So, to be able to spot an asteroid in the night sky, it should be bright enough and at the right distance from the Earth. Let’s dig into the specifics.
Asteroid brightness: In Astronomy, the apparent brightness of asteroids is described with the help of the same magnitude scale used for stars, planets, and other celestial bodies. This scale is inverse, which means that the bigger the number is, the lower the brightness. Therefore, negative magnitude numbers indicate very bright stars.
For example, the sun’s magnitude is estimated at -26, the Full Moon can reach -13, and Sirius, the brightest star in the night sky, has a magnitude between -1 and -2. In the clear night sky, our eyes can see objects up to magnitude +6. Vesta, the brightest asteroid seen from Earth, reaches magnitudes between +5 and +6 when it is at its best time for observations.
Even though the Main Belt objects have higher chances to be seen than the smaller asteroids crossing the Earth’s orbit, they are still not bright enough to be observed in the same way as stars and planets.
Another factor that comes into play here is how far the asteroid is from the Earth and the Sun.