The Jupiter Theft

The Lunar Observatory on Earth is picking up a very strange and unidentifiable signal from the direction of Cygnus. When the meaning of this signal is finally understood, it clearly spells disaster for Earth. An immense object is rushing towards the Solar System, travelling nearly at the speed of light, its intense nuclear radiation is sure to kill all life on Earth within months. As it moves close the humans can discern that it is an enormous convoy of some sort, nearly as large as a planet. And there is nothing anyone can do to divert such an enormous alien object. Then, unexpectedly, the object changes course and heads toward the dead planet of Jupiter but what could an enormous alien convoy want with such a useless planet?