Ascientist is tracking an object orbiting the sun that is found between mars and jupiter. in order to classify the object as an asteroid, it must be at least km.

The answer is: 1 km

If the size is small than 1km in diameter, the object would be classified as a a meteoroid, not an asteroid. Up until the year of 2018, NASA has spotted around  147,132 asteroids. From that number, around 8% of them had the risk of colliding with the earths.