Talking with the planets
Published on Feb 2, 2007 at 10:25 am.
4 Comments.
Filed under amusement, physics, planets.
A few days ago I was trying to get across to someone just how big the Solar System is. I have an exercise that I do with my students in which they build a scale model of the Solar System. I don’t mean making just the planets to scale, but also putting them at scaled distances from the scale model Sun. With Mercury just a couple millimeters across, it is nearly a football field length from the scale model Sun. On this scale, as we stretch out the Solar System, we wind up walking from one corner of the campus to the other, and still don’t quite fit all the planets on campus. Then it starts to sink in how big the Solar System is.
Well, I couldn’t do that exercise with the person I was talking with, so I thought of another way of communicating just how big it all is. I could tell them how long it took for light to travel between the planets. After all, light travels pretty fast. In fact, for most human experiences, it is practically instantaneous. Rarely does the average person get into a situation where they have to take into account the finite speed of light. But, astronomers have to think of that all the time. Light travels at 300,000 kilometers per second. That is fast, for sure. But, space is so vast that that is just a crawl when you are thinking of astronomy. The nearest star is so far away that it takes light over four years to get from there to here. Even the Sun is so far that it takes light on average just over eight and a quarter minutes to get from the Sun to the Earth. So, how long does it take for light to travel from one planet to another?
The planets all move around the Sun, and currently Mercury is the closest one to Earth, at 1.13 AU. At that distance, it takes light 9.4 minutes to make the trip from Mercury to Earth. If we had a spacecraft there and wanted to communicate with it, the round trip communication time (the shortest time between when you ask a question and get the answer back) would be just under 19 minutes.
Venus is somewhat on the other side of the Sun than Earth at the moment (not directly so, or we wouldn’t be able to see it in the evenings), but it is currently about 1.52 AU distant. That means that light would take 12.6 minutes to get from there to here, and the round trip communication time would be just over 25 minutes.
Mars is 2.24 AU distant. That means that it takes light 18.6 minutes to get here one way from Mars. When we communicate with the Mars rovers, we send a signal, and get the response over 37 minutes later. That is why they are designed to be somewhat autonomous. As they roll along, if they sense something in their path they can’t wait for instructions to arrive from Earth. By that time, they’d have already crashed into it!
Jupiter, at 5.82 AU, is 48.4 lightminutes distant. That means that when we talk with the New Horizons spacecraft, now near Jupiter, the fastest response is over an hour and a half later.
It you go out tonight a bit after sunset and look east, you’ll see the Moon. Right near the Moon will be Saturn. Saturn is currently 8.22 AU away from Earth. That means that the light that you see reflected off of Saturn about 68 minutes and 20 seconds ago. The Cassini spacecraft is now in orbit around Saturn. When we send it a query, it takes over two and a quarter hours for an answer to come back to Earth. This reminds me of a science fiction program that I watched many years ago in which scientists were getting these strange radio messages from someone who seemed to have all the answers to their research problems. They’d ask a question, and the answer always came back about two and a half hours later. They realized that they were talking to someone in the vicinity of Saturn.
Uranus is 20.9 AU distant. That means a one way light travel time of 174 minutes, just shy of three hours! And Neptune is even more distant, at 31.0 AU, resulting in a one way light travel time of 258 minutes, or 4.3 hours! As fast as light travels, that is simply amazingly far!
Compared with all of that, the Full Moon tonight will be only a bit over 396,000 km distant. That means that light only takes 1.32 seconds to get from there to here. That’s nothing compared with the planets! But, even that is noticeable. Look at footage from the Apollo missions. The controllers in Houston would ask the astronauts something, and it would be a while before they responded. They’d sometimes start talking at the same time. That is because the round trip light travel time would be close to 2.64 seconds.
-Astroprof
Ed Minchau on February 2, 2007 at 10:37 am: 1
The lunar distance is off by an order of magnitude.
Astroprof on February 2, 2007 at 11:01 am: 2
Thanks! I corrected it. The calculation was correct, but I just left out a zero when I was typing the distance into the posting.
Ed Minchau on February 2, 2007 at 11:06 am: 3
Never let it be said that my anal-retentive attention to detail never yielded positive results.
A Ler…-- Rastos de Luz on February 3, 2007 at 6:05 am: 4
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