How could a weaponized asteroid remain undetected until its too late?

Question

So earth is at war with mars. Earth sends a trojan rocket ahead of time and declares war when its too late for mars, but wait the trojan is headed for phobos and so mars rig up stuff and when it breaks up (and harmlessly lands in unpopulated land) some of the pieces get sent back to earth but they need to remain undetected until its too late for earth to stop them and bunch of people die earth surrenders book over. but how can i make it so that the attack remains unnoticed until say 2-3 weeks from it hitting earth. Here is some important info:

• assume 2100s level tech, bit more advanced than now but not too crazy
• lets say 10% of phobos is whats heading to earth give or take
• earth cannot have enough time to stop it whether it be 2 months or a day but id like it to be in the 2-3 wk range so earth has time to panic

so how could i realistically have it remain undetected? no magic and minimal hand waving.

Answer 1

Given 22nd century tech, I'd think there would be a lot of more precise ways to attack a rival planet. Your choices of what tech is present make this very mutable. But let's try these:

• Sensors from so advanced a society would detect anything that big not covered in some sort of stealth rig. (option 1 - use some sort of stealth tech)
• Asteroids would usually be so slow that there would be lots of time to see them (option 2 - make the asteroids very fast, so the approach time is extremely small).
• Earth would be monitoring the debris from a major body in the solar system breaking up for potential threats to Earth (option 3 - sliver-like projectiles of rock always presenting their narrowest parts to Terran sensors, mixed with seemingly harmless dust & debris).

Answer 2

2-3 weeks is really fast for Phobos to get to Earth, there's little Earth can do. And yes: you would know about it easily

Phobos is large enough to be seen by a telescope - in fact it was discovered in 1877 well before any modern age satellites.

It's orbit would be easily seen to alter as soon as it does - so you could accurately say (in particular in a time of conflict which would warrant increased scrutiny) that Earth would know about it immediately especially with current day technology, let alone future technology.

So given this - is there anything Earth can do? Well, if you're accelerating a mass of Phobos which is 1 x 10^16 kg such that it would get to Earth in 2 - 3 weeks (a distance say lets assume about 200 million km) it would hit Earth at 132 km / s if only constant velocity (not accounting for acceleration).

Even at this average speed Phobos would devastate Earth, and there is not much Earth can do about it.