Relativity question re. Voyager spacecraft

[Sam]

Carol

I was too young to remember the landings, but I was told that the landing was manual as the ground was too rocky where the original site was planned and that they only had a few seconds left of fuel when Neil put the lander down.

Cheers

Ian

There was a documentary on a while ago that mentioned that same reason as well.

[Talitha]

IIRC, Neil had to take manual control because they'd overshot the target area by several miles because of the mascons... that's how they wound up over the boulder field.

The main concern was having enough fuel left for an abort (if it came down to that), but they were still within the limits at touchdown. I'll dig into it and get back with some links if possible, ok?

But for now, back to the gravity..

Gurney, is a crack-the-whip effect what you meant by "Anything caught in a gravitational field, or even passing through, will accelerate." ? I also seem to recall NASA using some of the planets' gravitational fields to slingshot probes or something... same concept?

edit:

Mascon links:

1) Paragraph 3: Apparently, mascons, or areas of mass concentration, were gravitationally pulling the Eagle ahead of the planned landing site.

2) Paragraph 8: The main cause of the navigation error was our poor knowledge of the way the Moon's gravity varies over its surface.

[Sam]

edit:

Mascon links:

1) Paragraph 3: Apparently, mascons, or areas of mass concentration, were gravitationally pulling the Eagle ahead of the planned landing site.

2) Paragraph 8: The main cause of the navigation error was our poor knowledge of the way the Moon's gravity varies over its surface.

Very interesting links and certainly explains how the mascons influenced the landing errors. As an aside the diagram depicting Armstrong's and Aldrin's heart rates is quite interesting to.

[gurney]

But for now, back to the gravity..

Gurney, is a crack-the-whip effect what you meant by "Anything caught in a gravitational field, or even passing through, will accelerate." ? I also seem to recall NASA using some of the planets' gravitational fields to slingshot probes or something... same concept?

Lol...I'm afraid I don't know what you mean by a 'crack-the-whip effect'. Sorry.

But er maybe

What I meant was that a gravitational field has the basic effect of causing any object within it to accelerate in some manner or other. A satellite in orbit around a planet is in fact constantly accelerating towards the centre of the gravitational body, countered by it's radial velocity or orbiting velocity, but the acceleration is still there.

As a satellite passes through the solar system, ala Voyager, it will pass through a whole maze of interwoven gravitational fields, which are far from simple. Al the time being accelerated in one direction or another. Now when we talk about acceleration in those terms, we don't mean it keep changing direction and whizzing around like a pinball machine ball, no - it carries on it's way but with very minor deviations caused by gentle tugs one way or the other, with it's velocity and vectors just slightly altered by accelerations in various directions affecting it along the way.

[Paxo]

But for now, back to the gravity..

Gurney, is a crack-the-whip effect what you meant by "Anything caught in a gravitational field, or even passing through, will accelerate." ? I also seem to recall NASA using some of the planets' gravitational fields to slingshot probes or something... same concept?

NASA did indeed use gravitational 'slingshots' to accelerate voyagers' 1 and 2 on to their destinations, without it the probes would never have reached the outer planets (and beyond). One point to bear in mind though, as there is a little law commonly known as the conservation of momentum, a little 'slingshot' does (although negligibly small before anyone complains) alter the orbit of the body used to slingshot i.e we have altered Jupiters and Saturns orbit minutely to achieve this effect.

The cases of Voyagers was a little unique though, the alignment of the planets following the 1977 launch date only happens about every 170 years so this was indeed a fairly unique opportunity to visit all of the outer planets (bar Pluto) which would not have been possible without gravity assists. I am making the assumption that this is the crack-of-the-whip Carol is talking about.

It is a technique which is used widely now as it allows smaller lighter craft to be launched, the downside, it takes a lot longer to get there. Recent craft have often been accelerated via passes by the earth for example before speeding off to the outer reaches of our solar system.

Steve..

[Sam]

One point to bear in mind though, as there is a little law commonly known as the conservation of momentum, a little 'slingshot' does (although negligibly small before anyone complains) alter the orbit of the body used to slingshot i.e we have altered Jupiters and Saturns orbit minutely to achieve this effect.

Steve..

mmm... that could be interesting after a few million or so spacecraft flung buy by us.

[gurney]

Lol - I wouldn't worry too much. Of course if anyone is worried, they could always just make sure they slingshot satellites in alternative directions around the earth to counteract the effect of the previous one.

[Ajohn]

The 4 dimension thing is a bit misleading. They way I think it's referred to is a time line. 3d space moving in time. SEEMS it can be illustrated by the lift shaft thought experiment. Say your in a lift travelling at a constant velocity, no windows or external references so your completely un aware of it. Some one outside then shoot a high velocity riffle at the lift - high velocity so that the bullets trajectory is flat. The observer in the lift sees that the bullet entry and exit points don't line up and that the bullet appear to have followed a curved path.

:? I used the word "seems" because I don't see how this explains a time line at all.

Must have been a bad book.

John

[Kaptain Klevtsov]

The bullet would appear to go in a straight line, though not horizontally, not a curved trajectory. Thats assuming the lift is at a constant velocity as you stated. Any acceleration of the lift would turn the bullet's course into a curve.

Kaptain klevtsov

[Talitha]

Lol...I'm afraid I don't know what you mean by a 'crack-the-whip effect'.

Basically I think it's the same as NASA's slingshot, except we do it while ice skating on the lakes in the winter and use momentum (I think that's the right term).

A line of skaters hold hands and form a human 'whip'. The inside skaters go slower than the outside ones and when a signal is given, everyone except the skater at the end of the whip digs their skates in and launches the end skater at a pretty good speed. It's exhillarating, but you have to watch out for holes in the ice left by the fishermen.

[MikeP]

Reminds me of an interesting puzzle ....

If the USA decided to change the rules of the road so that vehicles drive on the left rather than the right, would the length of a day increase or decrease?

Mike

[Ajohn]

I think what the book really meant was that the path of the bullet holes wouldn't make sense. Say the shot was at the lift and it was higher than the shooter. The bullet hole would show that it entered at an angle and that it exited at the same angle but lower than where it entered. The only path that would make that sensible is a sort of S shape on it's side. ie Not straight.

Can't help wondering about what the lift passenger would think about the sonic boom as the bullet went through the lift too.

It's a fairly famous thought experiment but I've never come across an adequate explanation. The web no only seems to mention people falling down lift shafts, violinists and etc. ie Doppler effects.

John

[Ajohn]

I think the sling shot effect can be explained like this. A planet has gravity. That sets the escape velocity. If a craft is approaching a planet at the right speed and trajectory the planets gravity will speed it up and or deflect it's path. That can be arranged to put it into an orbit that will take it to the next planet it's going to visit and so on. These things don't go in straight lines. I came across some time to mars numbers based on that approach as against going straight there. Lot more energy versus time and more acceleration - deceleration problems too. The usual approach as I understand it its to place the craft on the same orbit as the planet and allow the craft to slowly catch up with the planet or the planet to catch up with the craft.

John

[MikeP]

Transfer orbits entry in Wikipedia - http://en.wikipedia.org/wiki/Hohmann_transfer_orbit

Mike

[lunator]

The way I have understood it is as follows.

In simple terms there are 2 types of energy Potential & Kinetic.

e.g. when I hold an Apple up it has a certain potential energy and when I let go some of that potential energy become Kinetc.

As voyager approached Jupiter it swapped it's potential energy for Jupiters Kinetic energy so it sped up.

Cheers

Ian

[Ajohn]

Sticking my tongue out :mrgreen: try

http://en.wikipedia.org/wiki/Gravitational_slingshot

Can't help laughing. Excuse me

John

[Rosanella]

The way I have understood it is as follows.

In simple terms there are 2 types of energy Potential & Kinetic.

e.g. when I hold an Apple up it has a certain potential energy and when I let go some of that potential energy become Kinetc.

As voyager approached Jupiter it swapped it's potential energy for Jupiters Kinetic energy so it sped up.

Cheers

Ian

Potential Energy is stored energy that can be converted into Kinetic Energy. Whereas Kinetic Energy is the energy an object has due to its motion (the work needed to accelerate a body of a given mass from rest to its current velocity).

Kinetic Energy is gained during acceleration (from rest to motion--i.e. Potential E. --> Kinetic E.) and it remains so unless there's a change in speed.