NASA has led seven such simulations, and the participants only fully stopped the asteroid once. It shows how challenging it would be to stop an Earth-bound space rock, even with ample warning. An international group of space experts have been bested by a hypothetical asteroid. Again. In a planetary-defense simulation that NASA led last month, more than 200 participants from about two dozen countries learned that a fictitious space rock was set to strike Earth in six months. They put their heads together to investigate every possible way to avoid the impact. But in the end, the group determined that no existing technologies could stop the asteroid, given the limited warning time in the scenario. The rock ended up razing eastern Europe.