# Tag Info

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### What assumptions were made about the sphericity of the Moon for the Apollo program?

Edit: I'm going to have to retract part of my answer. The 1967 farside maps made from Lunar Orbiter data show no elevation information. By the time of Apollo 8, the Lunar Orbiter program had mapped 99%...
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### How did theorists determine that the atmosphere attenuates enough to support unpowered orbits?

If 200 miles up counts as a low earth orbit height, then the appreciation is as old as Newton's Principia, that there was no atmosphere left there, and nothing like atmospheric drag to oppose motion. ...
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### What causes a satellite's orbit to decay?

If satellites are truly far beyond the atmosphere, their orbits do not decay, except for very small perturbation effects like solar radiation pressure from the Sun or tidal forces from the Moon. But ...
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### How feasible would this mission to Proxima Centauri be?

Unless I am mistaken, the amount of speed a gravitational slingshot can add is less than twice the orbital speed of the planet being used. Mercury's orbital speed is the fastest, at a little over 47 ...
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### What assumptions were made about the sphericity of the Moon for the Apollo program?

As mentioned in this answer, the far side of the Moon was not unknown by the time of Apollo 11. And in fact the early Soviet probes did send back photos. The first ever view of the lunar far side was ...
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### Earth re-entry from orbit by a sequence of upper-atmosphere dips to reduce kinetic energy?

Unpowered re-entry into an atmosphere is about trading orbital energy for heat. If the planet being orbited has radius $R$, then the additional specific energy of an elliptic orbit compared to being ...
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### Why does the point of burn on an orbit stay at a constant altitude?

When calculating propulsive manoeuvres, we usually consider them as impulses. That is, happening in a very short time. This is usually a good enough model for chemical rocket engines which deplete ...

### How did theorists determine that the atmosphere attenuates enough to support unpowered orbits?

The scale height is a useful way to describe how the atmospheric pressure decreases with altitude. From Wikipedia, For planetary atmospheres, scale height is the increase in altitude for which the ...
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### How do we define the aphelion of Earth?

The aphelion is usually considered to be the point of greatest actual separation, so this would include the (very minor!) gravitational effects of other solar system bodies. It's not the slight ...

### Distance from earth to another planet in solar system

Here's a short answer to get the ball rolling: However, orbital parameters do not indicate where the body was at a specified date and time to calculate current position... That's absolutely right, ...
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### What causes a satellite's orbit to decay?

The Wikipedia article on orbital decay lists a number of reasons, in addition to atmospheric drag: Tidal effects An orbit can also decay by negative tidal acceleration when the orbiting body is large ...
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### Earth re-entry from orbit by a sequence of upper-atmosphere dips to reduce kinetic energy?

When Does an Orbit Stop Being an Orbit Low Earth Orbit is (rough) 7.8 km/s. This is the lowest amount of energy you can have in an orbit of the Earth. If you try to create a lower energy orbit, you ...
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### Help with fictional scenario: trapped in Lagrange Point orbit

For The Sun-Earth Lagrange points, trajectories to L3, L4 and L5 are time consuming, in the order of several months even for expensive "fast" trajectories. Regular low energy transfers take ...

### What's the motion of two connected satellites orbiting the Earth after their separation?

Many good things here, though a few of the assumptions made are not safe. Before cutting the tether the velocity of two satellites is equal to the one of the system CoM This can not be assumed. ...

### Object slowest at periapsis - despite correct position calculation

If I understand correctly, your $p, q$ are essentially cartesian coordinates, and you're trying to get the velocity components in those two orthogonal directions. However, you're taking the derivative ...
• 148k
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### Minimum delta V required to return a piece of metallic asteroid 16 Psyche to Earth?

Given the relatively low inclination of 16 Psyche (3.095°), and the fact that the line of apside is relatively close to the node line anyway, this can be approximated as a planar transfer. Thus, this ...

### Help with fictional scenario: trapped in Lagrange Point orbit

It is so easy to die in space… Being stuck in a Lagrange point is a bit dull: like being adrift in the Sargasso Sea. Isn’t it more exciting to be sucked into a raging hurricane? A propellant-feed ...
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### How close do gravitational 2-body hierarchy levels get?

The zone of influence of a larger body over a smaller body is known as the hill sphere. Within this sphere the gravitational attraction of the larger body will dominate its satellite and satellites ...
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### When you burn retrograde, will you reach periapsis sooner or later?

It depends on where you are in your orbit. Specifically, it depends on where you are in the orbit relative to your periapsis when you start the burn. Visualizing orbits is difficult at first, but a ...
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### In circle orbit, give a delta V to a satellite, how to find final orbit altitude?

To calculate how high your satellite will go, you forgot 1 major thing. The hohmann transfer. I am going to show you how to calculate the transfer from a 600 km circular orbit to another circular ...
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### Help with fictional scenario: trapped in Lagrange Point orbit

While the question asks for a "Lagrange Point orbit" and current answers mention that Halo-type orbits (about Lagrange points) are slow, typically about half of the primary orbit (so of ...
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### Is it possible to create an analytical ephemeris from raw position and velocity of a Body?

It's possible, because that's partly how VSOP was made, but the amount of effort involved is immense. What you are describing is at the very least a doctoral thesis, if not several of them. Why not ...
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### What is the relationship between the radius of the Hill sphere and escape velocity?

This is where it goes wrong: $$\Delta v = \sqrt{v_2^2 - v_1^2}$$ You're taking the difference of the squares of the velocities at a low lunar orbit, and the perilune of the transfer orbit. But really, ...

### Earth re-entry from orbit by a sequence of upper-atmosphere dips to reduce kinetic energy?

"Earth re-entry from orbit by a sequence of upper-atmosphere dips to reduce kinetic energy?" A satellite in a long very eccentric elliptical orbit will have a greater energy than one in a ...
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### What are "transcritical, pitchfork, period-doubling, torus, and subharmonic bifurcations" in the context of three-body orbits? (e.g. butterflies)

I can attempt an answer by summarizing some of my favorite dissertations. Forgive some of the word vomit below, this is certainly not the most elegant or succinct explanation. I'll give a bit of a ...
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Since we're in an elliptic orbit as soon as we leave the mass driver, we can use the velocity we have to set up the vis-viva equation; $$v^2 = \mu \cdot \left(\frac{2}{r_{moon}} - \frac{1}{a}\right)$$ ...