Curiosity on Mars

[Pabmusic]

...According to the Guardian the time it takes radio signals to arrive from Mars is 14 minutes.

My maths! But the Guardian's using an average, which is a bit misleading. The distance between Earth and Mars changes constantly, varying from about 250m miles to 35 million miles, though for the larger part it varies either side of 140m miles. It would take light (or radio waves) something like 12 minutes to travel 140m miles in a vacuum (which space nearly is) - so the Guardian is right-ish. However, when Earth and Mars are closest together, light will take only 3 minutes or so to travel between them. But when they're farthest apart, it's about 21½ minutes. (Light takes about 8 minutes on average to reach us from the Sun, which is 93m miles away, or ⅔ of 140m miles - an average is generally OK since Earth's orbit is almost circular; Mars's isn't.) It only emphasises the complicated maths needed for this successful landing!

Newtonian physics would probably still have been OK at these (relatively close) distances. Einsteinian physics is nevertheless the more accurate discipline as distances increase much beyond this. Remember that general relativity first began to be accepted when it was able to predict the deflection of light at the 1919 Transit of Venus more accurately that Newtonian physics could - and the Earth-Venus distances are not too dissimilar to the Earth-Mars ones. Einstein predicted that light emerging from behind Venus would be bent by 0.0005 degrees (an angle equivalent to a candle, 1km away, being moved to the side by 1cm) - more that twice what Newtonian physics predicted. Arthur Eddington did the experiment and Einstein was proved right.

This illustrates the Earth-Mars issue more eloquently than any words:

[kernelbogey]

Thank you very much for this Pabmusic. I had a general idea of the complexity which the two planetary orbits would add to the Curiosity project but you have made this much clearer.

I often think about the distance of the sun from us and of how I can feel its heat even in winter (given clear conditions, of course) and how astonishing that is given its distance. I also frequently think about the miracle of life bestowed on this planet by our particular distance from the sun and the seasonal variations brought about by the tilt of earth's axis.

I was chatting with a friend the other day about this and asking if heat travels at the same speed as light or more slowly. He insisted that what I experience as the heat of the sun is that of its light, and that heat doesn't travel. I found this hard to accept and wonder if you can elucidate (appropriate verb!)?

[amateur51]

Thank you very much for this Pabmusic. I had a general idea of the complexity which the two planetary orbits would add to the Curiosity project but you have made this much clearer.

I often think about the distance of the sun from us and of how I can feel its heat even in winter (given clear conditions, of course) and how astonishing that is given its distance. I also frequently think about the miracle of life bestowed on this planet by our particular distance from the sun and the seasonal variations brought about by the tilt of earth's axis.

I was chatting with a friend the other day about this and asking if heat travels at the same speed as light or more slowly. He insisted that what I experience as the heat of the sun is that of its light, and that heat doesn't travel. I found this hard to accept and wonder if you can elucidate (appropriate verb!)?

Great posts, kernel & Pabs.

On a morning when I've heard the sad news about Robert Hughes' death, it's good to know that sentient humans are still grinding away at [Earthings'] existence's posers

[Pabmusic]

...I was chatting with a friend the other day about this and asking if heat travels at the same speed as light or more slowly. He insisted that what I experience as the heat of the sun is that of its light, and that heat doesn't travel. I found this hard to accept and wonder if you can elucidate (appropriate verb!)?

I'll have a go. What reaches us from the sun is electromagnetic radiation from a gigantic nuclear furnace, in which hydrogen is being transformed very slowly into helium, and helium into all the heavier elements. All elements heavier than lithium were created in stars like our sun – especially (for us) carbon – and distributed throughout the universe by exploding stars (supernovas). We are all made of stardust!

The electromagnetic spectrum is much wider than we can see, and in consequence we have got used to calling only a narrow section of it ‘light’ – the bit we can see, stretching from the short ‘violet’ end to the long ‘red’ end. But beyond these are shorter ‘ultra-violet’ rays that cause cancers, and longer ‘infra-red’ rays that burn us. Beyond these still are very short X-rays and even shorter ‘gamma’ rays, while at the other end there are longer ‘microwaves’ and very long ‘radio’ waves. It’s like this:

Radio waves (including Radio 3)
Microwaves
Infra-red rays
‘Richard of York gave battle in vain’, or ‘Rinse out your greasy bottles in Vim’
Ultra-violet rays
X-rays
Gamma rays

[Incidentally, for insects, visible light is shifted nearer the short end – they can’t detect red at all, but they can see ultra-violet (though we’ve no idea what it appears like to them). Any plant that has a natural red colour has evolved to attract birds or animals, not insects, whereas those that attract insects make strong use of UV, which we can’t see at all!]

Anyway, as you see, the radiation we associate with heat and cooking (infra-red and microwaves) is at the long end of the spectrum. And surprise, surprise … 99% of the Sun’s light that reaches us consists of radiation in the yellow, orange, red and infra-red part of the spectrum - in fact, mainly infra-red (the other 1% is microwaves and radio waves, plus anything from the ‘short’ end that hasn’t been stopped by the ozone layer). In particular, the infra-red radiation is very good at producing heat, which it does by exciting whatever molecules it hits. Those molecules (of the earth, say, or your face) become much more active, rushing about in all directions and bumping into each other, producing heat. The heating of the earth itself is very important, since it then becomes a secondary source of heat – actually, the primary source of what heats the air and makes the day feel hot.

You see that the cause of the heat we feel is not a substance – ‘heat’ – that comes from the Sun, but rather the action of radiation on any molecules it hits. Also, 'heat' and 'light' are aspects of the same thing, electromagnetic radiation, that travels at the speed of light/heat/electromagnetic radiation. Your friend is exactly right. It’s a bit like a ‘Mexican wave’ going through the crowds at a sports fixture. It’s caused by the momentary excitement of small, essentially static, particles (or fans).

This has been fun. Hope it helps

[kernelbogey]

This has been fun. Hope it helps

Glad you had fun - and thanks for the answer!

[Gordon]

....... Were I a Martian I would be . . . [respond in no more than three words].

Over the moons?!?

[John Wright]

Pabmusic, I like your layman's explanations (I suppose I'm not a layman, having a science degree, chemistry with physics, but I rarely have to 'teach' or explain science in layman's terms).


Anyway, shouldn't infra-red be on your list of sun's radiations, that would clarify the rest of your message?

[Pabmusic]

Pabmusic, I like your layman's explanations (I suppose I'm not a layman, having a science degree, chemistry with physics, but I rarely have to 'teach' or explain science in layman's terms).


Anyway, shouldn't infra-red be on your list of sun's radiations, that would clarify the rest of your message?

Of course it should (I mentioned in the text, too!). Now where's the 'doh!' emoticon?

[John Wright]

Of course it should (I mentioned in the text, too!). Now where's the 'doh!' emoticon?

Well, it's such a good message I think it's worth editing

[Pabmusic]

Well, it's such a good message I think it's worth editing

Duly done. Thanks. or (or both if you like).

[Ferretfancy]

Here's a nice little Martian coincidence. In Gulliver's Travels ( Pub.1726 ) Swift's fictional astronomers of Laputa knew that Mars has two small moons, and also made a pretty good guess at their periods of rotation around the planet. They were not actually discovered until glimpsed by the American astronomer Asaph Hall in 1877!

[Vile Consort]

In reply to pabmusic's otherwise excellent posts:

You don't need either Special or General Relativity to navigate around the solar system: Newtonian mechanics are perfectly adequate. Neither the speeds of inter-planetary travel nor the strength of the gravitational fields make the motion relativistic. Or so the books on relativity say. The time it takes a signal to get to the earth when landing on Mars is of no more consequence than the time it takes a signal to get to Mars (or anywhere else) when landing on the earth.

And was it not light passing the sun during an eclipse that Eddington observed, rather than light passing Venus? Venus has far too little mass to produce observable GR effects.

The delay has little to do with relativity: it is merely a consequence of the finite speed of light. Roemer established the speed that light travels in the 1670's by observing the motion of the moons of Jupiter. He wouldn't have been at all surprised at the 14 minute delay (which I think is the round trip time).

[Pabmusic]

[A] You don't need either Special or General Relativity to navigate around the solar system: Newtonian mechanics are perfectly adequate. Neither the speeds of inter-planetary travel nor the strength of the gravitational fields make the motion relativistic. Or so the books on relativity say. The time it takes a signal to get to the earth when landing on Mars is of no more consequence than the time it takes a signal to get to Mars (or anywhere else) when landing on the earth.

[B] And was it not light passing the sun during an eclipse that Eddington observed, rather than light passing Venus? Venus has far too little mass to produce observable GR effects.

[C] The delay has little to do with relativity: it is merely a consequence of the finite speed of light. Roemer established the speed that light travels in the 1670's by observing the motion of the moons of Jupiter. He wouldn't have been at all surprised at the 14 minute delay (which I think is the round trip time).

Thanks. I wasn't attempting a thesis, and I'm sure much that I said needs greater unpacking.

[A] You may well be right. Mars is still quite near to us in space terms, and I made the point that Einsteinian physics is the more accurate system at greater cosmic distances. An elaborate red-herring, I suppose, but I mentioned the 1919 Transit of Venus to demonstrate just how precise are some of the calculations have to be.

[B] Of course you are right, but I don't think what I said was different - "light emerging from behind Venus".

[C] I wasn't trying to say that the 14-minute delay is a feature of relativity (and I didn't mention special relativity at all) - though as I write this, I realise that Gallilean relativity between two 'sealed' moving objects crops up! I was more concerned to explain that the delay varies constantly because of the juxtaposition of two separate and unrelated orbits; and that '14 minutes' is an average that is rather unhelpful.

[prokkyshosty]

I love day-dreaming about the possibility of colonizing Mars. I find myself doing it a lot. From small ideas of building self-sustaining human habitats to big terraforming dreams of melting the polar ice caps, rewetting the soil, and recreating a thicker, sustainable atmosphere. Anyone have any guesses as to how realistic these sorts of things actually are? Do we have technology that would allow us actually colonize Mars, or is it all still just science fiction? I believe we were planning missions with 2 years of surface time to be conducted in the 2030s, at least before the Obama cuts kicked in. Now I don't know where the long term plans stand.

[kernelbogey]

Anyone have any guesses as to how realistic these sorts of things actually are? Do we have technology that would allow us actually colonize Mars, or is it all still just science fiction?

I don't wish to be disrespectful of your daydreams, prokkyshosty, but while we have enough food on this planet to feed everyone, people still starve. If you think of the cost of colonising Mars, then imagine those resources being applied to solving the problems of overpopulation and hunger here, there would be no need to go elsewhere.