A question on escape velocity

reply to post by liejunkie01


OK I deleted a post I was about to make but this post prompts me to make it anyway, as confusing as it may be but if you sift through my jumbled post you will find the Voyager spacecrafts didn't achieve solar escape velocity until they got a gravity assist from Jupiter, and possibly also Saturn.

So here goes.

reply to post by -PLB-

And I guess Illustronic was right all along, although he didn't worded it that well (its not pulled into solar orbit, it already was in solar orbit to begin with)

If it is in earth orbit it is in solar orbit by proxy, but not independently. JUNO was placed into earth parking orbit for a half an hour, before its Centaur booster re-fired it off into solar orbit well under 23,000 mph in respect of earth. It entered solar orbit because the combined speed (of earth's orbit and 23,000 mph) was not 42km/sec. 66,600 mph + 23,000 = 91,600 mph, 94,000 mph at 1 AU is what is required for solar escape velocity, (judging from past math calculations by CLPrime I have no reason to believe 42k/sec is off).

Now New Horizons spacecraft left earth orbit at over 36,900 mph (speed record) so it never entered solar orbit, it passed the moon's orbit in less than 9 hours from liftoff, passed Mars's orbit in less than 3 months, and passed the orbit of Jupiter in 13 months for a slingshot up to over 38,000 mph.

Interesting paragraph from Wiki;

New Horizons is often erroneously given the title of Fastest Spacecraft Ever Launched, when in fact the Helios probes are the holders of that title. To be more specific New Horizons achieved the highest launch velocity and thus left Earth faster than any other spacecraft to date. It is also the first spacecraft launched directly into a solar escape trajectory, which requires an approximate velocity of 16.5 km/s (36,900 mph), plus losses, all to be provided by the launcher. However, it will not be the fastest spacecraft to leave the Solar System. This record is held by Voyager 1, currently travelling at 17.145 km/s (38,400 mph) relative to the Sun. Voyager 1 attained greater hyperbolic excess velocity from Jupiter and Saturn gravitational slingshots than New Horizons. Other spacecraft, such as Helios 1 & 2, can also be measured as the fastest objects, due to their orbital velocity relative to the Sun at perihelion. However, because they remain in solar orbit, their orbital energy relative to the Sun is lower than the five probes, and three other third stages on hyperbolic trajectories, including New Horizons, that achieved solar escape velocity, as the Sun has a much deeper gravitational well than Earth.

That makes a little more sense if I add;

The Star 48B third stage is also on a hyperbolic Solar System escape trajectory, and reached Jupiter before the New Horizons spacecraft. However, since it is not in controlled flight, it did not receive the correct gravity assist, and will only pass within 200,000,000km (120,000,000 mi) of Pluto.

So what I am gathering from all of that is the speed New Horizons left earth orbit at a combined hyperbolic trajectory of 103,500 mph is near the minimum required for a solar escape velocity at 1 AU from the sun, which converts to 46.3 km/sec. But in all fairness I also find'

After the Star 48B burn, the payload had reached escape velocity not only with respect to the Earth but also relative to the Sun (The velocity was 16.2 km/s relative to the Earth and I estimate an asymptotic velocity of 12.3 km/s, corresponding to 42.6 km/s relative to the Sun and leading to a heliocentric eccentricity of around 1.05).

So which is it, 46.3 or 42.6?

Today there is at least one Apollo third stage booster still in solar orbit and a little over a year ago was spotted and at first mistaken as a NEO asteroid, until it got close enough to identify as being what it is.

The interesting fact of the Helios spacecrafts is their orbital apogee extends inside the orbit of Mercury which is how they reach a higher speed than Mercury at its perihelion, around 150,000 mph. I also understand a recent sun grazer comet was measured at reaching a speed of over 300,000 mph at its perihelion, which at the distance it missed the sun by is still not high enough to reach solar escape velocity.

So going back to the Apollo days, none of the Apollos reached solar escape velocity and that is why their flight trajectories were an arc, (a portion of its solar orbital trajectory). So if they missed the moon they would be in solar orbit today, and unpowered could never achieve earth orbit due to the speed they are moving.

Now all of this is relative to the sun or the earth but does anyone have any numbers on what is required to achieve galactic escape velocity? Where would one find data to support calculations for that?

I just want to add that previous post used exactly the maximum characters allowable for a post here, editing required, what do I win?

reply to post by Illustronic


Thats a pretty awesome ATS achievement unlock! Here, have a silly picture, you can print it out and wear it as a badge

Originally posted by Illustronic
I just want to add that previous post used exactly the maximum characters allowable for a post here, editing required, what do I win?

That made me laugh a little.

I was'nt really trying to proove a point about the voyagers.

I just wanted to give them their little moment in the spotlight

I think that they are fascinating.

Keep up the good work.(no sarcasm, I enjoy reading on this topic)

stars from junkie

Originally posted by CLPrime
reply to post by OZtracized

 

TheMindWar and justwokeup already answered the technical part of the question. I just want to add that the NASA page you linked, which is aimed at young kids, is wrong. It would only be true if the shuttle's engines shut off immediately after launch. The shuttle can go at a proverbial snail's pace and, as long as it has continuous thrust (providing continuous acceleration), it will leave Earth's orbit.

CLprime, justwokeup and others, thank you for your replies! That's basically what I was getting at (my internet has been down for a couple of days, I wasn't trying to start a thread then just ditch it).

The rocket/escape velocity question was the main thing I wanted cleared up, it seems what I suspected is true. I threw the black hole/rocket question in just as some food for thought. Skimming through the replies, nobody seems to have addressed that one in any detail. I haven't read replies thoroughly though so I apologise if I've missed something.

reply to post by OZtracized


To answer what I think is your question of why escape velocity is described as speed, and why can't a rocket escape the gravitational pull at a snails pace you just have to visually put that model into a working dynamic. Forget at first escaping the extreme gravity of a black hole for now.

Lets also leave out all of the scientific jargon and actual mathematical calculations for now to focus on the idea of counteracting the force of earth's gravitational pull on a mass. (One can escape the gravitational pull of earth slowly, it is the least efficient way to do so). Lets take a typical Harrier jet or VTOL taking off from the ground. When a Harrier jet slowly lifts itself vertically it is using its available fuel and thrust at its most inefficient operational mode. The jets usually takeoff as STOL (short take-off and landing) to allow their wings to also provide lift, (and those funny short elevated aircraft carrier runways add by providing a jump lift incline). This is why we will ignore helicopters, which are entirely dependent on thick air and their propellers act like wings providing lift, which lowers their service ceiling compared to Harrier jets and true thrust vectoring VTOLs. Jets simply operate in a horizontal flight more efficiently, conserving fuel and increasing its payload.

The same principle applies in space with rockets. In order to escape he pull of gravity without speed a constant trust must overcome that force of gravity though that force lessens the further away one is from the center of that gravitational pull, but never disappears entirely. So to slowly leave that gravity more force is actually spent at a constant rate instead of using orbital dynamics to overcome that pull. To illustrate, if you are geostationary at the altitude of the ISS in respect to a point on the earth's surface you have to have constant thrust to maintain that altitude or gravity will pull you straight down again, no orbital free ride. An orbit is actually a free fall, unpowered it is just the correct speed of a given altitude that keeps one missing the earth, and around you go, in a relatively stable orbit. The higher you go the less speed you need to keep missing earth, and the longer your orbit will last, unpowered.

So beyond the event horizon of a black hole there simply isn't enough power to overcome that force of gravity for any particle mass or massless wave.

Quite simply if a rocket taking off from the pad doesn't have more foot/pounds of thrust than its weight it simply will not take off, and that will continue as gravity and weight decreases. All of the speed generated is for the most efficient use of available fuel, which is why all rockets enter orbit the opposite direction as the earth revolves, to gain that near 1,000 mph relative speed of a point on the ground, meaning that if they took off heading west, they need near 2,000 mph more speed to reach the same orbit.

Hope that more directly addresses the question, which has been brought up here before in a different model.

Speed is just the most efficient way to defeat gravity and the most efficient use of fuel or power source. Besides if you want to space travel isn't gaining the highest speed job one?

reply to post by Illustronic


Truth be told, the main question I wanted cleared up is why the space shuttle etc apparently has to reach escape velocity to leave Earth's atmosphere. I have heard that factoid numerous times and it just didn't seem to make sense to me.

It doesn't. It's a misconception and a mistake on the NASA website.

I think I see what you're saying about a rocket's (in)ability to leave a black hole. To totally put it in layman's terms, the rocket simply becomes extremely heavy within the event horizon. Infinitely so. No thrusters could possibly hold such a heavy object up let alone make even the slightest gain on gravity in that situation.

Truth be told, the main question I wanted cleared up is why the space shuttle etc apparently has to reach escape velocity to leave Earth's atmosphere.

The simple answer to that is that it doesn't. The Space Shuttle never comes close to Earth's escape velocity. It only has enough fuel and thrust to go into Low Earth Orbit, and that velocity is only (roughly) 70 per cent that of escape velocity.

Stars, flags, and fireworks to the well-informed and silver-tongued posters here regarding 'escape velocity'. It's a pleasure to see folks at ease explaining clearly and accurately a challenging and unearthly concept. Attaboys all around for the good question at the start, and the super-plus-good answers.

The penalty in not getting to orbital speed as fast as possible is called 'gravity losses' -- it's wasted thrust that is eaten up maintaining altitude without increasing speed. To get into orbit at 25,000 ft/sec a satellite booster actually has to deliver something like as much as 31-32,000 ft/sec, to claw UP and to hold your own bootstraps before you're fast enough horizontally to 'fall over' the receding horizon.

Originally posted by CLPrime
reply to post by OZtracized

 

I just want to add that the NASA page you linked, which is aimed at young kids, is wrong. It would only be true if the shuttle's engines shut off immediately after launch. The shuttle can go at a proverbial snail's pace and, as long as it has continuous thrust (providing continuous acceleration), it will leave Earth's orbit.

I guess the next question is why did NASA post an obviously incorrect statement on its website? If it's aimed at kids, why go out of your way to feed the younger generation B.S.? Or is it that the person posting that info just has no idea? Either way, it sets a pretty bad example. Sorry to come across as pedantic (anally retentive) but let's start feeding our children "facts" as we know them. Not rubbish because we think that's all they can understand. Worse still, information from people who don't know what they're talking about.

reply to post by OZtracized


I was actually wondering the same thing, so it's not just you (though, I have been described as pedantic as well).
If you wrote them asking about it, the reply would probably say something about the site being aimed at kids and their attempt to connect the subtle concept of escape velocity to a well-known part of NASA - and, thus, the application may not be exact.
More in-depth sections of the site may detail the specifics a little more.

Originally posted by OZtracized

Originally posted by CLPrime
reply to post by OZtracized

 

I just want to add that the NASA page you linked, which is aimed at young kids, is wrong. It would only be true if the shuttle's engines shut off immediately after launch. The shuttle can go at a proverbial snail's pace and, as long as it has continuous thrust (providing continuous acceleration), it will leave Earth's orbit.

I guess the next question is why did NASA post an obviously incorrect statement on its website? If it's aimed at kids, why go out of your way to feed the younger generation B.S.? Or is it that the person posting that info just has no idea? Either way, it sets a pretty bad example. Sorry to come across as pedantic (anally retentive) but let's start feeding our children "facts" as we know them. Not rubbish because we think that's all they can understand. Worse still, information from people who don't know what they're talking about.

From what I read it is describing what escape velocity is, and that is all. It does not in that one sentence go into all of the dynamics necessary to achieve an altitude and stay there by other means, it simply is offering what the means of velocity is to escape velocity, and nothing else. Sort of like answering what a tomato is without a description of the hundreds of varieties or hybrids.

reply to post by Illustronic


They actually seem to be doing something a little more, shall we say, askew than that. They're doing the equivalent of the US government calling a tomato a vegetable so they can tax it. They're twisting a fact a bit to achieve a desired result.

reply to post by CLPrime


Funny, NASA offered nearly word for word the Merriam-Webster on-line dictionary description.

I believe that is the only purpose, not a conspiracy. The question wasn't asked that is speed the only requirement, it asked what escape velocity speed is. Straight forward answer.

reply to post by Illustronic


Yet that's what this whole discussion has been about. A rocket doesn't need to achieve escape velocity because it experiences continuous thrust. The implication is that the rocket experienced an initial period of thrust at liftoff to propel it to escape velocity, and then it coasts into space. This certainly isn't the case.

The rocket is only used as an example of something that goes into space, it can't be used to illustrate escape velocity on any legitimate scientific basis.

reply to post by CLPrime


A rocket doesn't use continuous thrust to achieve orbit, once it reaches the speed of a particular altitude above the surface it can then coast to orbit and shut its engine(s) off for a free ride. If the speed it reaches is at 200 km for a 250 km orbit it can shut down and get there. If it wanted to maintain the same proximity of a point on the surface (motionless), it then would require continuous thrust to defeat gravity. If there is nothing within a light year of the earth a rocket with no thrust would be pulled back to earth at even a light year distance. See, gravity is what escape velocity defeats, regardless of the relative speed.

The real problem is the question is a loaded one, so the answer addresses only the speed aspect. Gravity never equals zero, it just gets overcome by a larger gravity or inertial momentum.

reply to post by Illustronic


I said this before, but Shuttles would (when they did such things) continue under Main Engine power for about 10 minutes until just before the orbital insertion point. They have no direct concern for the escape velocity at any altitude.

They powered up to achieve reentry, (and they also released unspent fuel). In orbit the Shuttle was entirely unpowered sans maneuvers.

Of course they used power in orbit, one has to understand how vast an area it is to converge two different pins in a haystack for a rendezvous.

Besides the Shuttle was designed to achieve orbit and not escape velocity, we should redirect the conversation to Apollo spacecraft and not the Shuttle, or even space probes, one of my favorite topics.

reply to post by Illustronic


Except that the NASA site uses the Shuttle specifically, claiming it has to reach the escape velocity of the Earth. It doesn't. Ever. Orbit doesn't involve escape velocity.

Well, you know what they say: What goes up, must come down (except for a powered vehicle that can reach and maintain escape velocity and achieve orbital altitude)

HAR HAR HAR.