124

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 ...


110

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 ...


105

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 ...


90

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, without being ...


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 ...


73

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 ...


70

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 ...


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 ...


63

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 ...


58

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 ...


55

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 ...


54

@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 ...


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 ...


51

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 ...


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 ...


46

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

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.


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 ...


44

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, ...


43

All interplanetary probes that I am aware of were launched into a parking orbit, and then waited some time in that orbit before restarting a stage or igniting another stage to inject on the desired outgoing asymptote. This is done for convenience to allow long launch windows on days in the launch period. It is possible and slightly more efficient to launch ...


42

The Juno spacecraft has no means to directly measure and compute that it is in orbit. It did not send any such confirmation message. All it sent was an FSK tone indicating that it had completed the activities it was commanded to do. After the spacecraft turned back to Earth, it transmitted all of the recorded engineering data from the event, providing much ...


41

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 ...


41

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 ...


41

More or less. While the ISS is below the satellites use for TV transmissions, it is passing by so fast that the coverage will be highly intermittent, meaning that you would be able to watch a channel for only a couple of minutes, have black outs over the oceans, and repeat. Other notable differences would be: Normal satellites receiver are "fixed": The ...


41

There is very little to gain by going straight to the Moon, and as @Uwe has said, it makes the timing of the launch extremely demanding. Let me have my go at explaining why there is very little to gain. The most fuel efficient way for a rocket to get from Earth to the Moon is basically to accelerate as close to the Earth as possible until it is moving at ...


40

There are a couple of reasons. The distance from the L2 to Earth is only 1.5 million km away. The L4/L5 are 1 AU, or about 150 million km away. That leads to a reduction in link margin of 40 db, or 1/10000. That is quite significant. In order to compensate for that difference, you either need a bigger radio dish, more power, or a loss in data. As you ...


38

Understanding the Principle Let's start by understanding the mechanism of a gravity assist. As a spacecraft approaches a planetary body, it gets affected by the planets gravitational pull. Getting nearer, the pull increases, and eventually when the spacecraft passes the planet, the pull decreases. If you think about a stationary planet as an absolute ...


36

No, because there's nothing like water for a keel to work against. In water sailing there are two force vectors, the vector from the reaction of the wind against the sail, and the vector from the keel and rudder against the water. These vectors add together to propel the sailboat. This works for almost any direction on the compass except where the wind ...


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