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98

The comet's tail always points away from the Sun. Yes, even when the comet is heading back into the outer solar system. This is because the tail isn't a 'trail' of where the comet has been, like a rocket exhaust or contrail, but instead it's gas, ice and other debris blown off by the stellar wind. (There's actually two tails, one made of charged particles, ...


44

Wouldn't i inevitably spiral to sun surface even if i was faster than 0km/s ? No. On reasonable timescales, an orbit will have a fixed distance of closest approach, called "periapsis." (These timescales shorten if you're close enough to what you're orbiting that an atmosphere can drag you down). You don't really need to "drop in straight line" (which ...


22

One reason large rockets are launched directly up is structural. Cylinders are strong under compression, stacking cylinders on top of each other means the weight is symmetric, you need less structural weight to hold it all up. Launch it straight up and make gentle changes in direction and the forces are equally distributed through the structure all the way ...


18

It's a matter of optimal trajectory - pitch maneuver/gravity turn which depends on characteristics of the rocket, the atmosphere, gravity etc. In particular, for rockets with lower initial thrust-to-weight ratio, the trajectory starts almost vertical; "rounding" the angle to perfectly vertical makes the launchpad infrastructure and preparation process easier;...


18

You need below 2866 m/s of orbital velocity at 1 AU to crash into the Sun. You technically don't need to slow down exactly to 0 m/s relative to the Sun in order to crash into it. Let's calculate the approximate velocity required to graze the "surface" of the Sun. This is an excellent answer on how to calculate apoapsis and periapsis of an orbit. So first, ...


17

2kW is not that much on Earth You've mentioned radiation and convection in your answer (you forgot conduction). Turns out the properties of Earth's atmosphere make conduction and convection way better than radiation for moving heat around. For an illustration, consider the size of a portable, 2kW, oil-filled radiator: this one lists the size as ...


17

tl;dr: There's certainly some propagation of sound waves possible at 100 km altitude. With a density a million times lower than at the surface the mean free path of individual molecules will approach a millimeter, so ultrasonics might be impacted, but for Human or GoPro frequencies it will be much quieter, but still there. Till what altitude above earth ...


16

And note that if you want to hit the sun the cheaper (but slow!) way to do it is to head out. 12.32km/sec will take you to infinity, at infinity a burn of 0m/sec will kill your orbital velocity and you'll come straight in. Of course this will take infinite time, but even going only as far as Jupiter's orbit means you use less energy to drop your periapsis ...


15

I am Patrick Shober (the lead author of the study). Thanks so much for checking it out! If you check out Figure 10 in the paper, I have plotted the specific angular momentum in the Sun-centered frame. So this shows how the meteoroid (the rock) gained energy during the close encounter but then lost a fraction of it due to the atmospheric passage. This can ...


15

While those movies probably do it because that's how comets are commonly depicted, it might not be that inaccurate. Remember that the Earth itself is moving around the Sun, so if a comet is heading for Earth, that means it's heading for a point where Earth will eventually be, not where it currently is. It would therefore be possible to see such a comet be &...


15

If you're the comet, the way to hit Earth is not to head directly for it. That's because Earth is orbiting the sun: you need to aim at where Earth will be, not where it is right now. For example: By Phoenix7777 - Own work Data source: HORIZONS System, JPL, NASA, CC BY-SA 4.0, Link This shows a transfer orbit from Earth (blue) to Mars (green), but the basics ...


14

It still has the resistance of terrain against wheels (well, weaker than comparable terrain on Earth due to lower gravity - but then the terrain is pretty awful for driving), the same friction of bearings and so on - a car driven through loose sand on Earth will stop really fast due to the sand resistance, and not due to air. Now if instead of a lunar rover, ...


14

The title of the question asks Till what altitude above earth sounds can be heard? @uhoh gave a detailed answer to that question. I'll instead speculatively answer an implied question in the body of the OP, What is the sound in the linked video? The OP implicitly assumes the sounds in the video were transmitted through the air to a microphone. (Many ...


11

@OrganicMarble nailed it: ...it looks like it's the distance from the ecliptic plane. Yep, it's height above/below the ecliptic, a way to represent 3D in a 2D plot. At first I thought they might be thrust vectors like these but no, these are ballistic arcs. Instead I am 99.44% certain that these lines are use to indicate height above/below the plane of the ...


11

If you were able to change a booster rocket engine nozzle's area ratio at will, you would want it to expand, not contract1. Best performance is achieved when the exit plane pressure matches the ambient pressure. As the rocket ascends, the ambient pressure drops, and more expansion is needed, not less. Why don't they do this? The usual aerospace reasons: ...


11

1. Diamond. Its hardness is legendary. That it appears in liquid form on Uranus or Neptune hasn't been directly measured (no recent probes), but lab measurements in 2009 and 2010 of diamond's phase diagram still haven't been contested to claim that diamond can't be liquid there. On the contrary, in 2017 a process was demonstrated of converting diamond ...


9

We must remember that there is no spoon gravity here and that as soon as our astronaut is no longer in contact with the floor they must have an essentially straight line trajectory. Below is a simulation in an inertial frame moving towards Jupiter along with Discovery 1. Let's assume that the astronaut is initiall standing and has a tangential velocity equal ...


8

Quick summary of what a "Caplan" thruster is: A Dyson swarm collecting sunlight, shooting it back at the Sun to stir up mass as solar winds. Some system to collect that solar wind. Fusion reactors, using the helium from the collected solar wind. A fusion product jet of oxygen-14 pointed into space. A hydrogen jet pointed back at the Sun. Caplan's ...


8

I have been looking for the same thing. The only mention I found was on a blog post (https://www.neowin.net/forum/topic/1402753-spacex-starship-sn8-15km-test-flight/) which stated 66-68 m/s for SN8. If true, that would imply ± 320 m/s on Mars. Despite Mars' lower value for g (3.72), the much lower atmospheric density of 0.02 versus 1.2 kg/m3 results in a ...


8

Your formulas are using a base-10 logarithm, not the natural logarithm. This causes all your delta-v values to be off by a factor of 2.3 In google sheets, you can supply the base of the logarithm as a second argument to the LOG function. For other systems, the identity $log_a(x) = \frac{log_b(x)}{log_b(a)}$ may be useful if you need to obtain a logarithm in ...


6

From Viking '75 Spacecraft Design and Test Summary Volume 1: Lander Design page 76


6

Indeed, you are correct, it could reach escape velocity. The M110 can reach speeds of over 700 m/s, which is well above the escape velocity. Most guns actually don't need oxygen to work either, as the gun powder has the oxygen needed. So yeah, be extra careful where you fire a gun on an asteroid.


5

The atmosphere thins out exponentially as you go up. This means that the part of the atmosphere close to the ground is substantially more dense than higher up. In other words, air resistance is much higher when you're close to the ground. In order to minimize the amount of fuel lost to fighting air resistance, you need to minimize the amount of time you ...


5

Since a spaceship doesn't really burn much fuel during its voyage, except for at the start, at the end, and for any trajectory correction maneuvers in between, it doesn't really matter whether or not it has fuel for the most part of its flight. In other words, if it loses its fuel mid-flight, it would likely end up close to where it would have ended up if ...


5

It would move Down! By the definition of the Karman line on wikipedia, the lift force and the "centrifugal force" must be equal to the gravitational force and, therefore, each other. This gives the following equation: $\frac{1}{2}\rho v^2C_LS = \frac{v^2m}{R_e+h}$ Where $\rho$ is density, v is velocity, $C_L$ is lift coefficient, S is wing area, m is ...


5

Cars and also lunar rovers are slowed down by wheel bearing fricition and rolling resistance of the wheels on the ground. The rolling resistance of the wheels is reduced by a perfectly flat terrain with no dust but it will never be zero.


5

Summary: The time will depend on how the inside of the spacecraft is insulated, but if we assume that you are in contact with the metal shell of a spacecraft similar to the lunar module (and make a lot of approximations regarding convection in the Venusian atmosphere), you will get serious burns within 15 minutes. The assumptions I make break down as the ...


5

Do warp drives produce radiation? How much? Do small warps produce less? Physicists agree: We have absolutely no idea. Compared to the many unknown problems of an Alcubierre drive, dealing with radiation is a solved problem in general. You need either: Shielding Distance to not care about damages A layer of lead sufficiently many light years thick is known ...


4

Just to propose something specific in an answer: I would first guess Gallinstan (by mass: 68.5% Ga, 21.5% In, 10.0% Sn), at least if we're talking about near-room-temperature applications: Supposedly, it melts at -19°C, and has about a μPa or less vapor pressure at 500°C. (This is much lower than Hg, but I don't know if it's lower than pure gallium because ...


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