The time thing is interesting, but I feel like no one talks much about the appearance of passing objects. That is, I wonder how the image of a passing celestial object might distort due to length contraction and any other effects. I’m still trying to understand that. This article seems pretty digestible, so far.
Aside from the fact that anything with mass cannot travel at the speed of light… Lots of fun things happen as you approach the speed of light. There’s an excellent mostly-hard sci fi novel called Tau Zero that explores this concept in depth and, despite being older, is worth the read.
(1) Time dilation (the universe and you have different clocks).
(2) blueshifting of objects in front of you. At 0.95c, basically all visible starlight in front of you has been blueshifted into ionizing radiation. Fun fun.
(3) shape distortion. You become more needle-shaped – getting very long and skinny, as observed by the rest of the universe.
(4) you become a nuke. At .99c if you run into anything, your kinetic energy related explosion would be roughly 6x the Tsar Bomba (largest nuke ever detonated) for each kg of mass. Or, put another way, each kg of your mass would impact with the energy of 3kg of antimatter contacting 3kg of matter. Boom.
Sci fi always overlooks the last one. Near light speed combat is basically firing buckets of sand at planets and blowing them up.
Speaking of sci fi, Kim Stanley Robinson’s 2312 does a really good job of incorporating the existential dread and lurking horror of weaponized orbital mechanics.
‘Speed of light’ compared to what? is what you need to worry about. Most things in the universe won’t be moving at the speed of light compared to you (or whatever you’re inside of), and when you run into them, you won’t last for long.
If you’re zooming past the Earth at the speed of light headed straight at the Moon, you’ve got about 1 second to enjoy that before you make a very, VERY large crater.
If you change course and head straight at a frozen tardigrade, it will make a VERY large crater in you.
To actually reach the speed of light you'd be massless, so the only damage, would be from momentum transfer, at which point your particles would be reflected or absorbed like light.
But that aside, mostly I was referring to your statement:
'Speed of Light' compared to what?
Which is really not a concern. It's the speed of light for everyone with respect to everything, or it isn't the speed of light. Like, two beams of light going in opposite directions don't see the other light beam going at 2x the speed of light, just at the speed of light with lots of time dialation.
You already knew the answer to ‘What would happen if you moved at the speed of light’ was was “To actually reach the speed of light you’d be massless.” No shit. The question was already massless.
If you somehow got rid of your rest mass to move at the speed of causality, two things would happen: first, you'd experience no time; second, you'd instantly crash into your destination and die in a rather energetic way. That's the neat thing about photons; from a photon's POV time and distance do not exist. A photon, from its POV, is emitted and absorbed at the same time in the same place.
Maybe I missed this on the article but if somehow a human is moving at 186,000 miles per second they would also escape earth’s gravitational pull (and probabbly the sun’s as well) and within a second find themselves just over halfway to the moon and crashing into it a couple of second or two later with enough force for the impact to be seen with the naked eye from earth.
I don’t know all of the details of this mission, but it seems like they’ve just lowered the lowest point in its orbit - called periapsis - until it sits low enough in the atmosphere to get enough drag that the orbit slowly decays over a decade.
The lowest part of the orbit would only drop a little bit, but the highest part of the orbit woukd reduce more with each orbit. If you do it slowly enough, the orbit would circularise and then it would begin to decay more evenly. As it falls deeper into the atmosphere the orbit would decay faster and faster until it can no longer sustain orbit, and then it falls deeper into the atmosphere and burns up in just a few minutes.
The reason for this I can only guess at - it wouldn’t take a whole lot more fuel to just deorbit all at once. My best guess is that it has something to do with reentering at the lowest possible speed. If you fall from a high orbit and reenter, you have a lot more speed and have to dissipate more energy all at once. It’s possible this increases the risk that the satellite will fail to deobrit, and break up and send pieces off in less predictable orbits. If it breaks up from a low circular orbit, there’s no chance of any parts escaping back into orbit.
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