Invisible Impactors

[SunJumper]

What material made into an impactor to impact a planet would give those living on said planet the least amount of notice before impact. (Ie impact the most stealthily)

I'm guessing either glass/clear plastics to be transparent to visible light (although this may not be the case for non-visible light), or a substance that is completely black to as many wavelengths as possible (probably same issue as before).

Edit:

The object would either be rod shaped, or pancake shaped, and have a volume of ~10 - 50 km^2

Edited February 12, 2014 by SunJumper

[Ralathon]

Just fire it at 99.99% of c. That way, if you fire it from 1 lightyear distance they have about 50 minutes from detection to impact.

[Seret]

Depends, where is the point of origin? How fast is it going and where did it get that speed from? What sensors are being used to try and detect it? What countermeasures do they have? Does whoever sent the impactor have any assets on the ground?

[SunJumper]

Just fire it at 99.99% of c. That way, if you fire it from 1 lightyear distance they have about 50 minutes from detection to impact.

I'm presuming that the individuals don't have access to that amount of energy. By preliminary calculations, an object at 0.9999c has a kinetic energy of 12.2 times that of its mass energy.

Depends, where is the point of origin? How fast is it going and where did it get that speed from? What sensors are being used to try and detect it? What countermeasures do they have? Does whoever sent the impactor have any assets on the ground?

The object was created in the system's asteroid belt, and is set on a collision course with a small nudge. Hence, it is travelling at ~15 to 20 km/s.

Then again, thinking about the countermeasures, it may be easier to send in a few black holes in. The target is a planet sized city on a shell suspended over a gas giant (saturn sized and massed), with a population numbering in hundreds of trillions, armed with billions of satellites watching the sky for natural (but not artificial) asteroid phenomena. Ie, an asteroid losing some mass might not be noticed if it is ~10km across or less. Millions monitor these satellites, millions more monitor the sky. Defences include millions of nuclear lasers to redirect asteroids and fusion torch-ship missiles to nudge asteroids out of impact trajectories, of which thousands will fire at any one object if necessary.

[drakesdoom]

If you want to be real sneaky, send it as freight. Bulk freight has almost no checks on it since there is so much. Lay down enough bribes to get shipping manifests signed and stamped and you could get a zar bomb into NY, NY cheaper than any missile.

[panzerknoef]

Mirrors... No lasering that down!

[andrewas]

Make it black and it'll be visible by occulatation. Make it transparent and refraction effects make it visible, these might even draw attention to it. More importantly, moving it in the first place results in huge, hot jets that advertise its location, mass and new trajectory.

[Headhunter09]

Mirrors... No lasering that down!

No mirror could be made 100% reflective for the entire journey, and thus a combat-grade laser could melt it rather easily. Not to mention it's easily detectable.

[LordOfTheOrbits]

I would suggest trying to find a natural asteroid which has a flyby of the earth and wherever you are, hiding behind it and short before it has its earth flyby thrust a little bit(assuming that that's allowed) so you will impact the earth. Also something black wouldn't necessarily help because it would be visible on transits of other bodies.

[Idobox]

Make it of something really cold, and cover it with coal powder. That way it will emit almost nothing, especially in the infrared spectrum. The cold core evaporating could give you thrust, but might be visible.

You also want it as dense as possible, to reduce size, and hence cross-section. A chunck of iron in the middle would do the trick nicely, but if you have access to heavier stuff, use it. Uranium is a nice choice, especially if just sub-critical.

Then, give it very flat surfaces, like a cube for example, and avoid having one face perpendicular to the direction of your target, to reduce the radar cross-section. Also carbon will absorb a good deal of radio waves, but not all. A layer of microwave absorbing foam would decrease the reflection further.

Occultation will not be an issue,

[Soda Popinski]

Beware the Hun in the Sun. Put it on a trajectory so the sun/star is behind it.

[Rakaydos]

Beware the Hun in the Sun. Put it on a trajectory so the sun/star is behind it.

Or in front of it. Start accelerating it from behing the sun, aiming for a low solar flyby that you can adjust (once you have the sun at your back) to an impact trajectory.

[victory143]

Neutron Astroid? Super dense and has a HUGE amount of mass. Gravity (tidal forces?)would do the damage.

[SunJumper]

If you want to be real sneaky, send it as freight. Bulk freight has almost no checks on it since there is so much. Lay down enough bribes to get shipping manifests signed and stamped and you could get a zar bomb into NY, NY cheaper than any missile.

I actually like this idea more than my own.

[Palladium Corp]

You would also need something invisible to radar, such as on the B-2 stealth bomber. Ok, that is not entirely invisible, but it reduces the profile of the thing.

[Duxwing]

On an impending asteroid put a powerful shaped charge with its jet outward facing. When the asteroid nears its target fire the charge, instantly accelerating the asteroid onto a collision course.

Alternatively mail packets of a plague that lacks a cure; e.g., Ebola.

-Duxwing