# Tag Info

125

As you said, it varies. Imagine I'm in Chicago and you're in London. My little dog is running circles around me. Which is closer to you, me or my dog? While the correct answer is "it depends on where the dog is in its orbit around me", I'd argue that a better answer is "it doesn't matter" - the distance between you and me is so great that any little ...

114

In my former job I was writing educational software. In short, it's exactly what you described: we offered a paid version of what you could get for free by looking out on the internet, going to class, going to the library, ... And yet, I'm still incredibly proud of it, knowing I made a difference. What is the difference between well written software and a ...

108

Because the earth goes very fast around the sun. If you want to get to the sun, you need to slow down almost completely so that your speed relative to the sun becomes almost zero. If you don't slow down (almost) completely, your probe will miss the sun when you 'drop' it, so it will eventually come back and you'll end up in an elliptical orbit. Kind of like ...

106

Traveling from Mars's surface to Earth's surface requires less energy than traveling from Mars's surface to Luna's surface, but traveling from Luna's surface to Mars's surface requires much less energy than traveling from Earth's surface to Mars's surface. For the purposes of space-travel, the actual physical distance is much less important than the ...

99

Given a pair of objects that are gravitationally bound to each other, they will orbit around their common barycenter (center of mass of the system). The object to be most logically deemed the moon will be the one of lesser mass because it will be further from the barycenter than its companion. For example, Pluto has a gravitationally bound companion named ...

94

In several press conferences, employees of NASA or private space firms have been asked if they played KSP, and some answered with "Yes". NASA used patched conics to find candidate orbits for Apollo back in the days. With that being said, KSP strikes the balance between accuracy and simplicity. Patched conics give a good idea how space works, ...

79

Distances to Mars You can answer the question with Astropy, a Python library for astronomy. Here's a diagram of distances from Earth to Mars and Moon to Mars, between 2000 and 2030. You can see that the two curves are so close from each other that they look like one single curve. Here's a zoom around the last opposition, in May 2016: Relative difference ...

78

The Apollo spacecraft consists of three major parts: The Command Module (CM), a conical module where the three crew members live during launch from Earth and travel to and from the moon, and which re-enters Earth's atmosphere alone at the end of the trip; The Service Module (SM), a cylindrical section containing fuel, power, life support, communications, a ...

74

Gravity isn't just about mass, but about distance, too. Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius, holding mass constant. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its ...

72

I quite agree that it is not intuitive. However, orbital mechanics are frequently not intuitive, probably because we don't get to experience an orbital environment on a regular basis (if ever). Let's just assume we're talking about circular orbits for the remainder of my post, since you are a beginner in orbital mechanics. There is only one speed that a ...

72

In order to land on the Moon, you must, at some point, be moving towards the Moon (decreasing your distance from it, to be more precise, you may also be moving sideways) and close enough that the Moon's gravity dominates that of the Earth and the Sun. From that point on, your kinetic energy (relative to the Moon's centre of mass) can only increase as you get ...

71

Maybe some visual intuition for what actually happens in the Hohmann transfer helps? It's already very close to what you are describing. In the top arc, the spacecraft (yellow), is going a bit slower than Mars (red), so it's indeed "waiting" for the planet to catch up to it. It only touches the orbit of Mars in a point, but that's all we need if we time ...

67

Your orbit is uniquely determined by a current position (three coordinates) and velocity (three more quantities to give magnitude and direction). Going places involves changing your orbit. For instance, from a circular orbit about Earth, enter an elliptical transfer orbit to the moon, then circularize your orbit about the moon. Everything you do in space ...

62

Going directly to the Moon would require a very small launch window. The Earth orbit before enabled a launch window of about 3 to 4 hours, see this question. Abort from an Earth orbit was possible when the second ignition of the third stage of the Saturn V failed using the Service Module engine to initiate a reentry. Time in orbit was used to complete the ...

60

Imagine you have a very heavy book and a bookcase, and your goal is to put the book on the top shelf of the bookcase. How much time would you spend doing that? Maybe five seconds, maybe fifteen. Would going much slower help you? No, it would not, because simply carrying the book is exhausting to you. You would never be able to hold the book up for an entire ...

57

@SteveLinton's answer is right, no matter how gently you try, by the time you get to the surface the Moon's gravity will have accelerated you to something like 2,400 m/s. There are ways to use the gravity of the Earth and Sun to make a tiny reduction in this, but it's a very small effect. The simplest way to argue this is that rocks on the Moon don't ...

57

Yes. 1st scenario: A spacecraft orbiting the Sun at Earth distance vs. Pluto distance, shedding its orbital velocity The orbital velocity decreases with distance, according to the following formula, where $r$ is the orbital radius, and $\mu$ is the mass parameter (it's just a shorthand we use) $$v_{circular} = \sqrt{\frac{\mu}{r}}$$ The orbital velocity ...

53

I want to allow students to tinker around with basic central force motion and see the ways in which conic sections are altered by thrust, etc. Seeing/enacting an example of rendezvous (maybe in a CW frame?) would be neat too. I definitely think Kerbal Space Program is the right answer here. The ways in which it departs from real-world space flight (such as ...

52

Let's say you start rolling down from the top of a half-pipe skateboard ramp, and you plan to get back to your starting point. If you stop in the middle of the pipe, it is much harder to climb back up to your starting point then if you ride up the other side of the ramp and let gravity accelerate you back. Similarly, even if you planned to get into an orbit ...

52

What immediately springs to mind is the Martian moon Phobos, orbiting the planet in 7 hours 39 minutes. That's a fair bit quicker than the 24 hour 37 minute sidereal period of Mars. From the surface of the planet, Phobos and Deimos will therefore appear to cross the sky in opposite directions. Other solar system examples include the small inner Jupiter moons ...

50

All the other answers are great, but I think one explanation is still missing: how an interplanetary orbital transfer actually works in practice. The thing is, space is rather big, and things keep moving. At the same time, you're being tugged on constantly by all the other bodies in a planetary system (we can ignore other stars for interplanetary transfers)....

48

It isn't really feasible to launch your own rocket, unless you have a lot of money to spend. The required power is immense. Hitting sub-orbital might be possible, and has been done once by amateurs, but they received sponsorships and had a team dedicated to making it happen. The trick to an orbital rocket is not just to get high, but also fast. The speed ...

48

That's a mistranscription of OMS Burn, or Orbital Maneuvering System burn. The OMS system is how the shuttle changed its orbital characteristics. You can read about it here. One, two or more might have been used to fine tune the orbit, avoid space debris, rendezvous with the space station, etc.

47

Statement of the Problem The problem you want to solve is called the Kepler problem. In your formulation of the problem, you're starting out with the Cartesian orbital state vectors (also called Cartesian elements): that is, the initial position and velocity. As you have discovered, the only way to propagate the Cartesian elements forward in time is by ...

47

As was astutely noted by Hans, the period of the movement was about 25 days. It turns out that is the time it takes for the sun to rotate once. When I was grabbing the data from JPL Horizons, I listed the target (center) as "coord@10". I should have omitted the "coord", as that means coordinates, or in other words, a point on the surface. Without that, it ...

46

http://nbviewer.jupyter.org/gist/leftaroundabout/3955d27877e19be39d0f61fdafce069e Barely achieving escape velocity means you take a parabolic orbit. The thing with parabolic orbits is that they actually approach zero speed as you depart to infinite distance from the starting body. That is, zero speed with respect to the starting body's frame of reference, ...

45

Kerbal Space Program is somewhat of a medium fidelity simulation. It manages a few things quite well, and a few things not as well. Let me try and give a list (which might be a bit out of date): The Good: The orbit simulation is quite accurate, including how to change inclination, raise/lower orbits, leave a planet, and approach a new planet. The staging ...

45

In terms of distance, the two swap considerably. But perhaps a more interesting question is, which of the two is closer in terms of the energy required to land. For that, let's look at our friend, the delta-v table. Once one is approaching Earth from Mars, things only become different at the point labeledEarth C3=0 (See $C_3$). From there it is about 2.3 ...

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