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Mars orbiter FLOORS IT to avoid hitting MOON

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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.

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Re: Has the Moon

NASA wouldn't be the first to have forgotten about the Martian moons. Arthur C Clarke's Fountains of Paradise envisaged a Martian space elevator being set up to exhibit simple harmonic motion to avoid periodic flybys from the lower moon snapping the cables.

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Re: Has the Moon

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.

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Alien

Re: Has the Moon

"Has the Moon changed orbit recently. "

Depends on who's driving it...

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Re: Has the Moon

Actually, it's Deimos, not Demios (a contraction of Demis Roussos?).

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Re: Has the Moon

Probably not... bit someone should name something after him*; here's proof that he's earned it.

* Artemios Ventouris "Demis" Roussos (Αρτέμιος Bεντούρης Pούσσος), 1946-2015

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Re: Has the Moon

"Probably not... bit someone should name something after him*; here's proof that he's earned it."

Well, it can't be a piddling little Martian moon, It has to be something with substantial mass that goes on forever and ever and ever and ever :-)

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Alien

Re: Has the Moon

I like the ultra cool hi-res photo of this moon. Look at the amazing uncropped version:

https://regmedia.co.uk/2015/11/11/phobosincolor_pia10369.jpg

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Flame

humm?

Is that?....

Oh crap..!?

*out gasses*

*Glides past with minutes to spare*

And let's not speak of this again.....

-MAVEN probe

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Anonymous Coward

CAPS LOCK

It's not big or clever.

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Re: CAPS LOCK

Au contraire - I have a brain the size of a planet and am two separate gorillas.

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Re: CAPS LOCK

Pah! I wrestle poodles and win!

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(Written by Reg staff)

Re: CAPS LOCK

Which is why we use it SPARINGLY, rather than for every word.

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Re: CAPS LOCK

@smudge

Welcome Mr Apollo

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Re: CAPS LOCK

It's not big NOR clever.

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Headmaster

Re: CAPS LOCK

Caps Lock is big. Or at least on my keyboard it is, probably double the size of most of the keys, it's almost as big as backspace.

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Trollface

Re: CAPS LOCK

i tHINK iT'D aNNOY tHE pOSTER iF i wROTE tHIS sENTANCE lIKE..... this.

wOULDN'T iT?

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Headmaster

Re: CAPS LOCK

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.

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Anonymous Coward

Re: CAPS LOCK

Gorillas and Poodle wrestling? amateurs... I'm a lumberjack. And I'm OK.

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Re: CAPS LOCK / engaging SHOUTING AT PEOPLE mode

Coming soon: nuke launch via 3 a.m. tweet.

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Alien

" chance of hitting each other"

That's a bit like saying the sidewalk hit me in the face on that night. It conveys a similar size of the objects - at least to a non-native speaker.

(icon 'cause it is *their* fault)

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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".

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Pint

Re: " chance of hitting each other"

If the sidewalk hits you in the face, then you've had too many beers.

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Re: " chance of hitting each other"

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.

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"lithobraking"

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Re: " chance of hitting each other"

> "If the sidewalk hits you in the face, then you've had too many beers."

And I it hurt, you haven't had enough.

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Traffic Cops

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?

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Unhappy

strictly speaking

It means MAVEN was in risk of hitting a part of the nominal 30Km that JPL uses to model the moon.

Might have been Km away.

Might have been in touching distance.

Perhaps time to do some mapping of Mar's moons to refine the models a bit more?

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Re: strictly speaking

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.

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Coat

Re: strictly speaking

predicting the exact orbital path for the moons over time is non-trivial.

Maybe they should employ a rocket scientist?

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Pint

Re: strictly speaking

"...the tidal forces between the two moons..."

Must be essentially zero. Obviously.

"...and Mars..."

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.

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Re: strictly speaking

""...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.

"...and Mars..."

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.

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Re: strictly speaking

"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.

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Re: strictly speaking

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?

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Re: strictly speaking

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.

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Anonymous Coward

Re: strictly speaking

"[...] 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.?

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Boffin

Re: strictly speaking

" ...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."

And this is where I bring up Larry Niven's Neutron Star, because a) it's relevant, and b) J. P. need to read it.

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Unhappy

"If I remember correctly, the orbits of Phobos and Deimos are quite eccentric, "

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.

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Thumb Up

Re: "If I remember correctly, the orbits of Phobos and Deimos are quite eccentric, "

@John Smith 19

Ah, I see what you mean now.

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"Ah, I see what you mean now."

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.

Amazing times.

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Pint

Re: strictly speaking

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.

...

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Pint

Re: strictly speaking

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.

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Pint

Re: strictly speaking

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...

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FAIL

Re: strictly speaking

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...

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Re: strictly speaking

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.

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Unhappy

" Earth's equatorial spare tire and Luna's masscons"

Quite true.

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...

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Boffin

Re: strictly speaking

"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.

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Give it to us in Adamses

> therefore occupied the same volume of space

Presumably sector ZZ9 plural Z alpha?

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TRT
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Re: Give it to us in Adamses

Adamsian galactic sectors are pretty big. Earth is in the same sector as Barnard's Star, six light years away.

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Phobos phobia fortunately foiled by flame.

nt

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