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John M. Dollan
2005-Dec-19, 03:52 PM
I think this is the ebst portion of the forum to post this question in. If not, then apologies....

I've just finished reading Clarke's "Songs of Distant Earth" for the fourth time or so, and for some reason this question only now popped into my head. According to the book, the main drive of the ship was visible from Thalassa's surface (the main planetary setting) for years after the Magellan's departure. So a departing ship with some sort of viable interstellar drive would be visible. But there is also the whole process of the ice shield, which really made me wonder....

If a ship, moving at some 0.2c or so is on approach to your star system, and it possesses just such an ice shield, it is going to probably suffer some hits from stray bits of space dust and hydrogen atoms along the way. At such speeds, these imapcts will of course release a lot of energy. Would these impacts be visible to the people of the planet the ship is approaching, and if so, what would they look like? Visible light in telescopes? Some sort of gamma ray burster on a small scale? Anything?

...John...

bigbluestar
2005-Dec-19, 04:09 PM
I really like your question john. A couple things though I have never read this book and I have no idea what the ice shield is for or what it does. I was thinking of assuming but that would get us no were.....

But I want to weigh in on the question part at least if you don't mind John.
First off lets talk about speed here so >=c,or <c.

>=c ,,,,, NO I don't think you will be able to see this vesel comeing. I think a vesel traveling at c or higher is in essense a black hole. Not because of einstiens comparison with gravity and accelaration but because the photons that are been reflected from this vesel cannot leavs its surface because of its speed. Remeber measuring the light emitted from a fast speeding rocket gives us the same speed as light from a stationary object.

So if the vesel is traveling at C or more the escape velocity of the object is greater than the speed of light. Wow I heard that used to describe another object yes a black hole.

quick subquestion here would this vesel experience a form of photon buildup on the surface of the ship. Now when this vesel stoped lets say like Star Trek an all stop kinda thing. Photons should then be released from the surface from the ship at c. Since it has an unusually high number of photons it should suddenly look like a bright star for about 1 trillionth of a second?????

John M. Dollan
2005-Dec-19, 08:06 PM
I really like your question john. A couple things though I have never read this book and I have no idea what the ice shield is for or what it does. I was thinking of assuming but that would get us no were.....

The ice shield is placed in a rather flattened cone configuration at the bow of the ship, and acts as a method of protection against the odd interstellar dust grain or small rock. I forget the statistics given in the book about its size, but it was quite large and thick (the ship itself was, I believe, about 2 kilometers long), and by the time it stopped at the Thalassa system, 35 light years from Sol, it had been worn down to half of its size.


So if the vesel is traveling at C or more the escape velocity of the object is greater than the speed of light. Wow I heard that used to describe another object yes a black hole.

The ship itself (named Magellan) travels between 0.2 and 0.3c if I remember correctly (the speed was only briefly mentioned). Regardless, it does not come even close to the speed of light, and certainly not beyond it. So wouldn't some kind of radiation, be it visual or other, be able to radiate outward and be seen by... someone?

...John...

phunk
2005-Dec-19, 09:38 PM
Unless it was using some kind of exotic engine, it would need to point the engines forward to stop itself as it approached its destination. So in theory it should be just as visible on approach as it was on departure.

bigbluestar
2005-Dec-19, 09:53 PM
So wouldn't some kind of radiation, be it visual or other, be able to radiate outward and be seen by... someone?

...John...[/QUOTE]

For argument sake lets say the speed is equal to or greater that C. Any slower you will be able to see it. Just as a really spectraly and optically blueshifted object.

As long as you are the destination of the vesel no. You will not be able to see it coming towards you. You will not be able to use any sensors on it or retrieve any information from that vesel. As the vesel approaches it will collect all the photons or radiation on the front surface of the ship. When the vesel stops it will release all these photons.

The release should be equal to the amount that was collected but assuming the surface of the ship some may be absourbed as reflecting it is not an option.

If you are not the destination of the vesel say a neibouring star and the angle of the ship is off it is theoretically possible to see or retreive sensory data about the ship

atleast thats my opinion as this can never be verified

eburacum45
2005-Dec-19, 11:52 PM
Let's see; for a ship travelling at 0.3 c there would be a lot of radiation from hydrogen atoms hitting the shield, plus the detonations of kinetic energy when the ship hits a dust particle.
From here
http://www.centauri-dreams.org/?p=159

For a spacecraft traveling at 0.3c…even small grains of dust behave like very energetic cosmic rays as we collide with them. For instance, a relatively abundant carbonaceous dust grain 1/10 of a micron in diameter will have a relative kinetic energy of 37,500,000 GeV, and our interstellar craft should impact just over 13 of this size dust particles per second over every square meter of frontal area. Obviously, stopping or redirecting these dust particles is a major design goal.

This would create a small amount of luminosity, which someone might like to calculate (say for a shield of a hundred metres in area); we might observe such a ship by its luminosity alone in deep space before the ship starts to decelerate. I don't know what wavelngths this radiation would be most visible in, by the way.

Once the ship starts to decelerate we would see a bright and continuous exhaust from the motor; this deceleration would last a few months (the period depends on the thrust used), and would probably start at something like a light-month distant from the target star.
Of course this deceleration burn would get more easily visible as the ship gets closer. If the ship uses a magsail to decelerate with by friction with the interstellar (and later interplanetary) medium, then the need for a rocket burn would be diminished; but as the magsail effect is quite small, only a very low mass vessel could rely on such braking alone.

If anyone has any estimates for the luminosity of such a drive (a fusion drive perhaps) that would be excellent; I have a feeling that the approach of quite a substantial interstellar craft would be missed by the astronomers of today, unless it were observed by someone looking for comets or KBOs.

If anyone can fill the blanks in my rough imaginings with a few numbers that would be great.

John M. Dollan
2005-Dec-20, 12:15 AM
Hey Steve...

So it's possible that an approaching ship (or even one flying relatively close by) would appear as a new and dim star due to a constant visual radiance from the shield?

Obviously, if the ship is heaing for *us*, then it would become quite visible once it flipped around and began its deceleration. But if it were merely flying by (and the odds of that would probably be pretty great, I know), would we maybe see nothing at all? Would the ship have to be aimed directly at us in order to see the illumination from impacting dust grains?

...John...

eburacum45
2005-Dec-20, 02:12 AM
Now I come to look at it the Gev is a very small amount of energy; just from 10 micron dust impacts the whole shield emits only 8 joules per second. Impact with hydrogen would add some luminosity but only on the same order of magnitude.

So it might be difficult to see an interstellar ship after all, unless it hit a few larger dust grains. The impact of larger dust grains would be less frequent but much more damaging and visible; from the OA shielding page (http://www.orionsarm.com/ships/shielding.html), written by Anders Sandberg;

For 0.3 c, a one milligram impact would give 4.3456e+09 J of energy, equivalent to one ton of chemical high explosive.
(that is why I put big shields on the front of my interstellar ships of course.)

Perhaps the shield would look more like an intermittent sparkler than a glow-worm.