Instead of a puny little thing as few dozens miles wide asteroid, may I suggest a rogue planet at very very far distance instead? Basically George would want to:
put the fake planet at such great distance and direction that only Hubble Space Telescope or select few on Earth can see or detect (through hacked radio and infrared telescopes) it.
put the fake planet at extremely high speeds so that it would appear to come and collide with Earth in a matter of weeks.
Reasons For Choosing A Planet Over An Asteroid
Planets are really really large things. And considering that most planets discovered outside out solar system are several times larger than Earth, it would mean that if a planet is coming at you, it is serious business. With an asteroid (even miles wide), you still have the hope of deflecting it by ramming it with a large, fast moving spaceship or try to break it up with several hydrogen bomb explosions in space (man that would be cool!).
Plus, if an asteroid was coming at you, the scientists would go on in a long, stupid, boring debate about whether or not it is on a collision course with Earth. Then talk about the variables of composition, speed, direction and blah blah to try and measure the extent of damage possible to Earth. All this would mean that while it would create some stir, there would be really little panic factor involved.
Now think about the shock when you read or listen that a rogue planet is on a collision course with Earth. Oh man, the horror! And another important difference would be that there would be little math involved with deciding whether the said planet is on a collision course with Earth or not.
Even if the scientists decide that the said planet is not on a collision course with Earth and would instead pass near Jupiter (I can tell you that's a huuuge distance between Earth and Jupiter, nearly 3 times the distance between Earth and the sun), still Earth would be at serious risk. The planet might deflect the orbit of Earth. Even a small change in Earth's orbit would spell doom to the reign of these puny creatures known as homo sapiens. And even if Earth is unaffected, the planet can still cause lethal damage by disturbing the orbits of Venus or Jupiter or Mars. Heck! Even deflecting a few asteroids from the asteroid belt (between Mars and Jupiter) inwards would be enough to cause an extinction level event.
Size Of The Planet
Naturally George would want to make the planet as large as naturally possible, in order to increase the shock and hysteria factor. But larger planets are also easier to detect, and so the lie would not hold for long.
No, not really. You can have a Jupiter-sized planet while still making it impossible for amateur astronomers to see it.
Making The Planet Undetectable For Amateur Astronomers
Ever read or heard about TrEs-2b? I mean the planet, not the member of worldbuilding community. If not, then know that it is THE darkest (and also one of the most horrific) planets ever discovered. It is so dark that it reflects less than 1% of all light falling on it. And then again, its orbit is shorter than Mercury's orbit. What's more, it is a Jupiter-sized planet.
The important thing about TrEs-2b is its darkness. This planet is the darkest planet discovered so far, although it is revolving horribly close to its parent star. What about a rogue planet with an atmosphere like TrES-2b? It would be simply too dark to be seen from any and all telescopes by amateurs. Heck! It wouldn't even be seen by many professional telescopes operating in the visual light range. The only way to detect a planet such as this would be through infrared or microwave telescopes (which, luckily for George, are extremely sparse, even at professional level).
Velocity And Distance Of The Rogue planet From Earth
Scientists have theorized that an angry rogue planet can get shot out of its former orbit at a frighteningly high velocities. We are talking about 10 million miles per hour. That is more than 2700 miles per second. In 6 weeks, the planet would travel 1.008 x 10$^{10}$ miles.
Another important thing to keep in mind here is the radius of our solar system. Neptune is located 2.84 x 10$^9$ km from the sun (reference).
This means that George can place his imaginary planet 5 times away from Neptune and it would still come crashing into Earth within 6 weeks. ...and then they say train collisions are horrible to watch...
Direction Of The Rogue Planet
In order to offer least hope and create maximum frenzy among the scientific community, George would prefer entering the rogue planet from an angle where its path is least affected by the gravities of outer gas giant planets. An image is being uploaded to help George on this.
How Much Time It Will Buy For George?
At most 5 weeks (if the credibility of the functionality of the radio and infrared telescopes is not reassured) and at least 4-5 days (this much time would at least pass in ascertaining that all hardware and software are working flawlessly and that the system has not been hacked).
