"Get a Straight Answer" Site Map

Get a Straight Answer

Please note!

This is just a selection--

either

or else

To save space, this list starts from http://www.phy6.org/stargaze/StarFAQ17.htm
A full list is found at http://www.phy6.org/StarFAQsA.htm
and links arranged by subject are at http://www.phy6.org/stargaze/StarFSubj.htm.

Items covered:

Angle between ecliptic and Milky Way

(Continuation)--About the year 2012

396A Posssibility of Asteroid Hitting Earth (1)
396B Posssibility of Asteroid Hitting Earth (2)

305. About mountains

Dear Dr. Stern,

I would be very grateful if you answer my following questions in details?

Do mountains stabilize Earth?
How will be the Earth without mountains?

306b. Another question about the year 2012

Reply

I have written about it

307. Advice to graduating High School Student

I have just passed out my 11th exams (from the Gujarat state board in India) with maths, physics and chemistry as my main subjects, English as language and computer science as a side subject and I am thinking to take up aeronautics as my main subject (to be an aerospace engineer or designer) so which are good universities and which are good courses which you would suggest me to choose. I am a bit weak in chemistry.

Please give me some suggestions to improve it.

Your site have been really very helpful for my school studies and also in outer school examinations

Reply

I do not know enough about conditions in India to advise you. For instance: some very good universities exist in India, but they charge high tuition, have difficult entrance examinations, and much competition exists among those who wish to attend. Where you belong in this, I do not know.

One thing, perhaps. What will you be doing in the coming summer vacation? If you are interested in aviation, look for a technical job associated with it--perhaps at a local airport, at an airplane refitting or repair facility. If you can afford it, be prepared to work for minimal pay, or even no pay at all. That way you will be in contact with people in the profession. Choose carefully, and if you are lucky, you might find a good mentor.

One last note: whenever you write ANYTHING, make sure to re-read and edit your words more than once, to catch any mistakes in spelling (your message had a few) or in grammar. The way you write gives your correspondent the first impression about what sort of person you might be.

A similar letter, received earlier:

I'm a 23 year-old girl. Work in astronomy and being an astronaut are an old dream dating back to my childhood. Since I was a little girl I wanted to be in space and when I became older dreamt of La Silla, and being an astronaut like Story Musgrave as my hero fascinated me. I was very young and I didn't know which major should I study in high school. I thought that aerospace engineering was the best choice for me and I intended to study this major. So I studied mathematics and physics in high school, although at that time we didn't have aerospace engineering for women in our country, but I was very serious in my choice. After finishing high school I participated in university entrance examination to fulfil my dreams but unfortunately it was very hard and I couldn't pass the exam for my selected major and I had to study industrial management, despite my other interest. Finally a year ago I finished my studying but it doesn't satisfy me. I studied a lot but I couldn't achieve my dreams because in my country there is a university entrance exam for entering the university which is very hard and unfair. I cannot forgive the government because of this carelessness and injustice.

Anyway, now I have the chance to follow my dreams. I want to study aerospace engineering or engineering physics but I have a concern and it is about my health. I afraid of passing the time and money and then reaping nothing. Now I have bachelor degree in industrial management and I work in a good company and I can follow my education in my major in Europe (I have admission from one of the best universities from Sweden) . I'm worried about this problem. I afraid to quit everything and follow my dreams but it doesn't have any result for me.

Now I have a question from you and I need your help and guidance. I'd like to know about the medical and physical examination for selecting the astronauts. I afraid that I lose everything because of these kinds of examinations. Could you tell me about this examinations? I know medical tests include audio and visual examinations, dental examination, electrocardiography, pulmonary function test, and analyses of feces, blood and urine but I do not know anything about the physical examination. Also I'd like to know whether or not the person has curvature in his/ her spine can be an astronaut or not. Please guide and help me. I'd appreciate if you kindly answer my questions and raise my ambiguities.

Please accept my thanks in advance and I'm looking forward to receiving your early reply.

Reply

You may not like to read this--but in fact, you are asking a lot. In this world, many dreams end up being just dreams. As far as I recall, astronauts are selected from a very small group--either outstanding airplane pilots (usually military airplanes)--or people with outstanding professional records, usually in science. Many people have a dream like yours, but few are chosen. Also, almost all astronauts are US citizens, and those who are not, are chosen by their governments, in countries collaborating with NASA.

I should add that nowadays just a few space missions with astronauts are conducted each year, far fewer than what was once predicted.

I don't know what effect curvature of the spine has. May depend on how bad it is, how much it interferes with your normal life.

Aerospace engineering or engineering physics are more realistic goals, but take a lot of study and please remember, even there few jobs are exciting. I know a young man who dreamt of becoming a pilot (even though he studied computers), he studied flying, got his license and then found it was hard to get a good job. For a while he flew tourists to see the Grand Canyon and other small-plane missions (once even transporting convicts from one prison to another), but his company went bankrupt and now he is in Alaska, transporting passengers and cargo in small airplanes to distant native villages, or tourists who want to see some scenic areas. It is hard and uncertain work, and all but stops in the coldest part of the year.

In truth, the number of interesting jobs in aviation or space is small--much fewer than jobs for doctors, trained nurses, computer experts or school teachers. The same is true in astronomy--see "Advice to a would-be astronomer" on

http://www.phy6.org/stargaze/StarFAQ6.htm#q100

I would not discourage you from asking around a university (and joining a flying club, maybe?), but you will need persistence, ability to work hard and luck. I hope you have them all.

308. Could a (heat resistant!) ship float on the Sun?

Hello. I am Chris. I just stumbled upon your website when I was inquiring around the internet about the rotation of the earth. I found your website to be very interesting and informative. Thus, I would like to know your thoughts on this question.

This question may seem like an irrelevant and impossible excursion into science fiction, yet it is a question that has tickled my curiosity for quite some time. If we were to build a spaceship that could successfully reach the sun's surface, how would the ship behave on the sun's surface? Or, more simply, what would the consistency of the sun's surface be like? Would the ship fall right through the surface, or would it float on it like a rubber duck floating on water? Could it be something else? Although I heard that the sun is gaseous, wouldn't the immense gravity of the sun cause sufficient compaction of the gases to give it a more fluid consistency? I don't think anybody has a definite answer on the subject, yet I would like to know your thoughts on the matter. Thank you for your time.

Reply

Dear Chris

Matter comes in 3 varieties--solid, liquid and gas (viewing here plasma as a special gas). Solid and liquid form well-defined surface boundaries, e.g. the ocean surface. Gas just dwindles away exponentially, e.g. in the Earth's atmosphere (discounting temperature variation) air density drops by half with every 5 km of altitude, approximately. So at 10 km it is down to 1/4 the sea-level density, 15 km to 1/8 of the density... I think you get the point (the process ends around 100 km, where molecular collisions become rare).

For mathematical reasons, physicists use the "scale height" H, the distance where pressure and density are reduced, not by a factor 2, but by a factor e=2.7128... , But the physics is the same. Light objects can definitely float in an atmosphere--consider helium balloons--but the altitude can vary, they do not rise to any outer surface, but find an altitude which matches their density.

Same with a balloon near the Sun--if it were not instantly evaporated by the heat. Gravity is about 30 times larger, but molecules are lighter (hydrogen) and a lot hotter, so H is larger. In

http://www.phy6.org/stargaze/StarFAQ4.htm#q71

http://web.njit.edu/~dgary/321/Lecture7.html

309. Reducing the fuel weight of the Space Shuttle?

I have a question you probably could answer really easily. I know the escape velocity for the Shuttle is around 18,000 miles per hour. But the Shuttle is very heavy and the fuel tanks filled with propellant are very heavy too.

So if you had a craft that had some sort of electronic propulsion that did not require those tanks filled with heavy fuel, I gather the escape velocity might still be the same?

So if you had perhaps a craft with 2 or 3 different systems that were like 2 or 3 stage systems, the outlay of raw energy would be less but the required speed would still be the same. And basically, such a craft would be quite large enough–with using all the lightweight materials–to carry 6-20 astronauts (or a large family wanting to leave Earth for greener pastures).

Reply

Your scheme unfortunately will not work, Newton's laws of motion conspire against it. To understand how that happens, you will need read about them in "From Stargazers to Starships,"--sections 16, 17, various parts of 18, as well as 25.

The shuttle starts off with something like 2500 tons of fuel, NOT just because that fuel supplies it with the ENERGY needed for reaching escape velocity, but also, because it supplies it with the FORCE needed to accelerate it to that speed. By Newton's 3rd law, you cannot exert force without pushing against something. Forces come in pairs between two objects, if A pushes B, then B pushes A with equal and opposite force.

With the shuttle, "B" is the fast jet of burning gas: by pushing that jet backwards, the shuttle (or any rocket) is pushing itself forwards at the same time. It is like the recoil of a gun, which is another example of these laws in action.

Electric propulsion (sect. 33) is not very suitable here, it still would require something massive to be thrown backwards, and electric energy can't be stored as easily as chemical energy. As for "greener pastures"--no such thing exists, not for life like ours, depending on liquid water: Venus is too close to the Sun and hot enough to boil water, while Mars is to far from it, and water there would be frozen much of the time. Earth alone is in the "Goldilocks range" of distances: be grateful for its green pastures, forests and fields!

310. How do Rockets Land?

I am not a young student, but a medical doctor. I have been teaching some stuff about space to my young daughter. However, I am unable to tell her how a rocket lands, reason being I myself don't exactly know.

Could you please enlighten me? How do rockets land? Do they land at all or do they all burn up? Why do we see pictures of astronauts jumping from parachutes?

Could you advise a good website which can satisfy my doubts?

Reply

A major problem of an airliner returning to Earth is how to get rid of its energy of motion--kinetic energy--while still getting enough "lift" from the motion of air across its wings. If your daughter ever sat by the window of a landing airliner, she would see all sorts of auxiliary extensions of the wing sliding into place at the back of the wing--increasing air resistance and slowing down the airplane, and at the same time creating extra "lift" to hold the airplane up. The lift is much less efficiently produced than in ordinary flight, but that is all right, this unusually slow flight lasts only a short time and then the airplane is rolling on the ground.

A satellite in low Earth orbit must move at 24 times the speed of sound (or faster), which means its kinetic energy, the energy of motion, is at least 24 x 24 = 576 times the energy of something moving at the speed of sound, which is already more than the speed of an airliner. Weight for weight, a satellite has about 20-50 times the energy of a rifle bullet, enough to melt it, even boil it away.

Getting rid of that energy safely is the main challenge in landing. The astronauts returning from the Moon (at even greater speed) could not save their spacecraft but had to abandon it, and return in a "capsule" designed to stand a lot of heating, and meanwhile creating a powerful shock wave ahead of it, containing very hot air whose glow dissipated the energy. Then in the thicker atmosphere, at low speed, they used a parachute, and splashed down into the ocean.

Like the Apollo Moon ship, many research rockets are abandoned when their job is done--it would be too hard to bring them back intact.

The space shuttle enters the upper atmosphere (which is very rarefied) sideways, with its bottom forward: the bottom has heat-resistant ceramic tiles, and creates a great shock wave (in the "Columbia" some tiles broke away and the heat destroyed the shuttle). By the time it reaches the denser atmosphere, it has slowed down to about half the speed of sound, and it can land like an airplane--still, much faster than a jetliner, requiring accurate computer control.

Burt Rutan's "Spaceship One" similarly used wings, first as brakes (turning them to create air resistance), then to land as an ordinary airplane. However, since it only reached about 3.5 times the speed of sound, this was not as great a challenge.

311. The Earth's Spin reduced by Global Warming

I have read an article on internet which says that due to continue warming of earth, the length of day is becoming greater. Although the change is very small but with the passage of time it may perhaps become appreciable. Reference: http://space.newscientist.com

My question is: does there exist a chances that a time will come when due to change, the rotation of earth will stop?

Reply

I could not find the item on the site you have mentioned, and the only thing I can guess about it is the following.

As you know, the water level in the oceans is rising. The main reason (at least now) is not melting of icecaps, but warming of the water, which like almost any substance expands when heated. Therefore, since most of the Earth surface is ocean, if the water rises by, say, half a meter, the effective radius of Earth might increase by something like 35 cm.

In a rotating object, expanding away from the rotation axis slows the rotation (think about a rapidly spinning ice skater, extending her/his arms out and almost stopping). Expansion of the Earth will therefore slow down the rotation. But only by a tiny amount, since 35 cm is maybe 1 part in 20 million of the radius of the Earth. It will never become appreciable.

312. Circumnavigation of the Sun

A question came up in my family and interests me. I am indeed a new stargazer who is very interested in the workings of our solar system, galaxy, and the universe itself. The question I would like answered is...

"Why can't humans circumnavigate the sun?"

Reply

Not only have humans done so, but you yourself have done so too. Every year the Earth carries you on such a trip!

There even exists a bumper sticker, something like "Living on Earth is expensive, but you get a free trip around the sun."

Response

My question had nothing to with being on planet earth. I meant it as traveling in a space ship. Like the ones they send out now.

Answer to the response

There exists no good reason for such a trip. It is also a difficult thing to do: no one has yet supported a human in space for as long a year, the orbit must be chosen carefully to have a period of exactly one year (or else Earth won't be there on the return!), and there is not much a human can do in such an orbit that an instrument cannot do simpler and cheaper--observe not just light but ultra-violet, x-rays, energetic particles, magnetic field etc.

Such instruments have in fact orbited the sun aboard "Ulysses". The aim was not just orbit the sun, but do so above the poles of the sun, a region which cannot be observed from Earth. That is a difficult orbit to achieve: the spacecraft first approached Jupiter and used its gravity to rotate its orbital plane by nearly 90 degrees. It was primarily a European space mission, launched in 1990, it has accomplished interesting discoveries, and is still working. No human astronaut could so as much. More on Ulysses home pages, e.g. http://ulysses.esa.int/science-e/.

313. Are nuclear forces merely gravity at very close distance?

Is it possible that when we are speaking of strong nuclear forces, we are actually speaking of a tiny gravity space for the nuclei?

Are there any pages on this? I don't even know what it might be called.

I am just a layperson, who cannot master the math in my quantum gravity book, but I think --what I can read is rather accurate.

It just "looks" like we may replace the phrase strong nuclear force with gravitational field (with quantum restriction (?)) as holding the particles in place. Would this also apply to the sub particles, --and might it be the sub particles which allow the escape of the alpha particle in the first place?

Reply

Nuclear physics is not my field, so you might check what follows below with someone more familiar, and with a fresher education in physics-- I'm past 75!

The drawing in

section Q8.htm

electric repulsion

overcome their gravitational attraction

314. Changing the Earth's Rotation

I have a serious question

What is the energy required to either slow down of speed up the Earth's rotation by say, 1 msec? I have tried out the calculations but I think I am missing a parameter or two.

Reply

The energy E of the rotation of Earth is (I believe) (I ω²)/2 (check the "Hyperphysics" web site) where I is the moment of inertia of Earth, and ω is the angular velocity (a Greek omega). The value of I can be found on
http://scienceworld.wolfram.com/physics/MomentofInertiaEarth.html
and is around 8 10³⁷ kg/m²

The earth rotates in about 86000 seconds, so approximately
ω = 2π /86000 = 7.3 10^–5 radian/sec
ω² = 5.3 10^–9
and the energy E is around
E = 2 10²⁹ joule

Changing the period by 1 msec out of about 10⁵ sec means a relative change around
dω/ω = 10^–8
The energy change, found by differentiating the energy equation, is approximately dE = E 2dω/ω = 2 10^–8 E = 4 10²¹ joule.

A million power stations of 100 megawatt each would produce about as much in a year. Of course, it is not enough to have the energy--the Earth must also grab hold of something outside it, to transfer the angular momentum. The Moon and Sun do slow down the rotation, by raising tides on the Earth. But they do so at their own slow pace, and discussing this process would take us too far off.

315. Why are planetary orbits eccentric?

I was reading yours question/answer section wherein you clarified several doubts beautifully and interestingly.

Why would the orbit of any planet be elliptical? Any planet that orbits any star or other planet is being pulled by the gravity of the central object (star or planet). Shouldn't the orbit always be circle, as the force pulling the planet inside would want it to be?

Assume the planet encountered the star while moving somewhere in the space, and started orbiting it. Then due to its own initial motion, it has at first an elongated elliptical orbit. But that orbit would slowly become more circular as the speed of planet on the edge of ellipse would be reduced. If we go by the pendulum theory, as the planet is moving under the impact of gravity of the star, the distance it moves far from the star (the longer axis of the orbit) would eventually shorten and become equal to its shorter axis, resulting in a circular orbit.

In case of a planet orbiting another planet (like our moon), we may have to consider the motion of the center planet (the one being orbited) and that may reduce the speed with which the elliptical orbit becomes circle, but at the end the orbit should be circle.

I can't think of any other theory to convince myself with the truth that earth is still in an elliptical orbit, while orbiting an relatively stationary star.

If so, at some point of time, the Earth's orbit will become circle (considering for now that my ideas hold true). Then it's also interesting to think about what happens when there are not longer or shorter days on earth. All days measure exactly equal and there remains only one season through out the year.

Waiting to know the things I'm not aware of (not very good with Physics - didn't get a chance to study formally).

Reply

Copernicus thought orbits should be circles, because the circle had a perfect shape (or else, because he knew how to calculate with circles). But your question really relates to the way the solar system came to be. I don't have answers, but knowing some physics and astronomy (and having read a bit about the problem), I can guess a few things.

It is generally agreed that the solar system began as a cloud of dust and gas, pulled together by gravity (astronomers have observed such clouds). Planets were not encountered by chance, after they had already formed. At first the material of that cloud just moved (on the average) towards the center. A dust particle or gas molecule pulled towards such a center does have an orbit, but if it starts far away, this will be a very elongted oval. The width of that oval will depend on the distribution of the "sideways velocity" of dust particles (related to "angular momentum").

However, as particles and gas come closer to the center, they may collide and convert some of their energy to heat. Such energy loss slows them down, allowing gravity to clump them together. The clumps radiate heat into space and become very cold, so gas molecules and atoms hitting them are likely to freeze and stick (as may have happened in comets, whose motion remains very elliptical). Furthermore, if the cloud is not completely symmetric--extending further on one side than the other--more will circle in one direction than in the other (say more counterclockwise than clockwise, as seen from some distant star), causing the average of the cloud to rotate.

The loss of energy to collisions will prevent dust particles from returning to their starting region. It will also bring everything down to a rotating disk, explaining why all major planets are very nearly in the

plane of the ecliptic

in the end

circular orbits

rings of Saturn

planetesimals

protoplanets

(The "snowballs" in those rings cannot stick together, they are too close to the planet and its gravity prevents them from sticking together to form big moons--although some small moons do exist there. Look up "Roche Limit" in a search engine.)

imprints on the Moon

before all their orbital eccentricity

has completely disappeared

The process which you propose

length of days

Seasons

not

inclination of the Earth's axis

plane of the Earth's motion