposure to vacuum in 2001 was pretty accurate.) Long-term exposure, of course, is deadly, but you could recover from an exposure of, say, 15-30 seconds. You don't ...


27

The gas molecules in your bottle of air aren't just sitting still, they're moving around in random directions. From memory, the speed of oxygen and nitrogen molecules at room temperature is around 500 meters per second. When the bottle is closed, the air molecules hit the walls and lid of the bottle and bounce back, so the air stays in the bottle. If you ...


20

Suppose you move a small distance $\vec{dr}$ = ($dx$, $dy$, $dz$) and you take a time $dt$ to do it. Pre-special relativity you could say three things. Firstly the distance moved is given by: $$ dr^2 = dx^2 + dy^2 + dz^2 $$ (i.e. just Pythagorus' theorem) and secondly the time $dt$ was not related to the distance i.e. you could move at any velocity. Lastly ...


17

It will diffuse into space. Space is a near-perfect vacuum — its pressure is nearly zero and it has extremely little matter (in the empty parts, at any rate). On the other hand, your bottle has a relatively high pressure. When you remove the barrier (by opening the cap), the air naturally flows to the region of low pressure. Once there, it creates a ...


16

It's a great way to get gyroscopic stability. NASA has been using this technique for a long time. For instance, the Pioneer spacecraft used this method. Another example is the Juno spacecraft as well. I hope that answers your question sufficiently.


16

For the sorts of vehicles we're used to, like cars and aeroplanes, there are two contributions to drag. There's the drag caused by turbulence, and the drag caused by the effort of pushing the air out of the way. The streamlining in cars and aeroplanes is designed to reduce the drag due to turbulence. The effort of pushing the air out of the way is basically ...


16

Strictly speaking vacuum is the state of lowest energy. That means no matter or radiation (photons or any other particles). Note that space is not a perfect vacuum. Also note that, technically, a gas of planets and comets etc. has a pressure (there is usually little reason to care about it though). There is also radiation pressure due to the photons. ...


15

A brief history of the misapplication of magnetohydrodynamics to the analysis of the solar wind: 1959: Soviet satellite Luna 1 directly observed the solar wind for the first time and measured its strength. http://en.wikipedia.org/wiki/Luna_1 So as of 1959, by direct experimental observation, it was known that the heliopause was at least the radius of the ...


15

When you take the lid off, all the molecules that would otherwise hit it escape since there is nothing to hold them back. Although the molecules are going at a typical thermal velocity of roughly 500 m/s, the mean free path of molecules in air is about 70 nm and it therefore takes some time for molecules near the bottom of the bottle to "find out" that the ...


15

Two points that may help Think about what is required in order to accelerate. You have to throw something overboard.1 However your engine works you will eventually run out of fuel and at that point you are done accelerating. There is an exception to the "run out of fuel" claim and a possible loophole. The exception is a photon drive: just point a laser ...


14

Let's start by assuming you're in the shade, so you're not receiving any radiation (apart from the cosmic microwave background, which I think we can ignore). The amount of heat per unit area that you radiate is given by Stefan's law: $$ J = \varepsilon \sigma T^4 \tag{1} $$ The emissivity of human skin is allegedly 0.98, and the area of skin of an adult ...


13

When the cosmonaut sneezed they would start moving, and rotating, in the opposite direction, but when the sneeze hit their faceplate (ugh!) this would stop the motion. The net result is that the velocity of the cosmonaut would not have changed, but their position and angle would have. According to Wikipedia a typical breath is 500cm$^3$ and a sneeze ...


12

I think what would happen is that any water molecule with enough energy to escape the surface tension would escape. Because there is no air to provide the water molecule with a way of turning round and going back in, it would permanently leave. This means that the highest energy molecules would selectively evaporate, lowering the average energy of the ...


12

First of all, it is an elementary misconception that there would be a "zero gravity" environment in a tower that would only reach the top of the atmosphere. Most of the air molecules exist at a height smaller than 10 kilometers - and above 100 kilometers from the Earth's surface, the air is so diluted that it becomes undetectable. At the height of 10 ...


12

1) Most materials you use in everyday life contain far more moisture than you might believe. This is a major reason materials meant to be exposed to space are specially designed and tested. In a general vacuum, most fabrics and many plastic will outgas - all of the absorbed moisture and oils will work their way to the surface and boil off - which is a major ...


11

Dear GJ, "vacuum" and "empty space" is always the same thing, but one must always be careful what these two synonymous terms mean. General relativity implies that the only "information" that the vacuum carries at each point is the so-called "metric tensor" - a set of numbers that allow one to calculate the distance between any two nearby points. This is ...


11

First of all, there is gravity on the moon; it's just weaker than the gravity on Earth. Second, gravity has essentially nothing to do with whether a flag flaps. If you managed to create windy conditions in a zero-gravity environment, any flags placed in that wind would flap just as well as they do on Earth (or perhaps even better, since gravity wouldn't be ...


11

According to Universe Today, for the intergalactic medium they state a figure of only one hydrogen atom per cubic meter. As a point of comparison, the University of California, San Diego quotes an interstellar density of 1 atom per cubic centimeter. As to why it is important, all this material has photoionization effects on observations. Even though it is ...


11

Stabilization. Example: Pioneer Equalize heating (barbecue mode). Example: Apollo Deploy antennas & booms (via centripetal force). Example: IMAGE Maintain tension in a solar sail. Example: Cosmos 1 Test general relativity. Example: LAGEOS Create artificial gravity. Example: Gemini Simplify or reduce weight of sensors (e.g. star trackers). Example:...


11

I prefer to think of it that the Earth and Sun actually orbit around their combined center of mass, which just so happens to be very deep inside the sun. The same can be said for the Earth-Moon system.


10

As others have said, it's almost empty, but not quite, as there are gas particles and so on floating around. As wikipedia states: Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the voids, is probably nearly empty. Some theories put the average density of the ...


10

I might add some further notes to the actual material things existing in intergalactic space. One might wonder but the notion that there is space is already stating that there is more than nothing. It implies that there is at least vacuum which is a pretty interesting thing on its own. Classical harmonic oscillator Maybe you know that the harmonic ...


10

Turn on your rocket engines or configure your space craft into a rotating cylinder.


10

Yes, the ball would land in front of you. If you watch from outside the space station, the ball moves in a straight line at constant speed while you move in a circle at constant speed. That means the distance the ball takes to get from point A (where you release it) to point B (where it hits the floor) is shorter than the distance you take. Further, ...



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