# Tag Info

51

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 ...

31

Not a soft landing. A soft landing requires the spacecraft having a thrust-to-weight ratio greater than one (otherwise it just falls faster and faster). Ion engines have a very low thrust to weight ratio, much smaller than one. On the moon, the surface acceleration is 1.625m/s², so the thruster must provide at least 1.625N of force for every kg of spacecraft....

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Are there any known examples of this situation? Yes! In addition to Phobos mentioned in this answer and from Astronomy SE: How did “oddball” Valetudo, Jupiter's new prograde moon, end up in a wider orbit? Why are most of Jupiter's moons retrograde?

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Though it's a stretch, GNSS satellites in MEO like GPS, Galileo, GLONASS, etc... are examples of such "moons" orbiting planet Earth, since they're positioned at an altitude lower than geostationary orbit but are still high enough for atmospheric drag to be so low that they can maintain that orbit without needing additional sources of propulsion/...

7

This is a case of an answer originating before a question. Originally a spin-off problem from this question asking about satellite footprints, it doesn't fit very well as an answer there. Hence this separate question and answer. As it turns out, this has a straightforward geometric solution from observer latitude $\phi$ and inclination $i$: $$r_P < \frac{... 6 Sorry that I'm so late to this, but I worked with the butterfly family quite a lot when I was a student in Professor Howell's group so I feel compelled to answer! It looks like you found some good resources already, but I can expand a bit and show some more of the family :) I have quite a few visualizations of the L2 southern butterfly family in the Earth-... 6 You can get much of the software you would need from the same place you get TLEs. Go to https://www.space-track.org/documentation#/sgp4 , register for a free account, and download the file Sgp4Prop_small_v8.0.zip . Inside that is the official US government implementation of Simplified General Perturbations version 4, together with wrappers in C, C++, C#, ... 5 Changing only the data wouldn't do much, if it were just fed to the same old SGP4 we've been using for decades. However, to my delighted surprise, at long last, the US government has provided a new propagator for public use! I haven't seen it announced anywhere, but if you go to https://www.space-track.org/documentation#/sgp4 and download the zip file, the ... 5 Going backwards, and starting with 3. In my experience the two are used interchangeably. The Wikipedia article on frame of reference implies that coordinates are the orthogonal directions, while a frame of reference has a defined origin. The AGI help reference for STK treats the two terms as interchangeable. So the answer to 1 and 2 yes, they are both ... 4 From what I understand, you need to ensure that both spacecraft are phased by 180 degrees. There are different orbital elements used to assess the phasing of spacecraft. The main one is the true anomaly, but it's ill-defined for circular orbits (in which case I would recommend using the "true longitude," defined as AoP + RAAN + TA, it is unrelated ... 4 A body of mass can keep a spacecraft in orbit, if the spacecraft is moving slow enough. If the velocity of the spacecraft is too high, it escapes. This limit is the escape velocity. Escape velocity depends on both distance and the mass of the body:$$v_e = \sqrt{\frac{2GM}{r}} Where $G$ is the universal gravitational constant, $M$ is the mass of the ...

4

For coplanar orbits, a bi-elliptical transfer is more efficient than an Hohmann transfer when the ratio of the initial and final radii is greater than 15.58. When the ratio is less than 11.94, an Hohmann transfer is more efficient. (Thanks to notovny for correcting me.) A bielliptic transfer is effectively two subsequent Hohmann transfers. Section 6.3.2 of &...

4

It really depends on how pedantic you want to be. In the most extreme phrasings, there is a frame centered on the earth which is inertial, sometimes marked as "Earth inertial" and there is a frame centered on the earth which is fixed to the geoid of the Earth, sometimes marked as "Earth fixed." The key attribute of frames is that, within ...

4

One of the problem with drag models is that most are not very precise. The few which are precise are usually written in FORTRAN, and lots of tools just interface with the FORTRAN code instead of rewriting and re-validating the algorithm. The typical high fidelity drag models are the Jacchia Roberts models and the more recent NRLMSISE00 model. The former is ...

4

This has to do with ambiguities in the definitions used for certain terms, and software doing strange contortions to avoid breaking backwards compatibility. The USAF's family of Simplified General Perturbations propagators are based on mean element theories of orbital motion. However, there are multiple ways to define and compute mean motion, and ...

4

From the comments and the OP reply, it sounds as if this is "a good place to start": Hohman transfer Learn the equation for orbital velocity as a function of the apogee and perigee of the orbit. Determine those velocities for the start orbit and the finish orbit (step back from your homework problem here and just put any circular orbits in, just ...

4

I'll post this and then wait to see if the OP want's to take it from here and post an updated improved script, or would like improvements here. I'm no expert at this but I will give it a try with an approximate calculation and then mention ways to make it better. I'll start with positions of the Sun, Earth, Moon and 2020SO on 01-Nov-2020 from JPL horizons ...

3

Sun-synchronous orbits can be set to go over the same part of Earth every day with shorter periods. Wikipedia has a nice table, but it is entirely possible to have up to 16 orbits/ day, where they all will pass over the same ground track every day. The 16 orbits/day would require too low (274 km) of an orbit to be practical for long term use, however. It ...

3

Using Python code How do I determine the ground-track period of ... from uhoh: and Great Circle Distances in Python from Chris Webb, I wrote a program to plot the distance of the ISS ground track to a given reference point on Earth during 4 days after November 17, 2020. The minimum distance is 113.81 km, calculated every minute of 4 days. The ISS moves very ...

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LVLH and RSW are exactly the same, according to the book Fundamentals of Astrodynamics and Applications, by Vallado (with contributions by Wayne D McClain). The frame is also known as RTN frame. In addition, RIC is not the same as RSW, according to the same source. Chapter 3.3.3 is very clear on the definition of those frames, I strongly encourage you to ...

3

I add this here just for completeness: The General Mission Analysis Tool (GMAT) also can model the spacecrat drag. GMAT doesn't belong to commercial software, but it is available. GMAT has two atmospheric models in its stock distributive: Jacchia Roberts, which can be used for altitudes more than 100 km only, and MSISE90 (for any altitudes). Copernicus ...

3

$\theta$ is often used as the symbol for sidereal time e.g. in this Sidereal Time article on Navipedia. The subscript $_{GMT}$ is probably referring to Greenwich Mean Sidereal Time, albeit incompletely, at the Greenwich meridian rather than a sidereal time at a local meridian. As explained in the comprehensive USNO Circular 179 ("The IAU Resolutions on ...

3

A couple of considerations I replicated your hyperbolic Hohmann calculation for Earth, and got 3.46 km/s. Close to your value. But for the Venus part, the numbers seem off. Assuming a helocentric transfer orbit with apsis at the semi-major axis of Venus and Earth, and a circular orbit for venus, the relative velocity should be 2.71km/s, working out to a ...

3

To a first approximation, thrust doesn't matter. If it takes 3 minutes instead of 2 to apply the delta-V to go from Earth orbit to a Venus transfer, it makes no odds. Beyond that first approximation, there are differences mostly in the direction that lower thrust increases the total impulse (delta-V essentially) that is needed. The main one is the Oberth ...

3

You're in luck that you have the mean anomaly, that makes the calculations much easier. The mean anomaly is the angle expressing the fraction of the orbital period that has been covered since perihelion (and not the "real" angle between perihelion, the Sun and the object. That's the true anomaly and harder to calculate). So given a mean anomaly in ...

3

NASA's planned lunar space station is actually headed to an L2 orbit, namely a halo orbit (more specifically a Near Rectilinear Halo orbit, a subset of the halo family with better stability properties). NASA is choosing this orbit because it offers many benefits: comms coverage to the Earth, access to the lunar surface, good view of the southern pole, etc. ...

3

This is equivalent to asking whether it's possible for a moon to orbit lower than a geosynchronous orbit, and there are of course satellites in orbit around the Earth that are lower. It's also equivalent to whether it's possible for a planet to rotate slower than its moon orbits, and clearly there's no lower limit on how slowly a planet can rotate (at least ...

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OP's 'Answer' So I have spend a few hours couple days going down this rabbit hole and I thought I would give my findings of going from knowing little about orbital mechanics to someone who knows a little more... Many things could be wrong so it would be great if someone who actually knows what they are talking about could correct and explain to me why I am ...

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Changes due to the new fence: The new space fence will most likely lead to better orbit estimates for most objects currently in the catalog, as they will be observed more often. It will also detect more and smaller pieces of debris. They might only be detected if they pass at high elevation and less likely at low elevation. Consequently, their accuracy ...

2

I am no expert in this but I think this is right as far as it goes. Your question is about the use of a magnetometer in space with Arduino style sensors and algorithms, so I can provide some links which will be helpful. On earth in a local area like a room up to kilometer-sized flying radius, people try to use magnetometers to help establish a baseline for ...

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