Get a Straight Answer

I live in Guatemala, Central America, and I was wondering something.

    Do you know at which distance (miles)) and object in space gets attracted or caught by the Earth gravity, and brought down to Earth.

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    Supposedly, a very similar question occurred to Isaac Newton in 1666, possibly when he saw an apple fall from a tree. The apple was attracted to the Earth, which made it fall. How far from Earth did this pull extend? In particular, the Moon must also be attracted to the Earth--otherwise it would wander off into space. Did this same force extend all the way to the moon, and keep it in its orbit?

    Newton's answer was "yes." Assuming that the force decreased with distance r like r-squared, he calculated the orbital period the Moon should have, and came out with the correct number. You will find all this, including the calculation, on

Hello

    How far away from the Earth is the moon?

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    About 60 Earth radii, fluctuating a bit because the orbit is an ellipse. An earth radius is 6371 km, and you can do the multiplication yourself.

    A much more interesting question is "How do we know that"? A clever Greek astronomer, Aristarchus, figured that out more than 2000 years ago--without telescopes, using just a few simple observations and logic. See

    Help me please ! I am trying to help my young sister in-law with some research for a project and I cannot find the information I need on stars. I need to know their locations in the sky, their sizes, what they are made up of and what impact they have on the Earth. I need this information as soon as possible as she has an assignment to hand in Friday the 6th of September. If you can help me I know we would both be so grateful...

   Thank you

[Received from Australia, 5 September]

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(1)     Stars are seen all over the sky. Those are the closer ones. Our Sun belongs to a disk-shaped wheel of stars, the galaxy, and most of that wheel is so far that to the eye, its stars blend into a fuzzy glow. That is the Milky Way.

    Other galaxies can be seen, usually using time exposures on telescopes. The Hubble telescope photographed two small regions with 10 day exposures ("deep field") and saw a great number of faint distant galaxies.

(2)     Sizes--mostly around the size of the star, say from 20 times as bright to 1/20th. Of course, there may exist faint "brown dwarves" too dim to see. Very bright stars burn away rapidly, so not many are around.

(3)     What they are made of--mostly hydrogen (basic fuel) and helium (what is left from burning hydrogen. Heavier elements are rare--on the Sun, but not on Earth, of course. The heaviest are produced in an instant, when supernovas collapse.

(4)     What impact? We exist because a star--the Sun--provides us energy, allows plants to live and thereby makes all other life possible. Our distance from the Sun is critical, it's just right for water to be liquid. A bit closer and the oceans would boil away, a bit further and they would freeze solid.

Hello,

    I was reading your explanation of the seasons of the year. Are you aware of any "hands on solar system tool" that would assist me to explain your information to my daughter?

    As a student I recall a tool where the students could move the earth around the sun. That tool really helped me to understand day, night and the seasons.

    Thank you for any help.             :)

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Hello, Kelly Jo

    I am not aware of any such tool, although one could imagine it. The traditional way in class is have one student hold a big ball (or a small lamp) representing the Sun, and another holding a small ball (Earth) walk around the "Sun" A globe is also useful, but not many people have globes.

    I read your webpage about the 'Far-out Pathways to Space', where you state that a cannon could not be used to send humans into space.

    I have recently conducted an experiment that I think you will find interesting. I accelerated some shrimps and crabs up to 83 G with success. They were not harmed at all. I am quite certain that humans would also survive this if we could find a way for them to survive with water in the lungs for an extended period of time. If it turns out that humans could survive at 100G, then a 2km gun would enable a 2km/s start velocity. If the projectile also had a rocket engine....

    What I'm trying to ask here is, would an initial velocity of 2-3km/s be sufficient for this kind of launch to be economically feasible? Weighing the problems with the launch, the fuel saved, cost of launch system, etc., would it still be a 'Far-out Pathway to Space'?

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    I suspect the method remains "far out." It is true that water-creatures can stand more acceleration (though you did not accelerate them for 2 full seconds!), but filling human lungs with water is a bit hard. There do exist experiments of filling lungs of small animals with oxygenated fluids, though it is very hard for humans to push it in and out. Also, the water would weigh a ton, and I wonder how much damage it could inflict in 2 seconds. Humans have stood up to 25 g in a compressed water suit, I believe.

    The rocket would also have to be rather massive, to stand an acceleration which briefly increases its weight 100-fold. It won't be a small rocket, either: from 2 km/sec to orbital velocity there is still a long way.

    A better way to reach an initial 2 km/sec velocity would be with a hydrogen ramjet. It is attractive from the point of view of fuel economy, because the greatest part (8/9) of the mass of the fuel would be drawn from the atmosphere, and ramjets do work up to about Mach 6. They do have to be first pushed past the speed of sound--perhaps by a very big airplane, burning some of the hydrogen as fuel. It seems more practical than a 2-km cannon, with an internal diameter matching that of the rocket.