Several telescopes today monitor space for potentially harmful space debris, such as comets and asteroids. These pass by the Earth all the time, and NASA currently considers about a thousand asteroids ranging in size from basketballs to mountains to be potentially hazardous. This means they will probably pass very close to the Earth in the foreseeable future and have a chance of impacting the Earth. If an item about 2 kilometers (1.2 miles) across impacted the Earth at the proper velocity, the explosion would raise so much dust that it would block out the sun, an event that would cripple agriculture to the point that humanity may starve before the dust settled. According to the passage, how many potentially hazardous asteroids is NASA monitoring?