Answer:
The gravitational effect when spacecraft flies close to the asteroid.
Explanation:
The gravity of a planet, or in this case an asteroid, is determined by two factors: it's mass and it's size. The bigger the mass, the stronger the gravity is. But the bigger the size of the mass is, the smaller the gravity is, this because of the distance from the center of the mass. As you can see by measuring the gravitational effect when a spacecraft is close to an asteroid can help determine it's size.
Although this is helpful, there are some other ways scientists are measuring asteroids' sizes. NASA uses infrared to determine in an accurate way their sizes.
