Greetings once again,

I posted on here many months ago on a question regarding Earthshine. It involved research for a fantasy novel that I am writing. The responses were very useful. I now find I have a second astronomy question that I'm hoping you all might be able to assist with.

In the second book of the series that I am writing (the first one isn't published, or even finished yet, but I'm planning out the series) the world in question is hit by an interstellar object. This object originated from outside the planet's solar system... it in fact was sent from another planet.

So I have a few questions on the reality of such a scenario.

Would this be classified as a comet? It is not in orbit of any star... as I said it was propelled through space with a particular destination in mind, so what would be the closest classification for such an object. If it matters, it's about half a mile in diameter.

This object's travel time is intended to be about 2,000 years, from a nearby solar system... let's say that it's about 20 light years away. Is it probable that a comet-like object could move fast enough without being overcome by forces (would it even, since space is a vaccum?) to travel that far in that amount of time?

Third question, assuming a size of about 1/2 mile in diameter, how soon do you think it would be before this object could be seen with the naked eye at night? Might I assume that to the uneducated medeival society that it would first appear to be a star, until it got close enough to be larger than 'just a star'?

Last question. The object would need to be large enough to survive atmospheric entry. I'm assuming that 1/2 mile in diameter would be large enough to survive entry without the core of the object being destroyed. I know that would depend on the material that composed the object... I haven't thought about it too much, but it is not an object made of ice, it is essentially rock.

Let me know if I need to provide any more details on this. Like I said, I haven't done a lot of planning of the details, but I'm just doing preliminary research and want to make it fairly accurate.

Thanks,

Zach

Would this be classified as a comet? It is not in orbit of any star... as I said it was propelled through space with a particular destination in mind, so what would be the closest classification for such an object. If it matters, it's about half a mile in diameter.

Well technically, I think that would make it a missile. As for the comet/vs asteroid definition, I think Asteroid would be the more appropriate choice. Comets tend to have a lot more ice, which is what leaves the trail. Comet Hale-Bopp was naked eye visible for several months before it arrived. I cant seem to find a size for it though.

An asteroid would be much harder to detect. One about 1/8, mile (200 m just passed very close to the Earth and never got much brighter than Pluto.

This object's travel time is intended to be about 2,000 years, from a nearby solar system... let's say that it's about 20 light years away. Is it probable that a comet-like object could move fast enough without being overcome by forces (would it even, since space is a vaccum?) to travel that far in that amount of time?

Voyager 2 is the fstest moving abject over launched from Earth and is currently moving about 9.7 miles per second. That's really fast but no where near your 1/100 light speed, which would be 1862 miles per second. Look into the Orion Project from the 60s for a way that would be possible to get a Big Freaking Rock moving a lot faster.

Third question, assuming a size of about 1/2 mile in diameter, how soon do you think it would be before this object could be seen with the naked eye at night? Might I assume that to the uneducated medeival society that it would first appear to be a star, until it got close enough to be larger than 'just a star'?

Depending on how far away it was, it would appear to move across the sky a bit, If it was not seen until very soon before impact it might just be a brighteneing in the sky. If they could see it, it would probably be considered a sign from the gods or a harbinger of doom, or it may go completely unnoticed.

Last question. The object would need to be large enough to survive atmospheric entry. I'm assuming that 1/2 mile in diameter would be large enough to survive entry without the core of the object being destroyed. I know that would depend on the material that composed the object... I haven't thought about it too much, but it is not an object made of ice, it is essentially rock.

Make it iron and a great deal of it would make it through, but at that speed it may still shatter when it hits that planet. Most BFRs in our system move round 40 km/s at the most. That's about 25 miles per second.

Let me know if I need to provide any more details on this. Like I said, I haven't done a lot of planning of the details, but I'm just doing preliminary research and want to make it fairly accurate.

Zach[/quote]

Other's will likely have better info, see below for corrections.

__________________
A Nerd can figure out how long it will take the original Enterprise traveling at warp 6.5 to travel from Regulus to Antares.
A Geek will think he can use that to pick up a girl in a bar.
A Dork knows he can't pick up the girl with it, but will hang around for hours anyway, just in case she asks.
She might. You never know.

If it was deliberately propelled, I would classify it as "projectile" ...in any case, the definition of a comet hinges more on the composition ("iceball"), than on a particular trajectory.
To travel 20 ly in 2,000 years, it must move at 1% c, that is 3,000 km/sec. That is not nearly enough to erode a half-mile comet over 2,000 years (I assume that's what you mean by "overcome by forces", but see below...
If it is indeed a comet, it will develop a gaseous coma and a tail like every other comet. On a world without artificial light, a half-mile comet may be first seen perhaps 4 AU (600,000,000 km) away -- as a barely visible star, -- and will have a distinct tail at half that distance. At 3,000 km/sec, it will go from barely visible to impact in only 55 hours.

If it is a rock rather than ice, and does not outgas, it will be visible only for 1-2 minutes before impact.
See above. Like any other comet.
Ordinarily yes... but not at that speed. Ordinary comets and meteorites hit at about 30 km/sec. We are talking 100 times that -- meaning 10,000 times kinetic energy. Nothing can survive that -- at least nothing made with what we think of as "chemistry". At that speed a half-mile object with density of rock would make an explosion of about one million gigatons. More than enough to sterilize a planet. There will be nothing left of the medeival society, or even of bacteria.
As I said, at that speed, material makes no difference.
Then it's not a comet. No tail. Someone lucky enough to look in the right direction will have about one minute to puzzle out their doom.

__________________
Fiction has to be plausible. Reality is under no such constraint.

Thanks for the comments so far.

Since it's a fantasy novel, I feel I have some liberties I can take. ie. Magic is a part of the universe, so the projectile could begin slowing down via 'other forces' once it gets within a reasonable distance.

As far as the classification of the projectile. What would an object be called if it was a fragment of an exploded planet? The best example I can think of is Krypton from Superman. Would the pieces of the planet that hit Earth (and bringing the kryptonite to the planet) be classified as projectile if they were not deliberate?

If I think of any other questions, I'll let you know.

Thanks,

Zach

If this is a magic universe, then surround the projectile with a force field, send it by levitation, or even teleport to near your target planet.
THAT would make it hard to spot.
But then you would have to explain why your planet's people cannot use telepathy, 'far-seeing', telekinesis and precognition to defend themselves. A magic universe MUST be as consistent as a real universe, or else your readers will throw it down in disgust at another 'deus ex machina' event. Terry Pratchett and Larry Niven (The Magic Goes Away series) can do this, why not you?
Though Clarke's conjecture (magic= advanced science) can be invoked.
John
PS "fragment of an exploded planet" and "propelled through space with a particular destination in mind" would need a pretty big magic. The fragments of an explosion, with the exceptionn of shaped charge (now there's an idea for you!) are a good example of a chaotic system. J.

This may be something you've already thought about but I think may be important. Why is the aggressor planet launching a BFR at a world, knowing that it will take 2000 years to hit their target?

__________________
A Nerd can figure out how long it will take the original Enterprise traveling at warp 6.5 to travel from Regulus to Antares.
A Geek will think he can use that to pick up a girl in a bar.
A Dork knows he can't pick up the girl with it, but will hang around for hours anyway, just in case she asks.
She might. You never know.

JohnD, good comments. Yes, the magic system does have to be just as plausible as the science. And I’m working on the rules of the magic system as I develop the world and universe.

Tog_, you raise a good question that I have already personally addressed. And it is based on the assumption that this projectile is a weapon. The idea is that about 2000 years ago a dragonrider society departed World A via a magical gate. Before the whole society could depart to World B, the waygate collapsed. The waygate requires a certain material to construct, and there is none left on World A. The dragonriders on World B decide to send an abundance of this metal to World A so that they can reconstruct the waygate and bring the descendants of their kinsmen to World B. The only means that their extensive, but limited magic will allow is to propel this material through space, knowing that it will take thousands of years to reach the planet, and that it will be necessary that the projectile make impact with the world. These dragonriders are fairly advanced as far as the study of the stars is concerned.

Now, in the book, the world sees this object in the sky for about a year before impact, bringing up all sorts of doomsayers, etc…, during which the story builds, and “stuff happens.”

Here’s something else I thought of... if this projectile is traversing between two stars, it couldn't really be detected at all because as a person looks to the night time sky, one could not discern the projectile from the star, so it would essentially be undetectable. It would be like a lightbulb placed in front of another lightbulb... looking directly at one, you couldn't see the other. Make sense?

I think what I may need to do is have this projectile orbit the planet for a short time to allow for the doomsaying, tracking of the object, etc… The magic would have to be advanced… even perhaps an intelligent system so the object could decelerate, maneuver into orbit, and eventually de-orbit in a controlled manner to avoid too much destruction… not much sense in sending this projectile only to destroy all life on the planet.

The other thing I’m thinking is that I may be trying too hard to make this work. The other option is a random collision by a comet that happens to contain the required material. I’m thinking that may be the best solution… fewer things to explain and accomplishes essentially the same goal.

Thoughts?

Zach

Ahhh, okay, I was seeing this as an attack sort of like the one in Starship Troopers.

The problem with the "can't see it if it's moving between two stars" bit is that as it nears the original planet, the rock will get brighter and angular distances will increase. If the rock is anything more than about 1 arc minute from the star (1/60th degree), the people on the planet can see it as being a different object. As it gets closer, the orbit of the planet almost guarantees that this distance will be exceeded.

There also seems to be a destruction factor worth considering. A rock that size, hitting that fast will do damage. These people are likely to interpret the impact as some sort of divine wrath at first. Once they become aware that it was a "gift" from ancient ancestors they will probably be just a bit upset. It's been 2000 years. They don't call, they don't write, and when they finally do make contact, it's a gift that wipes out a good portion of the southern continent. Build that gate, Grab your axes. Let's go say "thanks".

A potentially less destructive method (someone correct me if I'm wrong about the "less" part) would be to plan the orbit so that the rock enters orbit in about the same position as the planet. The world will see it like a small moon for a few days, every few months until it is captured by the planet's gravity and is ripped apart by tidal forces. This would cause a spiral of debris to rain down over an extended time, but in much smaller chunks. Probably.

Some of the Seers on the world may get visions of a "ring" that has the power to transport people to new places. This could be misinterpreted to be a ring in the jewelry sense, as opposed to the the Jupiter sense.

I'm not sure if that will fit your idea, but it's a way to slip some semi valid astronomy into the story.

__________________
A Nerd can figure out how long it will take the original Enterprise traveling at warp 6.5 to travel from Regulus to Antares.
A Geek will think he can use that to pick up a girl in a bar.
A Dork knows he can't pick up the girl with it, but will hang around for hours anyway, just in case she asks.
She might. You never know.

At 1862 miles per second, it would be barely naked eye visable one second before impact, so likely no one would notice, until it entered the atmosphere about 1/20 th second before impact. The last few milliseconds it would be so hot it would emit gamma rays, but it would still be cold below the surface which would ablate rapidly: Perhaps one inch per millisecond. On contact with Earth surface, it would likely penetrate the ground for several milliseconds. 3 billion tons of matter in the stony asteroid would be converted to plasma = hotter than vaporized. Ground zero might be as much as a million square miles. Severe earthquakes would shake much of Earth. Neil