# Tag Info

47

Nearly all the velocity is cancelled by atmospheric deceleration of the descent module, before its parachutes are deployed. ISS orbital velocity is around 7700 m/s. An initial retro-burn of the Soyuz engines, of something like 115 m/s magnitude, is sufficient to lower the perigee of orbit into the uppermost part of the atmosphere. The orbital module and ...

36

The drop in acceleration around 40s into the flight is the shuttle throttling down to reduce the aerodynamic load on the vehicle. It then accelerates when past this point. The drop in acceleration at 2 mins into the flight is due to the solid rocket boosters running out and being discarded. Acceleration then continues to build, as the thrust from the engines ...

32

Does this logic make sense? Has anyone thought of this before? Yes, it's been considered. In the literature it's known as the "incentive trap". There are a couple of academic papers on it, notably Andrew Kennedy's 2006 Interstellar Travel - The Wait Calculation and the Incentive Trap of Progress and René Heller's Relativistic generalization of the ...

31

Rockets are much faster than airplanes for most of their flight. Here's a graph of a Space Shuttle launch: The red line is speed. It's in ft/s, 1000 ft/s is 1097 km/h. So At about 45 seconds, the Shuttle flies 1000 km/h which is faster than an airliner. At about 1:40 it crosses 3000 ft/s which is about Mach 3 (the speed of the fastest aircraft). ...

25

Physical First and foremost, the physical reason is that objects accelerate as they approach massive bodies and decelerate as they recede: Parker Solar Probe achieves its peak orbital speed (almost 200 km/s eventually) at its closest approaches to the Sun - as it falls inwards towards the Sun on each orbit it speeds up then slows down again on the way back ...

25

The process is described here, which answers nearly all of your question. The reentry burn removes about 120 m/s of velocity from the capsule (that's your 1) and the final impact is 15 miles per hour (about 6 m/s). That's your 3. That leaves about 7.5 km/s for part 2. The only remaining question is the split between 2a and 2b, ie the velocity when the ...

23

@user3715778's answer is correct. Throwing will not make enough of a difference in the orbit to reenter Earth's atmosphere de-orbit promptly. Let's run the math using the vis-viva equation: $$v^2 = GM \left(\frac{2}{r} - \frac{1}{a}\right)$$ periapsis apoapsis semi-major apoapsis altitude (km) altitude (km) ...

21

A rocket isn't automatically fast - a small firework rocket may be no faster than a car. The important point is that rockets carry their own oxidiser and aren't limited by the need to interact with the air. Most aircraft engines need to develop lots of thrust at low speed for take off, and they have propellers or large fans that cause drag at high speeds, ...

17

The manned Apollo Lunar Roving Vehicle from Apollo 17 holds the record of 17km/h achieved by Eugene Cernan. Apollo Lunar Roving Vehicle was also present on Apollo 15 and 16. For traditionally understood rovers - unmanned mobile landers, the fastest historically would be Lunokhod 1 and 2, capable of 2km/h. At current time only two rovers are still active ...

15

Currently functional and proven technology is limited to basically no interstellar travel at all. To reach one of our stellar neighbors (like Proxima Centauri), one of the fastest space probes we have now, New Horizons, would take 54000 years. There are multiple proposed methods of sending spacecraft interstellar distances (in shorter time spans) such as: ...

15

Uwe's comment on the question is spot on. The characteristics of the flow through the nozzle depend critically on the pressure ratio - the two pressures being the pressure at the entrance to and exit from the nozzle. Above the critical pressure ratio flow through the nozzle is subsonic and it is not choked at the throat. Below the critical pressure ratio, ...

14

NOTE: This answer was provided for a different, very basic question which didn't specify the WGS84 ellipsoid; it's illustrative of the basic principle as applied to a spherical Earth. Still, not bad for an answer written a year and a half before the question was asked... At any latitude, the Earth completes one rotation per day. At the equator, the ...

11

Your premise is incorrect. In no case does "skipping off the atmosphere" leave you going faster than you arrived, engines on or not.

11

Entering the atmosphere introduces drag, which could only reduce your energy. That is, reduce your speed relative to the planet. If you hit the atmosphere at 18,000 mph at too shallow an angle you could bounce off, but not with more energy than you had on approach. You'll fall back, but your landing point may then be outside of your control. You may be ...

10

We get information from the Voyagers just about every day for 8-16 hours per day. Getting the information requires that a dish antenna be pointed accurately in Voyager's direction, so that gives two position dimensions with decent accuracy. To get the third dimension (distance) accurately, the spacecraft can be pinged. A single ping takes ~2 days and is not ...

10

There is a great video by Scott Manley, which specifically addresses your question: Could An Astronaut Throw Something From Orbit To Earth? The short answer is no, humans can not provide enough delta-v by muscle power to deorbit an object by throwing it from ISS.

10

What is the required burn to keep a satellite at a Lagrangian point? tl;dr: typical station keeping delta-v for a halo orbit around Sun-Earth L1 or L2 points are of the order of 2 to 4 meters/sec per year based on a very old spacecraft (SOHO) and a future spacecraft (JWST). I'll address the two closest and most used Lagrange points; L1 and L2. Generally ...

10

Rockets don't actually mostly go up, they try quite hard to go up as little as possible. While flying, gravity is always accelerating you downwards at 9.8 m/s^2. This means that any fuel spent accelerating upwards is wasted, as gravity will pull you back to earth eternally, no matter how much fuel you burn (unless you reach escape velocity, but it will ...

8

This is a great question! tl;dr From a circular orbit, a little more or less velocity just makes your orbit slightly elliptical. If your orbit happened to be very close to the Earth's surface (LEO or low Earth orbit, like the ISS), then a little less velocity (at a given height) would put you deep into the atmosphere and you'd burn up. But that's not ...

8

You can't open an airlock hatch to vacuum until the pressure inside is essentially zero. The hatch opens inward and is held closed by tons of force if there is any appreciable delta pressure. Airlocks are designed for safety and reliability, not circus stunts.

8

You have probably seen funnels like the above in shopping malls. Drop a coin in the funnel and it will move slowly at the edge and move faster as it nears the center. This is a good model of a gravity well. Stuff moves a lot faster in the inner solar system.

8

0 m/s At the altitude of the ISS, atmospheric drag—the effect of particles stealing your momentum—will decay most if not all orbits. The ISS has to make regular adjustments to it's orbital speed to keep it in the proper orbit. Without such adjustments, it would most certainly decay. Now, if you wanted the object thrown to decay immediately, it would have to ...

8

The highest speed was recorded by an Apollo Lunar Roving Vehicle: The rovers were designed with a top speed of about 8 mph (13 km/h), although Eugene Cernan recorded a maximum speed of 11.2 mph (18.0 km/h), giving him the (unofficial) lunar land-speed record. Unmanned rovers have much lower top speeds. Their speed is limited by the amount of terrain ...

8

If you want speed, look for mass. Things closest to the Sun will tend to be moving the fastest. For example the Messenger spacecraft reached almost 63 km/s when in an elliptical orbit who's perihelion matched Mercury's position. In the 2nd plot you can see the red line (speed wrt Mercury) drops to a very low value in 2011, that's when it entered into ...

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 ...

7

Which is the reference point? is not answerable because we are free to choose any point and velocity as a reference to calculate the position and velocity of an object. There isn't any universal reference point. What defines the reference? You do! Some points are much more useful for certain types of calculations, but it's purely a personal choice. ...

7

You are travelling in days a distance that planets travel in months. From a trajectory perspective, that means that neither the velocities of Earth and Mars, nor the gravity of the Sun are particularly relevant any more. That is, "point and thrust" trajectories. You will be limited by either delta-v or acceleration. Delta-v limited case. To ...

6

The only absolute limit imposed by physics is the speed of light, but with our current technology -- reaction engines based on Newton's laws of motion -- we're limited by practical engineering issues and Tsiolkovsky's rocket equation to speeds on the order of 1/10000 of the speed of light.

6

According to Wikipedia's Geographic_coordinate_conversion#From_geodetic_to_ECEF_coordinates The 3D cartesian coordinates $X, Y, Z$ in Earth-centered, Earth-fixed coordinates assuming an ellipsoidal shape is given by: $$X = \left(N(\phi) + h \right) \cos\phi \cos\lambda$$ $$Y = \left(N(\phi) + h \right) \cos\phi \sin\lambda$$ Z = \left(\frac{b^2}{a^...

6

tl;dr: Can we get close to the speed of light with that? No, at least not very easily. The terminal velocity $v_{\infty}$ is only about 0.2% the speed of light if you start at 1 AU using a 10 nanometer thick sail, and scales only as the inverse square root of the distance to the Sun where you start accelerating (as well as sail thickness), so you'd melt ...

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