# Tag Info

84

Cory, here's a different way of thinking about gravity assists that may help: First is my short answer for readers in a hurry: What is really going on is a giant game of pool, with fast-moving planets acting as massive cue balls that impart some of their energy when they whack into tiny spacecraft. Since you can't bounce a spacecraft directly off the ...

61

Can photons push the source which is emitting them? Yes. If yes, will a more intense flashlight accelerate me more? Yes Does the wavelength of the light matter? No Is this practical for space propulsion? Probably not Doesn't it defy the law of momentum conservation? No In fact that last question is the key one, because photons ...

44

Start by considering what is seen by the people watching you from the Earth. Nothing can travel faster than the speed of light, $c$, so the quickest you could get to Kepler 186f would be if you were travelling at $c$ in which case it would take 490 years. In practice it would take longer than this because you have to accelerate from rest when you leave the ...

35

As other answers say, if someone just jumps off of the international space station(ISS), they would still be in orbit around the earth since the ISS is traveling at 17,000 miles per hour (at an altitude of 258 miles). Instead of just jumping, imagine the astronaut had a jet pack that could cancel that speed of 17,000 miles per hour in a very short time ...

33

Energy is in fact conserved, even in gravitational slingshots. After the slingshot, the velocity of the spacecraft may indeed change, which means its kinetic energy will also change. If this happens, the energy increase (or decrease) will be made up by a commensurate decrease (or increase) in the kinetic energy of the planet. In plain English: The planet ...

20

It's all a question of if they need it. Most that are staying within a couple AU of the sun can get sufficient power from solar panels. It's when they start getting further away that they use an RTG. For example, New Horizons, which launched in 2006 (which is considered to be 'modern' when you only launch a few probes per year) is going to Pluto, so it ...

17

The basic tragedy of space travel is expressed by the Tsiolkovsky rocket equation, which says that the amount of reaction mass you need grows exponentially with your $\Delta v/v_e$, where $v_e$ is the exhaust velocity. The advantage of antimatter propulsion is high energy density, but energy density doesn't have any direct, major effect on the amount of ...

16

Can photons push the source which is emitting them? Yes, photons have momentum and momentum must be conserved. The source is pushed in the opposite direction of the photons. If yes, will a more intense flashlight accelerate me more? Yes, more photons means greater momentum. Does the wavelength of the light matter? Yes, shorter wavelength ...

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.

15

edit: I originally had some points about the inefficiency of RTGs, but after some more research prompted by @Jeremy I found that it's not really a valid point when they're used appropriately for the spacecraft's mission. The RTGs used by Galileo at Jupiter generated 300W of power, whereas the solar panels that will be used by Juno at Jupiter will generate ...

15

Somebody has to put the pieces together somewhere. If you do it in the ground, you can work in shirtsleeves, with easy access to supplies, tools, equipment, and other workers. If you need a tool, you can probably walk next door to get it, or ask someone to bring it to you. If you drop a tool, you can pick it up. In orbit, you have to work in a bulky ...

12

It depends on how you define the problem. Humans have re-entered the atmosphere from the International Space Station many times, by riding in either a Space Shuttle or a Soyuz capsule. Someone re-entering without a spacecraft of some sort would obviously have to wear some kind of pressure suit (as Felix Baumgartner did in his jump). How elaborate is the ...

12

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 ...

12

Why can't a space ship accelerate infinitely? Because a space ship needs to carry fuel, and because that fuel needs to be contained in a fuel tank. That need to carry the fuel needed to make the spacecraft accelerate leads to the very nasty ideal rocket equation, $$\Delta v = v_e \ln \left( \frac {m_{\text{initial}}} {m_{\text{final}}} \right)$$ The ...

12

Deriving the relativistic equations for constant acceleration would be a formidable problem for most non-physicists. If you want to see how it's done then look at Gravitation by Misner, Thorne and Wheeler, chapter 6. For most of us the best option is just to look at John Baez's excellent article on the relativistic rocket. The relevant equation is: $$d = ... 11 The real problem with RTGs is that the US stopped making Pu238 in the 80s and has been very slow to start up production again, purchasing all our spacecraft Pu238 from the Russians (who have now also run out). I don't know about the byproducts from the breeder reactors, but Pu238 itself is actually not that dangerous to handle, and only toxic if ingested. 10 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). ... 10 The link you posted refers to a design which would supposedly make use of faster than light travel. After reading this, I immediately stopped reading, since this is not possible. I don't care if it's a NASA-affiliated person who says this, it is simply misguided! Before continuing, I should draw your attention to the fact that I will not be discussing the ... 9 The reason the donut is heavier when it's spinning is that it contains more kinetic energy, and this energy has mass. (Or at least, it does if you use the word "mass" to mean "relativistic mass" rather than "rest mass", which is somewhat out-dated language, but I'm going to keep using it anyway.) The question is, where does that energy go when the donut is ... 9 One aspect is the concern for if the spacecraft were to fail to launch correctly and ended up crashing back to earth. In such cases, the nuclear radiation pollution could be severe if it ended up crashing in inhabited areas. 8 In a lot of ways this is a technology---rather than physics---question, but lets look at some limits imposed by physics. For rockets there are two numbers that matter: the velocity relative the spacecraft with which the fuel can be expelled (called the specific impulse) and the fraction of the original mass that is fuel. For very high mass fractions the ... 8 I wrote a semi-popular book on this subject a couple of years ago: http://www.amazon.com/Can-Star-Systems-Be-Explored/dp/9812706178/ref=sr_1_3?ie=UTF8&s=books&qid=1297567209&sr=8-3 A lot of this was worked out. I illustrate the relativistic rocket and the photon sail. The photon sail is clearly the most reasonable of these two. I also ... 8 The distance between Earth and Alpha Centauri is 4.4\,\text{ly}. Dividing by 60\,\text{years} it's approximately 22000\,\text{km/s}. The relativistic factor, (I mean \gamma = \frac{1}{\sqrt{1-v^2/c^2}}) for this is almost 1. If we take a constant acceleration of 2g (it's possible) it would take only 320\,\text{hours} to reach this speed ... 8 If you are not interested in relativistic effects, the answer to your question is easy to workout. According to Wikipedia, Alpha Centauri is 4.24 ly away (4.0114x10^{16}\mathrm{m}). So to get there in 60 years (1892160000\mathrm{s}). So your non-relativistic answer is v = \frac{d}{t} = \frac{4.0114 \times 10^{16}}{1892160000} = 21200000 ... 7 As long as you have useable energy in your ship, you can use it to accelerate indefinitely your propellant in the opposite direction you want to accelerate; this is how rockets work. The propellant is basically something that carries away linear momentum in one direction so that the ship can gain momentum in the opposite direction. Indeed, as you realise, ... 7 Suppose the spaceship was accelerating constantly at 1g, what would that feel like? Well Einstein gave us the answer to that: it would feel exactly like standing on the surface of the Earth where the acceleration due to gravity is 1g. This is (one statement of) Einstein's equivalence principle. If the acceleration were continued for many hours that wouldn't ... 7 Gravity assists don't change speed in the two body problem. An object approaching a lone gravitating body will enter and leave the vicinity of that body with exactly the same speed. All that a lone gravitating body can do is change the direction in which the object is heading. The body that provides the assist needs to be moving with respect to the target ... 7 1.When they travel to the watery planet, they say that 1 hour on this planet is 7 yrs om earth. How is this possible? Is the planet moving at a speed close to c? Or does strong gravitational field influence time? Sure. This is gravitational time dilation. It's due to the gravitational field of the black hole. You can calculate it using$$\frac{d ...

6

Your assertion that only relative speeds matter is absolutely correct. However, you might want to look at the velocity addition of special relativity for space ships or whatever else travelling at relativistic speeds. For speeds high above our everyday experience, two things which, relative to us, travel in opposite directions with a speed $v$ will not see ...

6

Right, well someone check my math but starting from equations 4.16-4.17 and 4.73 of the link Ben Crowell posted I work out that the delta-v of a boost/plane change/circularize relative to the direct plane change is  \frac{\Delta v}{2v\sin(\frac{\theta}{2})} = ...

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