105

Because the earth goes very fast around the sun. If you want to get to the sun, you need to slow down almost completely so that your speed relative to the sun becomes almost zero. If you don't slow down (almost) completely, your probe will miss the sun when you 'drop' it, so it will eventually come back and you'll end up in an elliptical orbit. Kind of like ...


35

TL;DR: it is inefficient. You should play some Kerbal Space Program and see for yourself the effects of travel in this way. Assuming, of course, you didn't really want to enter the orbit, but wanted to e.g. go to the Moon or deep space probing. Especially in conjunction with Wikipedia's note about not having to attain escape velocity to leave gravity well....


33

Changing orbits requires delta-v. To reach the Sun, you need to subtract delta-v such that your velocity relative to the Sun is near zero, which allows you to "fall straight down" into the Sun - your required delta-v is nearly equal to your orbital speed. To escape the solar system, you need to add sufficient delta-v in order to reach escape ...


20

Escaping the solar system requires adding orbital velocity to the spacecraft. Similarly, getting closer in the solar system requires removing orbital velocity. It turns out Earth is more out of the Sun's gravity well than it's in it. In other words, the simple answer is that Mercury is "farther away" in terms of the change of velocity that's ...


19

Why zero excess velocity? Well, with almost zero excess velocity you can stay near Earth, but not too near. For example, the Spitzer space telescope did this to communicate with Earth while avoiding radiant heat from Earth. It's been drifting away, but slowly enough that other factors first reduced its effectiveness.


13

Based on the calculations presented by @uhoh I generated a plot showing the necessary delta-V for a fly-by mission, i.e. entering into a Hohmann transfer with a far point intersecting the orbit of a planet to get into a circular orbit at the same radius as a planet Note that this does not include any methods to save fuel (aero-braking, swing-by) and ...


12

Why would one want to choose zero excess velocity upon escape? If you aren't in a great hurry, and you have a small delta-V budget, and you want to visit the Trojan points L4 or L5, you can do so by getting just outside of Earth's sphere of influence, then lowering or raising your solar orbit very slightly to get ahead of or fall behind Earth. You wouldn't ...


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 ...


11

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


10

Your question is, as I understand it, pointing out that there are two ways to get from the surface of the Earth to the surface of the Moon. Way one: Burn upwards until through the thickest part of the atmosphere to avoid aero drag. Burn sideways to attain orbital velocity and raise apogee and perigee into space. From Earth orbit, burn prograde to attain ...


8

Many quantitative questions about orbits can be answered using the vis-viva equation $$v^2 = GM\left(\frac{2}{r} - \frac{1}{a} \right)$$ where $a$ is the semi-major axis, $r$ is the current distance to the central body and $v$ is the velocity at $r$, and the vis-viva equation comes straight from the principle of conservation of total energy which is the sum ...


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

Parabolic escape trajectory is only theoretical, it only "works" in a two body system, and in a two body system "escape" is a meaningless practice anyways. In a multi body system the forces from other bodies, especially around the edge of the gravity well, make parabolic escape impossible: before you'd have zero velocity the other body would've already ...


5

For getting to the moon specifically, there's an extra problem, on top of what's already been mentioned: presumably, your objective when you get to the moon is to either orbit it or do a nice gentle landing on it (if your objective is to turn yourself into a cloud of shrapnel spread over a large area of regolith, feel free to take your approach). That is: ...


4

Do you have to be on a parabolic escape orbit in reference to the origin planet to perform the first HT burn? The Hohmann transfer ∆v equations normally assume you aren't in orbit around another body. The following all assumes we're doing some sort of "patched conic" approximation, where we define a sphere of influence for each body and pretend it's the ...


4

Not a planetological exposition in sight so, I'll add my two cents to this rather theoretical discussion. Amongst exoplanetologists, the consensus has emerged that 1.6 Earth radii and 5 Earth masses is likely to be the upper limit to rocky planets¹. Simulations have shown that above these figures, the bodies develop increasingly Mini-Neptune² like ...


4

In addition to the answers above, one should add that assuming you can get to Mars very, very fast, you will therefore arrive there with a very high velocity relative to Mars. This makes the problem of landing on Mars much harder. Also, if something goes wrong at Mars insertion, the Martian explorers are hurtling off into deep space with no way back. The ...


4

Going to a celestial body, entering orbit, then having to shed all that orbital velocity. If you were travelling slow enough as it is, and slowly decelerating all the way, counteracting gravity, it would take you loads of time probably, but surely there must be another reason why it can't be done, else why wouldn't we do it. Since others (specially Starfish ...


4

I can't think of one. For mission planning there is nothing special about parabolic velocity. There is something special from the perspective of teaching orbital mechanics as it is the boundary between closed and open orbits, but from a practical mission point of view it looks just like a very long ellipse or a barely open hyperbola for many, many years. ...


4

According to Wikipedia: Escape velocity is actually a speed (not a velocity) because it does not specify a direction: no matter what the direction of travel is, the object can escape the gravitational field (provided its path does not intersect the planet). It may seem nonintuitive that an escape trajectory directed “downward” could escape just as easily ...


3

As everyone else has mentioned, there doesn't seem to be a mission for a true parabolic escape, especially since an exact parabolic trajectory is a target of zero size and therefore there is a zero chance of hitting it exactly. Also, a true parabolic orbit only makes sense in a two-body model. Once you consider the gravity of anything else, the orbit ...


2

The short answer is that ruling out world-building thought experiments to increase mars' escape velocity; nothing can be done to prevent water vapor from escaping which is precisely why it has so little water compared to Earth. With that said, Mars' water escapes on astronomical timescales while the goal of any type of terraforming activities would be to ...


1

What is the thing that prevents you from... going fast and then steering up... Isn't it the atmosphere? Going up first then speeding up makes spaceflight possible. No rocket could accelerate to mach-20+ at 1 atmosphere and then sustain it all the way to space.


1

If by "particles", you mean atoms and similar size things, it's not really just about escape velocity. (From WikiCommons) Once the atoms get more than a half-dozen or so radii away from Earth, they're directly in the solar wind: Atoms out there have mean free paths that are short enough that they collide, come to equilibrium, and move outward with the ...


1

When thinking about escaping a moon most people use the moon's 2 body escape velocity, sqrt(2GM/r). However reaching the edge of the moon's Hill Sphere can be sufficient. The Saturn Mimas L1 (SML1) is at an altitude of about 332 kilometers. In a 2 body scenario it would take .14 km/s to get an apo-apsis altitude of 332 kilometers. However tidal forces tend ...


Only top voted, non community-wiki answers of a minimum length are eligible