# Tag Info

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### How can we determine the most appropriate central body for orbit propagation?

Several years ago I had an intern investigate this very problem. A good intern task is an interesting but nonessential problem. This qualified as such; he even managed to turn that internship work as ...

### Orbiting Earth before heading to Moon

Going from Earth's surface direct to Moon orbit without stopping in LEO would offer negligible savings -- depending on your assumptions, maybe 20 m/s worth of ∆v (and 2-3 hours worth of consumables).1 ...

### Does the speed of ISS slow down at the time of a spacewalk or does it become stationary?

Let's look at Newton's first law: Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force ...
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### How to choose the best direction to leave Earth's sphere of influence?

Your intuition is quite correct. The Hohmann transfer orbit is a bi-tangential orbit, so at the point where the spacecraft leaves Earth, it is travelling in parallel to us. In the case of Mars, we ...

### How does a Mars lander reach a chosen landing site?

There are typically five planned trajectory correction maneuvers on the way to Mars, referred to as TCM-1 to TCM-5. (Also there is a slot for an emergency TCM-6 a few hours before entry, but it is not ...

### What are the practical uses of a state transition matrix?

From the top of my head I could think of the following practical applications for state transition matrices. Note that the application referred to in the question is captured in point 3. Also, I don't ...

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### What is the amount of Delta-V required to enter lunar orbit following Trans-Lunar Injection?

Strictly speaking, the minimum ∆v is zero: from a ballistic capture trajectory, marginally stable lunar orbits at about 20,000 km altitude can be reached for zero or nearly-zero ∆v cost; an orbiter ...

### How is the trajectory of spacecraft transferring between planets monitored?

In most missions, and especially interplanetary missions, spacecraft position and velocity is determined by calculating the range (i.e. straight-line distance) and range rate between the spacecraft ...

### How can we determine the most appropriate central body for orbit propagation?

A common approach is to calculate the sphere of influence of the celestial objects whose gravity you're accounting for in your propagation. $$r_{SOI} \simeq a \left( \frac M m \right)^{\frac 2 5}$$ ...

### Are there any sources with high-confidence state vectors for GRAIL-A?

Partial answer too long for several comments: tl;dr: The drift you are seeing is just as likely (if not more) to be a difference in your acceleration models as it is an "inaccuracy" (...

### notation to describe an elliptical orbit? (3000km x 40000km)

That's a common convention. The smaller number is the periapsis altitude, the larger is the apoapsis altitude, normally both given as height above a planetary surface rather than radius from ...

### How big a nuke would be needed to break Phobos out of orbit?

Simply to lift get Phobos out of Mars orbit you would need to increase its orbital velocity by a factor of $\sqrt{2}$ (this is generally true for any object in circular orbit). Phobos orbits at about ...
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### Calculate time to impact on elipisoid Earth

In the general case you will need to solve numerically for the radii at which the orbit intersects the ellipsoid. Then you can solve analytically for the times at which the orbit is at those radii. ...

### Orbiting Earth before heading to Moon

Achieving orbital velocity on earth's surface is not practical due to earth's atmosphere. First a ship must get above the atmosphere and then achieve orbital velocity. Once altitude is gained, the ...
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### How to derive the polar form of the equations of motion?

There's probably a simpler way to do it, but here is how you can obtain these equations from the basics. Let's start with equations in Cartesian coordinates: \begin{align} \dot{x} & = v_x;\\ \dot{...

### What liquids last the longest in space?

Just to propose something specific in an answer: I would first guess Gallinstan (by mass: 68.5% Ga, 21.5% In, 10.0% Sn), at least if we're talking about near-room-temperature applications: Supposedly, ...
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### Does Kepler's Method for Determining Time of flight between two True Anomalies break down with eccentricities approaching 1?

As noted in the comments, both methods potentially break down. Kepler's method is mathematically exact, but as formulated here it becomes ill-conditioned as the eccentricity approaches 1. The mean ...

### Where can one find the accelerations due to high-order terms of the geopotential model?

In Vallado's Fundamentals of Astrodynamics and Applications, Section 8.6.1: Gravity Field of a Central Body, he derives the equations to calculate perturbations due to aspherical bodies. In Section 8....

### Why are uncertainties of orbital state vectors provided as covariance matrixes?

The full covariance matrix is useful in itself for much more than just drawing ellipsoids, and it can be a lot bigger than 6x6. Covariance is essential to the basic operations of orbit determination, ...
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### Why is the eccentricity vector used to describe near-circular orbits?

The question is "how are singularities avoided for $e→0$ by defining $(e_x,e_y)$, which is still a function of $ω$, thus still affected by the singularity?". (Worth noting first is that here ...