Has the Moon
changed orbit recently. It seems as though this should have been seen well in advance and set off proximity alarms long ago. Glad it was caught all the same. Now find something mind blowing.
The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft last week made a hasty burn to avoid a likely collision with Martian moon Phobos. NASA says that without the burn, the probe and the moon stood “a good chance of hitting each other on Monday, March 6th”. NASA's Jet Propulsion Laboratory (JPL) figured that out about …
Later authors talking about space elevators on Mars have taken to calling them "Clarke oscillations".
And no, Phobos hasn't changed course recently, but planets and moons aren't the point masses we tend to model them as (and there's Demios as well making it a three-body problem), so there's a degree of uncertainty about position and velocity preventing them from running their predictions forwards.
My Capslock key acts as the Compose key.
I do have a way of engaging SHOUTING AT PEOPLE mode. A bit like a nuke launch, I have to engage both shift keys.
Occasional all caps words are acceptable for emphasis. It's a bit hard to read a large passage that way. Hence the monks invented lower case (miniscule). The Romans might only have had capitals. Probably that's why the Celts, Huns, Goths, Jews etc thought them arrogant.
Well, if your face hits the sidewalk after having jumped from the top of the Empire State Building, then yeah, it's a bit like that.
I'm guessing that any collision in space is going to have rather disastrous consequences, and a collision with a moon is more likely to be called "pancaking" than simply "colliding".
Surely it's more about speed relative to a given frame of reference, rather than relative size.
If someone is daft enough to stand on a railway track it's likely they'll get hit by a train. Even though the train is much larger it's the one doing the hitting, since it's the one moving relative to the likely frame of reference (the ground).
If we take the frame of reference as Mars then both Maven and the moon will both be moving and at similar speeds, so hitting each other is the appropriate description.
I'm sure, or at least I hope that there are international agreements and protocols for positioning of earth orbiting satellites by the various organisations that launch things. Is it the same for Mars or do they take a look at what's there already and make a 'sensible' decision about where to put theirs?
Perhaps time to do some mapping of Mars' moons to refine the models a bit more?
If I remember correctly, the orbits of Phobos and Deimos are quite eccentric, and the tidal forces between the two moons and Mars mean that predicting the exact orbital path for the moons over time is non-trivial.
""...the tidal forces between the two moons..."
Must be essentially zero. Obviously."
No, the tidal forces between two objects of finite size are never zero.
Hmmm... Not a lot of moving mass on Mars (oceans), specifically mass that is moving around in a lossy fashion due to those flea size moons. So seems a bit dubious."
No, it doesn't seem dubious at all. You seem to be labouring under the rather odd misconception that tidal forces require fluids to exist. That is not correct; tidal forces are simply the result of differential gravitational forces - the force on one side of an object is greater than it is on the other. Obviously this is always the case as long as the object is not a zero-dimensional point, since one side will be closer to the source of gravity and therefore experience a greater force. When surface fluids are present this can lead to all kinds of interesting effects such as tidal bulges and what we see on Earth as tidal flow and the like. When dealing only with solid objects, it still leads to all kinds of interesting effects, especially on rotation and orbits - the Moon has become tidally locked to the Earth, for example, despite not having any oceans. And of course, when dealing with solid objects subject to large enough tidal forces it can lead to said objects becoming rather less solid than they were, as seen with Jupiter's moon Io, and in the extreme case with spaghettification near black holes.
I do not think tidal forces are in this case relevant. The problem is pertubations away from two body orbits caused by the gravitional forces between the moons.
Incidently there is an argument that tidal forces exist even for point objects. It is the result of the gradient of the force which exiists even in the limit of an object of zero diameter. This can be see by considering that a drop of water whose shape is governed by surface tension and tidal forces would not be speherial when orbiting mars but be elongated towards and away from the planet and that the elongation would be present however much the droplet is shrunk.
"[...] the elongation would be present however much the droplet is shrunk."
There must be a minimum droplet size at which there aren't enough molecules to form a sphere? Is the "sphere" actually only an approximation of how the molecules arrange themselves even in the absence of gravity? Is a single water molecule inherently lop-sided given that each oxygen atom has a much higher atomic weight than the hydrogen one.?
Me. "Must be essentially zero."
You. "No, ...never zero."
You obviously understand the Queen's English. I shouldn't have to explain the meaning of the word "essentially". And you're not allowed to disregard my inclusion of it. It was intentional.
Mars' moons are very small. By any rational meaning of the words, the tidal force interaction between them is ESSENTIALLY zero. Not mathematically zero, but essentially zero.
Hell, even the first order gravitational effects between two tiny moons are extremely modest. Tidal effects? Puh!
My first point stands.
You regarding my words, "tidal forces require fluids to exist."
Point granted. I was intending to refer to the EFFECTS of the tidal forces between Mars and each of its moons. I failed to make it explicit that I meant the effects on the moons' orbits.
Do you support the Alister's statement that tidal forces make predicting the orbits of Mars' moons difficult?
That's the point I was addressing.
"tidal forces require fluids to exist."
That rather brings us into a discussion of how you define a fluid, because the (earth's) moon is known to deform ever so slightly due to the Earth's gravity/tidal force.
In similar news, the surface of the Earth is better modelled as a (very viscous) fluid than as a traditional solid - mountain ranges are actually more like ripples on the surface than anything else, so again, don't assume that _these_ moons (and the planet itself) don't have any fluid properties.
Hmmm... Not a lot of moving mass on Mars (oceans), specifically mass that is moving around in a lossy fashion due to those flea size moons. So seems a bit dubious.
You don't need liquid to perturb orbits. Earth's equatorial spare tire and Luna's masscons do a fine job of nudging orbits.
Apollo 16's subsatellite PFS-2 was placed in an almost perfectly incorrect orbit that exposed it heavily to lunar surface mass anomalies. Within 2.5 weeks of release, its elliptical 89x122 km orbit had varied to within 9.7km of the lunar surface. It seemed to back off to 50km, but after just 35 days it performed an unscheduled lithobraking maneuver. Later, it was found that Luna had four "frozen" orbits much less influenced by lunar gravitational anomalies.
Mars is a fairly lumpy world, with differences between northern and southern hemispheres that should make long-term predictions of MAVEN's orbit interesting.
The "tire" is IIRC why satellites in polar orbit shift the point where they cross the equator on a regular basis.
Still wondering what those masscons are though. I'd love for someone (anyone) to drop a lander on one of them built to try and find out. <sigh>
Oh well, perhaps some other decade...
"If I remember correctly, the orbits of Phobos and Deimos are quite eccentric,"
Actually, the Explanatory Supplement for the Astronomical Almanac gives their eccentricities as 0.015 and 0.0001. The moon is 0.055 so they're much more circular than the moon. But you'd expect that for lighter objects.
Hmmm, maybe eccentric is the wrong word, if you take it to mean how circular it is.
My understanding however, is that however circular their orbit, the moons' track across the planet (is it called the orbit footprint) can change in quite a random fashion, dependant on their interaction with each other?
Alister "...can change in quite a random fashion, dependant on their interaction with each other?"
You're searching for the "3-Body Problem" concept.
In principle, it's "impossible" to predict the long term orbits and their stability of three bodies. Except in about 20-some specific cases (a batch were recently added to the list)
In practice, there are iterative algorithms that can get the job done to any reasonable degree of accuracy required. Although course corrections are still useful...
It occurs to me that numeric models with finite accuracy would be perfectly adequate in this case and indeed any sane operator of hurtling celestial hunks of metal would be expected to employ them for the benefit of a well-fucking-over a week of advance notice of potential unscheduled landings. Then again, that's just me...
You misunderstand me.
Phobos is modeled as a 30Km sphere.
It's not. IIRC it's more like a knobbly sausage.
So depending on it's actual attitude, rotational axis and rotational rate relative to the flight path of MAVEN it could have been metres away or kilometres away at intersection.
Hence my suggestion it might be time to refine the model of the moons shape and dynamics.
JPL are very good at trajectory planning and up to now the 30Km sphere has been an adequate model, but things change and Mars has more objects in orbit around itself. Historically a pair of orbiters around Mars was a busy time. Now we're talking 8+ vehicles.
Yes, space is so vastly, hugely, mind-bogglingly big it is (you may think it's a long way down the road to the chemist, but that's just peanuts to space) and since there are only a finite number of space probes in space then the probability of one meeting another is so infinitessimally small then this occurrence can be ignored
See http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html. It is subtitled "A Tediously Accurate Map of the Solar System". It's not that accurate - it lines all the planets up in straight line, and has a lot of extra text to relieve the monotony. It is very tedious if you try to scroll all the way through (I gave up)
I was a bit surprised to see, at 7.2 light minutes out from the Sun a comment about being half way home. My understanding is that Earth is about 8 light minutes out from the Sun, so not sure quite which laws of Physics are being followed by "Josh" here.
That's a crappy repository for code dependencies and a crappier associated build system. No surprise they sent it out to Mars, but I don't get why they then saved it from oblivion!
(We used Ivy instead - so much better but requires water, oxygen and sunlight to grow so not so good for Mars visits)
So now we are cluttering up the space around another plant with junk. Before any company/government/agency puts anything into an orbit (Earth, Mars, or anything) there should be a plan in place for them to remove the object before end-of-life occurs. With smaller satellites around Earth that can mean planned orbital decay where they can burn up in the atmosphere, but this would not work on Mars. Options would be:
1) use thrusters to leave orbit and head out into the void
2) remain in orbit as a future navigation hazard, or
3) have the space junk scattered over the surface of Mars.
Any other solutions?
4) Teraform Mars, creating a nice atmosphere, then drop the junk satellites out of orbit to burn up.
And use more cubesats so they burn up more completely.
5) Create a rift in the time-space continuum, then shove all our space junk into that hole, then seal it up!
I see no problem what so ever with that method. :P
There are 21000 significant objects in Earth orbit - I'll ignore the untrackables - with no roundabouts or traffic lights, and we don't have major problems with satellite collisions. How come under 20 "vehicles" around Mars appears so much more difficult to manage?
There's going to be some obscure orbital mechanics answer isn't there ...?
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