Hot answers tagged

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, ...


56

This was tested nearly sixty years ago. Using a very large cup filled with 95 tons of water. An empty second stage of a Saturn I under test was used. Only the first stage should be tested but with full payload of a water-filled dummy upper stage. Dummies with the tanks but without the expensive engines. The dummies should have the same shape and mass as a ...


43

The squat end of the spectrum has little to do with solids versus liquids and everything to do with aerodynamics. Spherical tankage is most weight-efficient, so you'd expect squat stages in cases where aerodynamics don't dominate such as your Mars Ascent Vehicle (flying where atmospheric drag is on the order of 1% what it is for Earth ascent) or the Apollo ...


34

The pendulum fallacy is the belief that rockets would be passively stable with engines at the top, with the rocket "hanging" from them. The error lies in expecting gravity to pull the body of the rocket down while the engines pull it up. In reality, gravity acts on the body of the rocket and the engines equally, exerting no torque (except for ...


31

It would not freeze into a block. It would quickly expand and boil, but not in a rolling boil. Without pressure, bubbles would form throughout the coffee and expand rapidly, causing it to spray out of the mug as soon as you released the catch that would have been necessary to hold the lid on. But evaporation causes cooling, so millions of frozen coffee ...


30

After writing my comments, I started writing a new answer. That got long, so here's a shorter one. The "energy of an orbit" may be poorly defined and depending on the definition, is not subject to energy conservation laws. Let's ignore it and focus on the kinetic energy. Kinetic energy is not necessarily conserved. Momentum is. Momentum (derived ...


23

In the inverted pendulum problem: gravity exerts a vertical force on the pendulum, at the center of gravity the support of the pendulum (like the finger under the pencil) exerts a vertical force on the pendulum, at the bottom of it In a rocket: gravity is the same engines exert a force along the long axis of the rocket, where the engine is (which doesn't ...


16

The sum of all mechanical energy will be the same after your ideal burns. The difference will be that the portion given to your exhaust will be greater for higher burns. For a burn farther away from the planet: The craft will be higher, so the PE of the exhuast will be greater. The craft will (usually) be slower, so the KE of the exhaust will be greater.


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 ...


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 &...


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

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 ...


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

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

Which is nowhere near the speed the rocket has here near the stage separation. The video is reporting velocity in km/hour, not in m/s. 4383 m/s is 15780 km/hour -- over twice as fast as the Falcon 9 is moving at stage separation. In general, the rocket equation isn't going to give you directly usable results for the initial ascent phase of a rocket. Drag ...


7

Discrete Fourier techniques introduce errors in their terms that track with the sinc function. Any target in the image will produce side lobes like those in the following graph. Strong reflections will create side lobes that have a higher amplitude than the background of the image. This is why the target appears to have a larger spatial extent than it ...


6

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 ...


5

The answer has to be 'not necessarily', because, in general, as you go along, you're free to adjust the solar sail angle and thus the trajectory. In addition, the trajectory need not lie in a single plane since the sail can produce out of plane forces. I posted an analysis in the comments yesterday for a weak solar sail and a shallow spiral orbit and ...


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 ...


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

This is an Oberth maneuver, getting the most out of your delta-v budget by adding velocity on top of an already high velocity. Nowhere in the solar system does one achieve greater orbital velocities than during perihelion of a Sun dive, as close as thermal management allows. At that ~100km/s velocity, every km/s of velocity added corresponds to ~14km/s at ...


5

The magic words you're probably reaching for are "radiative cooling". If your ball is in a vacuum far from any star and doesn't generate any heat of its own, this isn't too difficult a problem to solve. There's even a handy calculator for it (and no doubt many others elsewhere) but I'll summarise the key bits here. The cooling rate in terms of ...


5

To start, I want to make sure that you are aware that the magnitude of a gravity assist can just as easily be in the positive (accelerating) direction as it can in the negative (decelerating) direction. So the ballpark numbers you see for what delta V you can get from a gravity assist when accelerating are the same for decelerating. Next, I think the ...


4

I'm leaning towards the "just coincident" explanation here. The semi-major must be >54.5 million km (and even that is too hot), as the aphelion must touch the orbit of Venus to enjoy the repeated gravity assists to reach this orbit, and the perihelion must be outside the sun. So ~57 million km is right were the trajectory constraints would ...


4

Thanks to @astrosnapper's comment I've looked at section 3.2.1. of Murphy et al. 2007 APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections. APOLLO uses the Apache Point 3.5 meter telescope for both transmit and receive, a rotating transparent disk with a mirrored spot on it rotates 20 times per ...


4

Let's look at the ballistic coefficients of a bunch of reentry vehicles. This is the mass divided by the cross section area. The drag coefficients of all these blunt bodies will drop from very roughly 1.0 while hypersonic to 0.5 near terminal velocity, except for the Pershing II MARV which being a sharp cone is smaller. Dry Mass Wet mass Length Diameter ...


4

A quick way to remember this is thrust is force. Assuming that the thermal blackbody radiation is isotropic it averages to zero so we can ignore that. Then for a spacecraft emitting EM radiation in one direction we can say that the thrust is just: $$T_{EM} = \frac{dp}{dt}$$ where $dp/dt$ is the amount of momentum per unit time produced in one direction as ...


4

It's all a matter of link budget. Globalstar, for example, has satellites at 1400 kilometers, usable from a satellite phone with a low-gain omnidirectional antenna, as are Iridium's satellites at 780. Both provide narrowband to midband internet access. Inmarsat, Thuraya, and plenty of others have satellites in geostationary orbit at 35,786 kilometers that ...


3

On a sufficiently large spin ship the result would be indistinguishable from normal gravity, at least to human perception. The idea behind a spin ship is simple: instead of gravity constantly pulling your body downward towards the ground as would happen on a planet, the ship rotates such that the floor is being constantly pulled "upward" towards ...


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