Are we really orbiting the sun at 66,000 mph? If so, why can't we get
rockets to go that fast?

Roughly correct. The average velocity is perhaps slightly higher than that, but that's close enough.

They all do.

Any rocket launched from Earth will be moving at the same velocity as Earth relative to the sun, until and unless the rocket alters its velocity relative to the sun.

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The Leif Ericson Cruiser

You are on a train which is going at 100 km/h.

You then run from the back of the train to the front.

Compared to the ground you are going 100 km/h + your running speed.

Compared to the train you are only going at your running speed.


A rocket launched from Earth can only add or subtract it's speed to that of Earth; but it will always start with Earths speed (whatever that's compared to). A rocket does not simply "drop off Earth" - it has to start with Earths speed.


(You can jump off that train to quickly lose your "train speed" and go the "ground speed" (zero). There is no equivalent for rockets. Space won't change the rockets speed the same way the ground changes your speed when you jump off the train.)

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I am not versed in space travel physics, but isn't the fastest rocket
in space now only doing 50,000 mph?
Also why is Voyager 1 and 2 only going at aprox 40,000 mph?

What am I missing?
Please forgive my thinking.

50,000? 40,000? measured from where?

I'm sorry, the new horizons space craft heading toward Pluto is the
fastest rocket ever launched from Earth. Its only going 36,000mph.

Yes, but 36,000 mph compared to what?

Earth?

The Sun?

The centre of the Galaxy?

Think about the train example. When you run along the train, what is your speed? (It matters who is measuring it!)


New Horizons was launched from Earth (which was moving) and is headed towards Pluto (which is moving) and that path from Earth to Pluto is not "as the crow flies". This stuff is complicated. You can't simply compare the speed of Earth as it orbits the Sun with the speed of a probe launched from Earth and headed somewhere.

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I suppose the speeds are being measured against the Sun. Roughly, New Horizons and the Voyager spacecraft all left Earth at a bit faster than Earth itself, relative to the Sun. From the Sun's point of view, they left Earth while adding a bit of extra speed.

From there, the Sun's gravity has been pulling on them, so the further they go the slower they've been going. Gravitational boosts from planetary encounters could increase the speed, by stealing a bit of kinetic energy from the planets they passed, but outside that they'll go slower and slower the further they get from the Sun.

(All relative to the Sun.)

I don't know, NASA launched it. Ask them.

Relative to what? Any spacecraft in orbit around Earth will be going, on average, the same speed as the Earth around the sun - so around that 66,000 miles per hour figure you mentioned. The Messenger spacecraft (which will eventually go into an orbit around Mercury) is moving well over 100,000 mph in its orbit around the sun. But essentially everything in the solar system is moving roughly half a million mph in orbit about the galaxy.


You seem to be thinking of velocity in absolute terms.

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Half a million mph? Now thats cuttin a chogy!

Thanks for all the replys!!

When your in a car that is going 60 mph, you are also going 60 mph. The rocket is like you and the earth is the car.

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Van Rijn,

He may well be thinking of velocity in absolute terms, but it doesn't appear to
matter for this question. The speeds he quotes do appear to be relative to
the Sun.

Your wording makes it sound as if the slowing were dependant on distance
from the Sun, such that the farther it is from the Sun, the more it is slowed.
Of course the actual dependancy is on time: The more time goes by, the
more it is slowed. Plus, the closer it is to the Sun, the more rapidly it is
slowed.

-- Jeff, in Minneapolis

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rw4pt6,

The spacecraft are moving slower than Earth is (relative to the Sun) for
exactly the same reason that an object you throw up into the air slows,
stops rising, and falls back to the ground. In the case of the object you
throw, Earth's gravity continually pulls it down. In the case of the
spacecraft, the Sun's gravity continually pulls them down. The one big
difference is that the spacecraft have enough speed that they can keep
on rising forever, even though the Sun is slowing them. That is possible
because the Sun's gravitational pull weakens with distance. As spacecraft
get farther and farther from the Sun, the Sun's gravity has less and less
effect on them. If you could throw something at 25,000 mph, it would
slow as it rises away from the Earth, but it would be moving fast enough
that it would never stop rising. Except that it would still be in orbit around
the Sun, along with Earth. So it would end up in an elliptical orbit around
the Sun, different from Earth's nearly circular orbit, and would occasionally
come close enough to Earth that eventually it would be likely to hit it.

-- Jeff, in Minneapolis

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were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

All motion is relative. You must define your coordinate system in which you are measuring velocity. Initially, a rocket’s velocity is measured relative to its launching point on Earth, a point that is actually rotating around the Earth’s axis and revolving around the Sun. Once it leaves the atmosphere, its velocity is normally stated relative to a non-rotating approximately inertial system centered on the Earth until it roughly gets beyond the orbit of the Moon. Afterward, the velocity is usually stated relative to a non-rotating approximately inertial system centered on the Sun. Then when it approaches another planet, its velocity is usually stated relative to a non-rotating approximately inertial system centered on that planet. If it descends and lands on the planet, then its velocity is measured relative to the planet’s surface. Velocity figures given by the popular media are meaningless unless the coordinate system being utilized is mentioned.

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That rocket is no longer propelling the spacecraft which is now coasting. It only provided the initial push. When you throw a ball upward, it decelerates as it rises. The same happens with a spacecraft coasting away from the Sun.

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But, he said "further", not "farther", so he must've been talking about time...

Some folks may be confusing the terms velocity and acceleration. Velocity is the rate of change of displacement (distance) relative to time, while acceleration is the rate of change of velocity relative to time. For instance it is quite possible for an object to still be increasing its velocity while its acceleration is decreasing (albeit still positive.) In the case under discussion, the coasting spacecraft is slowing in velocity relative to the Sun, but the rate at which that velocity is slowing (deceleration) decreases as it moves further from the Sun.

I worked in TV financial news. I can’t begin to tell you how often writers would state that some economic measurement was at its highest for a year, without indicating whether its actual level was its highest for the year, or the monthly gain was the greatest increase of the year, or the increase in the increase was the greatest.

Those who know differential calculus understand that I’ve been discussing the differences in meaning among a value and its first and second derivatives.

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It is.
That's not true. Look at an elliptical orbit. In time, the speed both slows down and speeds up. It depends on whether the satellite is going closer or further away from the Sun.

It's a matter of conversion between kinetic energy and potential energy. Going up means converting from kinetic energy to potential energy. Going down means converting from potential energy to kinetic energy. This works for elliptical orbits, parabolic orbits, hyperbolic orbits...it works for all two body orbits.

Of course, the paths of our space probes aren't quite two body solutions--using gravitational flyby assists, other planets get into the mix momentarily. A flyby assist of Jupiter let's you get a boost of extra energy from Jupiter rather just converting between your own kinetic and potential energy.

Hi rw4pt6,

I think there is some confusion about two different ways of measuring velocity.

For spacecraft orbiting the earth, the velocity (or more correctly, its speed - velocity includes direction) is usually measured relative to earth, ignoring the earth's motion around the sun. So let us suppose that a spacecraft is orbiting the earth at 20,000 km per hour. That means if we are standing on the surface of the earth, and we see the spacecraft moving overhead, we will see it moving at 20,000 km per hour. When performing this measurement, we ignore the fact that we (standing on the earth) are actually moving around the sun at about 100,000 km per hour. It is similar to pzkpfw's example of motion on a train. If an observer on the sun were to measure the speed of the spacecraft, it would be somewhere between 80,000 and 120,000 km per hour, depending on whether the spacecraft is moving (relative to the earth) in the same direction the earth is moving relative to the sun, the opposite direction, or at some oblique angle.

But once a spacecraft leaves the neighborhood of the earth, it is not very useful to measure its speed relative to the earth anymore. So if a spacecraft is moving at about 60,000 km per hour as it is speeding out of the solar system, this is probably a measurement relative to the sun. As IsaacKuo says, this spaceship must have been launched at a speed even faster than earth's velocity around the sun, or it wouldn't have gotten very far. But as it goes to higher and higher altitudes (greater distance from the sun), it looses speed, as Jeff Root and IsaacKuo describe.

But any spacecraft in orbit around the earth is moving (relative to the sun) faster than 60,000 km per hour - we just don't measure their velocity relative to the sun, we measure it relative to the earth.

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Nearly all the comments are correct in my opinion. Since speed is independent of direction, speed is confusing after you leave Earth's surface. Even airplanes travel at air speed or ground speed which is often double or half. Near the equator Earth's surface has a speed of about 1000 miles per hour which adds if the orbiting satellite is traveling West; subtracts, traveling East (I may have that backwards) Over the poles, the speed of Earth's surface, neither adds nor subtracts. Depending on the direction of travel, an orbiting space craft can have a velocity (or speed?) less than the velocity (or speed?) of Earth around the Sun. Velocities of practical space craft typically add vectorially, but subtract is possible. The last phrase is likely technically incorrect as subtract is indicated by one of the speeds being negative. Neil

Not really. The further it is from the sun, the less it is slowed in a given amount of time.

The speed is always slowing. Otherwise it would escape from the sun. Also, the closer an object is to the sun, the faster it has to go in orbit, because it is being slowed more quickly.

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Yes really. The equation is this:

v = sqrt(mu(2/r + 1/a))

mu is the gravitational parameter of the Sun; a is the semi-major axis of the hyperbolic trajectory (a constant determined by the specific orbital energy of the vehicle).

Notice that this function depends on r, the distance from the Sun.
The reason the speed is always slowing is because it is going further away from the Sun. If it were going toward the Sun, it would be getting faster.
Huh? What I say is true of all solar orbits, regardless of whether it's a true orbit (circular/elliptical) or whether it's on an escape trajectory (parabolic/hyperbolic). Comets on parabolic/hyperbolic escape trajectories do indeed speed up if they're moving toward the Sun, and then slow down after they pass the point of closest approach and then move away from the Sun.
No. An object in a circular orbit isn't being slowed down at all. Why not? Because it isn't going further away from the Sun.

This is the basic physics of freefall. Going higher in the gravitational potential well means going slower. Going lower into the gravitational potential well means going faster. It's conversion between kinetic and potential energy.

I'm not sure if I'm really contradicting you or if we are just saying the same thing but in different ways.

But let me put it this way. Take Mercury and Mars. Suppose that the gravity suddenly stopped existing at all, then I think it is clear that both Mercury and Mars would in fact move away from the sun. So therefore, they are losing that outward velocity and being pulled into the sun. And I think we both know that if gravity were suddenly to disappear, Mercury would fly away from the sun faster than Mars would. So I would argue that Mercury is losing more of that outward velocity than Mars is. At any instant in time, I think there is an out outward velocity. Isn't there?

I'll have to give more thought to the thing about eliptical orbits. Orbital mechanics are not my forte, to say the least.

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Definitely saying things in a different way, but I'm not sure if you are contradicting or not.

Mercury and Mars are already at different speeds. If gravity stopped, they would retain thier own speed, but thier trajectory would become a straight line rather than an ellipse.

I think the problem is this thing you call "outward velocity". If it's what I think you mean, then it is only a measurement of one aspect of the forces of an orbit.

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Well, the acceleration of Mercury towards the sun is greater than that of Venus towards the sun. But when an object is in a circular orbit, its velocity vector is always perpendicular to its acceleration vector. As a result, its velocity is always changing (in direction), but remaining constant in magnitude.

The change in speed is not just based on distance from the sun, but also angle of the velocity vector and the vector towards the sun.

The conservation of energy principle works well here. Ignoring third-body effects, the energy (kinetic plus potential) of an object in orbit remains constant. But kinetic energy gets converted to potential energy sometimes, and sometimes the reverse happens. (IsaacKuo talked about this somewhere.) An object in circular orbit is always the same distance from the star, and therefore always has the same potential energy; it must also have the same kinetic energy, to conserve energy. So it must always move at the same speed. Its velocity is always changing in direction, but constant in magnitude.

An object in elliptical orbit (or parabolic, or hyperbolic) is at a changing altitude, so its potential energy is always changing. As it gets closer to the star, it loses potential energy, but it gains kinetic energy, speeding up. As it moves farther away from the star, it gains potential energy, but loses kinetic energy, slowing down. At is closest and farthest approach, it is momentarily neither gaining nor losing altitude, and is therefore also neither gaining nor losing speed.

What really determines how the speed changes is a combination of how far the object is from the star (because the force of gravity changes with distance) and how rapidly its distance to the star is changing. Two objects could each be 100,000,000 km from a star. If one of them is in a circular orbit, it is neither gaining nor losing speed, but if one of them is rocketing away from the star at a high rate of speed, it will slow down.

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