ok, say a craft launches from earth with crew, to mars and earth travels at a fast rate through space around the sun, so does mars, how do the crew judge launching, and landing from one planet to the other, considering both planets are revolving around the sun so fast, also spinning on an axis. how can the crew judge the landings?,



peace,

Newton gravity.

ok, say a craft launches from earth with crew, to mars and earth travels at a fast rate through space around the sun, so does mars, how do the crew judge launching, and landing from one planet to the other, considering both planets are revolving around the sun so fast, also spinning on an axis. how can the crew judge the landings?,



peace,

It's all a matter of computations, Chi, and we've known how to do that for years. If there was a reason to do so, they could tell you what the precise location of each and every planet would be on 1 Jan 2050.

It's exactly the same math and formulas they used to get to the moon and to place every probe exactly where they wanted it to go. The planets orbits are VERY predictable to any date you want. Precise landings are figured out in the same way.

The relative positions of Mars and Earth have been worked out years in advance, so that Nasa (or whoever) knows down to the hour or minute when to launch missions. You launch in the direction of the Earth's rotation to get to the outer planets, and against the Earth's rotation to get to Venus or Mercury. On Mars, you'd launch with the rotation to get to Jupiter et. al., and against (therefore slowing down and dropping inward) to get to Earth.

the crew doesn't judge the flight path. it is continuosly updated by a computer.
for example, on the apollo flights at launch they had to deal with wind shear where the wind would push the rocket to one side or the other when that happens the rocket doesn't correct by vectoring the rocket but by the computer recalculating the entire flight path by saying, ok the wind pushed me over this much now how much thrust and in what direction should i thrust to keep me on target. humans on a rocket without a computer going to mars, i believe to be impossible. even the voyager craft had ground based computers.

The relative positions of Mars and Earth have been worked out years in advance, so that Nasa (or whoever) knows down to the hour or minute when to launch missions. You launch in the direction of the Earth's rotation to get to the outer planets, and against the Earth's rotation to get to Venus or Mercury. On Mars, you'd launch with the rotation to get to Jupiter et. al., and against (therefore slowing down and dropping inward) to get to Earth.

No, Xylene, that's not correct. It's a myth that shows up now and then. All launches are made in the direction of rotation regardless of destination. The purpose is to use as little fuel as possible while achieving escape velocity as quickly as possible.

Perhaps I should also point out that it's not a matter of conserving fuel for later uses. The weight of the fuel load has been carefully calculated along with the weight of the vehicle at the time of design. So it carries no more fuel than needed and lifting off in the direction of rotation means a lighter fuel load right at the beginning.

he is also wrong about decelerating to gain a lower orbit.to gain a lower orbit you must accelerate.to gain a higher orbit you must decelerate.

he is also wrong about decelerating to gain a lower orbit.to gain a lower orbit you must accelerate.to gain a higher orbit you must decelerate.

No, that's not correct either. ;) You are thinking in terms of an object orbiting a planet (or something similar) and maintaining an orbit. There are actually a couple of different things involved depending on what your goal is.

If you want to maintain a geosynchronous orbit, going farther out (higher) means you must increase the orbital speed in order to maintain position - exactly like getting farther from the hub of a wheel. Going lower means going slower - but you are limited because you have to maintain enough speed to keep from dropping out of orbit.

But going back to the thought of interplanetary travel it doesn't even figure into the effort. Headed inward - toward the sun - or outward, you accelerate as much as possible (fuel limitations) to get the shortest possible travel time regardless of the direction.

that was what i was talking about arocket orbiting the earth.but you are right tho it really doesn't matter.

Not the story I was looking for, but here's one route;

http://news.bbc.co.uk/1/hi/sci/tech/876112.stm

and there's a graphic showing the outward journey on this one;

http://news.bbc.co.uk/1/hi/sci/tech/3381531.stm

Somewhere there is a proposed mission timeline, but I couldn't find that quickly.

Anyway, it will take months to get to Mars, and then we'll have to spend months there, waiting for a good return opportunity, and it will take months to get back home.

All launches must take place while both planets are in proximitity, crossing the solar system is a long way round.

I think Bush really underestimated how far it is. how costly, and how how difficult, when he made his Kennedy rip off speech.

A geosynchronous satelite orbits the Earth in a time, by definition, equal to the length of a day ( and night ). Pretty much 24 hours. It is orbiting at a relatively great distance, therefore the Earth's gravity is relatively weaker, allowing its velocity to be relativitely low.

A satelite in low orbit may have a period of a fraction of a day, in relatively strong gravity, demanding that its velocity be relatively high.

Ubiquitously, an orbit farther from a primary must have a LOWER velocity and an orbit closer to a primary must have a HIGHER velocity.

I hope this disertation has not been too HEAVY. ;)

A geosynchronous satelite orbits the Earth in a time, by definition, equal to the length of a day ( and night ). Pretty much 24 hours. It is orbiting at a relatively great distance, therefore the Earth's gravity is relatively weaker, allowing its velocity to be relativitely low.

A satelite in low orbit may have a period of a fraction of a day, in relatively strong gravity, demanding that its velocity be relatively high.

Ubiquitously, an orbit farther from a primary must have a LOWER velocity and an orbit closer to a primary must have a HIGHER velocity.

I hope this disertation has not been too HEAVY. ;)

Although you're correct about the low Earth-orbit, you've failed to consider an important element in the case of a high-orbit geosynchronous satelite - it's realtive distance from the Earth requires a good deal of speed in order to maintain it's position.

Although you're correct about the low Earth-orbit, you've failed to consider an important element in the case of a high-orbit geosynchronous satellite - it's relative distance from the Earth requires a good deal of speed in order to maintain it's position.

I stand corrected - sorry!!!

A quick check showed that sync orbits are usually established at 35,800 km and orbital speed is 3.1 km/s at that altitude. And as altitude decreases, the speed for all satellites increases.

Again, sorry. (Brain dozing here.)

No problem. That's just strike one. Get back in the batter's box. Even the best major league hitters have trouble with the high fastball. :cool:

It is more complicated, but the concept is the same as shooting small game. You aim at where the rabbit (Mars) will be when the bullet (space craft) gets there, not at where the rabbit (Mars) is when you pull the trigger (launch the space craft).

In one sense I understand that a low geosynchronis orbit must be faster (in order to keep from crashing into the planet), but doesn't a higher orbit need to travel further per day in order to remain above the same location on Earth's surface (and therefore need a greater velocity)?

I feel like I'm missing something obvious here, because I used to understand this! Also, I have two conflicting notions about GS orbits :bugeye:

Jack,
any orbit altitude has a corresponding strength of Earth's gravity there, and thus a certain speed that must be met to remain there. Every different altitude of orbit has a different orbital speed. Since geosynchronous means just one speed in particular, it also means one altitude in particular, regardless of what is orbiting. Hope I said this right.

On a different note, I remember when the moon shots were happening and people would hear about a "window" of time the launches had to make. Some thought there was a section of our atmosphere that must be used for the launch. But of coarse, NASA was talking about timing the trip, or so I always believed.

1] On Geosynch orbits:

"...doesn't a higher orbit need to travel further per day in order to remain above the same location on Earth's surface..."

You're looking at it the wrong way. You're starting with the altitude and asking what the speed needs to be. Turn the problerm upside down. Starting with the speed and figure out what the altitude needs to be. You want the satellite to orbit the Earth at one revolution per day (so that it maintains geosynched over one spot). At what altitude will it need to orbit so that its orbital speed works out to one revolution per day? It turns out that the altitude required is about 35,000km.

P.S. I've never heard of "low" geosynch vs. "high" geosynch. Seems to me there is only one geosynch altitude. Can someone elaborate?



2] On interplanetary travel:
"...You launch in the direction of the Earth's rotation to get to the outer planets, and against the Earth's rotation to get to Venus or Mercury..."


Yes, you always want the boost of Earth's rotation to achieve Earth escape velocity, but correct me if I'm wrong (and maybe this is what Xylene was trying to get across) - to plan trips to inner system bodies such as Mercury and Venus, you want to exit your orbit against ***the Earth's revolution about the Sun***.

There has been some good and and a lot of bad information in this thread. Two winners are

It is more complicated, but the concept is the same as shooting small game. You aim at where the rabbit (Mars) will be when the bullet (space craft) gets there, not at where the rabbit (Mars) is when you pull the trigger (launch the space craft).

Good analogy. Hitting a planet is a bit more complicated than hitting a rabbit with a single shot. Think of it in terms of hunting rabbits while riding a Tilt-A-Whirl. Only you aren't carrying a normal gun. You are carrying a big gun that carries a medium-sized gun, which in turn carries a small gun. Each gun fires in sequence; it is the small gun's shot that finally sets the bullet on the way to hit the rabbit.

1]to plan trips to inner system bodies such as Mercury and Venus, you want to exit your orbit against ***the Earth's revolution about the Sun***.

Exactly. You also want to time your exit just so (see above) so that the bullet hits the rabbit.


Now for the rest.

he is also wrong about decelerating to gain a lower orbit.to gain a lower orbit you must accelerate.to gain a higher orbit you must decelerate.

That is exactly wrong. To gain a lower orbit you must decelerate. You accelerate to attain a higher orbit. In fact, you do so twice. The first deceleration/acceleration puts you in an elliptical transfer orbit that intersects the original and target orbit. The second deceleration/acceleration occurs when you have reached the desired altitude. However, you do end up going faster (in a lower orbit) or slower (in a higher orbit) than you were going originally. It does seem a bit paradoxical, but that is how it works.

Headed inward - toward the sun - or outward, you accelerate as much as possible (fuel limitations) to get the shortest possible travel time regardless of the direction.

Up until now we have sent unmanned spacecraft that accelerate as little as possible and still get to the destination. If that meant taking ten months to get to Mars or many years to get to the outer planets, so be it. Getting to the destination quickly is too cost prohibitive to justify.

"That is exactly wrong. To gain a lower orbit you must decelerate. You accelerate to attain a higher orbit. "

Well, it's still not that simple. You *first* have to climb out of the gravity of Earth. That requires accelerating, no matter what you're doing. (You cannot *decelerate* to escape velocity of Earth orbit.)

As you're coming out, you need to decide whether you want to fall down the Sun's gravity well towards Mercury and Venus or to climb up the sun's G-well to the outer planets. If falling inward, you would put your newly-acquired velocity against the direction of orbit, which will have the effect of causing you to fall inward.

Looked at another way, it depends on the reference point - Earth or Sun.

No mater where you go in the solar System, you must accelerate with respect to Earth to break away.

But where you want to go in the SS will determine whether you decelerate or accelerate wrt the Sun.

"That is exactly wrong. To gain a lower orbit you must decelerate. You accelerate to attain a higher orbit. "

Well, it's still not that simple. You *first* have to climb out of the gravity of Earth. That requires accelerating, no matter what you're doing. (You cannot *decelerate* to escape velocity of Earth orbit.)

As you're coming out, you need to decide whether you want to fall down the Sun's gravity well towards Mercury and Venus or to climb up the sun's G-well to the outer planets. If falling inward, you would put your newly-acquired velocity against the direction of orbit, which will have the effect of causing you to fall inward.

Looked at another way, it depends on the reference point - Earth or Sun.

No mater where you go in the solar System, you must accelerate with respect to Earth to break away.

But where you want to go in the SS will determine whether you decelerate or accelerate wrt the Sun.

My response was to Leopold, who was "talking about arocket orbiting the earth", not escaping from the Earth's gravity. The Earth's orbit around the Sun is more-or-less irrelevant when discussing changing orbits about the Earth itself.