See subject. So I can get my worries over the Sun’s supernova into perspective.
Luna is receding ever so slowly away from Terra.
So, no.
No it won’t decay. In fact, the Moon is getting farther and farther away from the Earth.
What do you think makes an orbit 'decay"? The reason an orbit decays is if the body is low enough that friction from the tenuous atmosphere slows it, which means it falls lower, which means more atmosphere, which means more frictional slowing, and eventually the object falls to earth.
Even if the Moon wasn’t getting farther away, its orbit wouldn’t decay, because it is very high above the Earth’s atmosphere, such that atmospheric drag isn’t just essentially nil, it is nil. And also, the Moon is very very massive compared to a satelite, and so even if it were being slowed by atmospheric friction it would take a lot of friction to slow it down.
I guess people have a hard time accepting that a body in motion will stay in motion unless acted upon by another force. We’re used to the notion that things in motion eventually stop moving and fall to the Earth. It took until Newton for scientists to figure out that it takes energy to make things stop or start moving, but it takes no energy for them to keep moving.
The sun can’t go supernova either. It’s too small.
IIRC, in theory given enough time (many, many billions of years) the Moon will continue to slowly move away from the Earth as it is now, then eventually the direction of drift will reverse, its orbit will decay and and the Moon crash into the Earth. The Moon radiates a small amount of energy in the form of gravity waves as it orbits; this serves as a form of “drag” that slowly drains energy from its orbit.
But it’s quite likely it will instead be destroyed by the Sun expanding into a red giant, which will happen much sooner.
As I understand it, it does in fact cost energy to keep moving in a curve; objects doing so radiate energy.
Thanks.
So, (of course I’m not arguing, just asking)
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Why is the moon “drifting” away?
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Why did Skylab fall?
Sorry, strike question 2 above. Lemur told me the answer. It never occurred to me that the atmosphere at Skylab or Space Station height was still significant enough.
Given that, doesn’t the Space Station have to add little force nudges to avoid Skylab’s fiery fate?
It was way closer to the Earth than the moon, and atmospheric drag did it in.
Actually there is some decay in the moon’s orbit. The earth moon pair are emitting gravitational waves since they are accelerating around their common center of mass. Eventually this loss of energy would make the orbit decay. Though as pointed out currently they are receding from each other as angular momentum from the Earth’s rotation is transferred to the Moon’s revolution by tidal forces. And this is many times as important as the gravitational waves.
The decay in the orbit won’t start until the day and month (that is the moon’s orbital period) are equal in length so that the moon is apparently stationary overhead one spot on the earth
Yes. Its orbital decay amounts to 2 km per month, and money is being to spent to counteract that.
Atmospheric drag, as Lemur866 described.
Gravitational waves for the Earth-Moon system are beyond negligible. I’d actually bet on aerodynamic drag from the interplanetary medium having a larger effect.
And yes, the Space Station does need occasional boosts to its orbit to keep it up. All satellites you want to keep around for an extended time do-- The Hubble is another good example. And the Russians didn’t want the expense of continuing to do this for Mir, which is why they deliberately brought it down.
Basically the tides. The earth bulges because of the water. That bulge and the moon are gravitationally tied, of course, which results in some of the rotational energy being transferred to the moon.
Energy and momentum must be conserved. In the moon’s case, it results in a higher orbit. In the earth’s case, the length of the day is slightly lengthening (i.e. the earth’s spin is slowing slightly) as rotational energy is transferred.
If the earth were a bit more rigid, this effect wouldn’t be observed.
I should add that, if it weren’t for that pesky sun going nova some day, eventually the earth would become tidally locked with the moon. So, the moon would be locked over the same bit of earth all the time.
As for the ISS, yes, its orbit is maintained with periodic boosts. The wiki article has a good description of the process.
As stated by Exapno Mapcase above, it won’t.
Yeah, but it will bloat out to, and probably beyond, Earth’s orbit, so I’m pretty sure that it will still be pretty miserable.
Just to be clear, there is a big difference between nova and supernova. The OP said supernova, and our sun is much too small for that, buy at least nine times. A nova on the other hand needs two components. A white dwarf to go nova, and a companion star to donate matter to initiate the nova. White dwarf is a main sequence stage for stars that include stars of the mass of our sun, and our sun will probably eventually become a white dwarf. But we don’t have a companion star, so still no nova.
There are many satellites in low earth orbit that don’t have thrusters for raising the orbit, and are expected to last many years. Of course their lifetime depends on how high the original orbit is. For example, ROSAT, launched into a 580km high orbit in 1990, reentered last year.
And of course, atmospheric drag isn’t a factor for geostationary satellites. Though they do need thrusters to stay in the designated spot, otherwise they may drift slowly along the orbit.
Yeah, my statement there was a bit too sweeping. It’s still mostly true for low Earth orbit, depending on what you mean by “an extended time”, but there are higher orbits that don’t need boosting for any human-relevant timescale.
I wasn’t aware of that. Thank you.
What about Nemesis! Nemesis (hypothetical star) - Wikipedia