FAQ: "From Stargazers to Starships"

Questions and Answers

by users of "From Stargazers to Starships"


Items covered:

  1. About asteroids hitting Earth.
  2. The swirling of water in a draining tub.
  3. Dispensing water at zero-g.
  4. Robert Goddard and World War II.
  5. Asymmetry of the Moon's orbit.
  6. Measuring distance from the Sun.
  7. Who owns the Moon?
  8. Acceleration of a rocket.

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  1. About asteroids hitting Earth

    A friend asked if I could find anything on an asteroid heading for earth & a laser that supposedly is in space that will eliminate the asteroid before it hits earth. Is there any such thing, or is he reading too many sci-fi books?

    Reply

    About asteroids heading for Earth: the best account I know is a section ;The Shoemaker Comets in the book First Light by Richard Preston. As for lasers capable of destroying one, they are (at least right now) pure sci-fi.

    The above book makes an interesting point: it would be very hard to spot an asteroid heading for Earth. Astreroids are usually detected in photographs of the sky (via a telescope) by the fact they move across the line of sight, leaving a streak rather than a spot. If they move across the line of sight, they are not going to hit Earth; if they are heading straight for Earth, they leave no streak and attract no notice.

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  2. The swirling of water in a draining tub

    I am a grandmother, soon to be 55 years old, who often gets into heated arguments in her maillists! My question, to avoid the argument by having facts, is: The direction of draining water in the tub, sink and toilet is said to be the opposite in Australia. Someone said the Coriolus Effect governs this, and it is a myth. Someone said the CE has nothing to do with it, and it is a myth. Then again someone said it is NOT a myth. I am on the fence with this one, as I cannot argue something I do not have one idea about!! Can you help me? Thanks in advance!

    Reply

    Your first respondent was right: the Coriolis effect governs it, and it is a myth. The Coriolis effect can govern the swirling of fluid flows, and where it does, the swirling is opposite in opposite hemispheres. However, it is only appreciable on a very large scale. Hurricanes obey it: tornadoes, which are much smallers, do not, and neither do kitchen sinks, which are much smaller still.

    For details and explanations, look up on the world-wide web here.

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  3. Dispensing water at zero-g.

    I am a student currently studying for a degree in engineering. As part of this degree we have been given the task of designing a water heater and dispenser for use in zero gravity. It has been suggested to use a bladder in a pressurized container heated using microwave or Radio Frequency technology. The heater must heat approximately 100 ml to 80 degrees celsius, and the entire system can use no more than 12 volts. How would you suggest a layout for this type of system might look? Any information that you could provide would be most appreciated.

    Reply

    Our research group is concerned with plasmas and magnetic fields in the rarefied medium between here and the Sun. We have no expertise at all in zero-g and space-station hardware.

    This does not stop me from speculating about your request, of course. The key word is "dispensing": what do you mean by that? You cannot just have a tap and let out the hot water--it will form globs that drift away in zero-g and ultimately contaminate your circuitry or mess up your living quarters.

    So you need three elements--a container where the water is heated, a tank from which the water is obtained and a container for the hot tea or whatever you want to make with the water.

    The first and third may well be plastic bladders, whose volume can adjust. The reservoir would have an outer bladder with water only and and inner bladder filled with air, and as the astronaut with a squeeze-bulb pumps air into the inner bladder, water is squeezed out.

    The heating vessel--you could use RF heating, but I suspect it will be somewhat heavy, will need stepping up the voltage from 12 volt and also will have to be shielded from radiating. In an environment where every superfluous gram costs a great deal, wouldn't a simple resistive heating element--with a thermostat, of course--be simpler? It could be in a cylindrical container with a spring loaded piston which is initially at the bottom. When refilling it from the reservoir the water enters from below and pushes the piston up, against a ratchet, and when the astronaut wants a drink, he or she releases the ratchet and the spring loaded piston pushes the water out, into the third container. The trick is to never mix any air with the water--once they are together, they are hard to separate.

    This opinion comes to you with no warranty by NASA or anyone. Have fun! Now, let me go back to serious work...

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  4. Robert Goddard and World War II.

    I just came upon your website Stargazers to Starships. Great site! I am researching a school project on Goddard and found the information here useful. I have a few questions, perhaps you can help me.

    1. When the U.S. was spending money on the a-bomb during WWII, do you think this prevented the government from providing Goddard with money to research rockets for the military?
    2. The German's developed the V-2 rocket, and Goddard believed they copied his design. I read that Germany sent spies to observe Goddard. Did Goddard know he was being watched?

    Reply

    To answer your questions:
    1. During WW-II, the government also spent money on rockets. The A-bomb project did not cut into this. Goddard was part of that effort, but the biggest rocketry effort was probably at Caltech, with Theodore Von Karman and Frank Malina. See the section in "Stargazers" on the evolution of the rocket.

      Part of the problem was that Goddard preferred to work alone, while the Caltech people brought in bright students and had much better engineering support.

    2. I never heard about the Germans spying on Goddard, and it seems very unlikely. They too had much better engineering support and took Goddard's ideas--DeLaval nozzle, liquid fuel, using the fuel to cool the engine, steering vanes in the exhaust etc.--and developed them beyond what Goddard himself was able to do.

      A similar thing happened in WW-I. The Wright brothers invented the airplane in 1903, but the Europeans took their work and expanded it greatly, so that the German, British French and even Russian airplanes in that war were far superior to the ones America produced. After America entered the war, its pilots all flew British and French machines.

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  5. Asymmetry of the Moon's orbit.

    Subj: Moon's perigee/apogee

    I have just read your article in "Stargazers", but what I am trying to dertermine is the observed variations in the time between events of perigee and apogee. For example it may be 14 days from perigee to apogee and say 12 days from apogee to perigee. Then at a further time the periods can be reversed. I am seeking an explanation of this dynamic variation.

    Reply

    I looked up the ephemeris tables of the Moon, and you are right: counting only the times between minimum and maximum distance from Earth, those distances ARE variable, more than one would expect for, say, an Earth satellite in a long elliptical orbit.

    All I can give you now is a guess. The motion of the Moon is really a 3-body process, influenced by the Sun as well, with further perturbations perhaps due to Jupiter etc. The orbit is close to a circle, which means that a pull of a few 1000 km this way or that can shift the time of largest and smallest distance by a great amount, in contrast to what it would do to a high-eccentricity orbit.

    The literature comments;The orbit of the Moon is complicated and I think your question illustrates that complexity. If you look at page D-46 of the US Astronomical Almanac, for instance, you will see that even the low precision formulae" for lunar motion are alarmingly long, and better approximations (found for instance in Astronomical Algorithms by Jean Meeus) are even longer.

    So the bottom line (as they say) is that Kepler's laws still hold, but actual motions may be complicated by additional factors.

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  6. Measuring distance from the Sun.

    I hope this isn't too dumb a question, but when a planet's distance from the sun is given, should that be assumed to be from the center of the sun to the center of the planet, or is it a measure of the surface of the sun to the surface of the planet?

    -- and if it's surface to surface, then what is considered the surface of a gas giant?

    Reply

    Your question isn't dumb, and it has a simple answer: from the center of the Sun. A spherical mass--Sun, Earth, red giant or whatever-- pulls objects outside it with the same force as it would, if all its mass were concentrated in its middle. As far as gravity is concerned, the position of the surface makes no difference.

    By the way--the Earth does not orbit the center of the Sun. If the solar system contained only it and the Sun, the two would orbit their common center of gravity. Of course, the Sun being much more massive, that point is very close to the center of the Sun.

    With more planets, the system orbits around the common center of gravity, which I suspect is close to the center of gravity of its heavyweights--Sun, Jupiter and Saturn. Viewed from some other solar system, far away, this would make the Sun's position wobble a bit, in response to the motions of the planets. In recent years, astronomers have observed such subtle wobbles in the motions of quite a few nearby stars, and concluded that like the Sun, they had planets, too--big planets, like Jupiter. It is still too hard to detect the effects of lightweights such as Earth, but progress is being made.

    Keep up your interest!

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  7. Who owns the Moon?

    Dear Gentlemen,

    If you be so kind as to reply, please tell me, is it true that the Moon has a formal proprietor and who is this man?

    Thank you in advance for your kindness.

    Reply

    I do not know who told you differently, but the moon belongs to all of us together, even you, even I. When Neil Armstrong stepped onto the moon he said "We came in peace in the name of all of mankind" and that still holds true.

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  8. Acceleration of a Rocket

    I've looked at your site and have taken some information but I need more for my project. In my project I want to search on the G force on the rockets at launch.

    Reply

    I really do not know. The g-forces on a rocket vary with the design. Manned rockets stay under about 5g, unmanned scientific satellites may be launched at up to 10-12g, small sounding rockets with strongly built instruments sometimes reach 30g, and missiles can also accelerate very rapidly. The greatest acceleration is usually not at launch but just before burn-out, because the thrust of the motor changes little (or not at all), while the mass goes down as fuel is burned.





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Author: David P. Stern