Moon's South Pole-Aitken basin, illustration. The South Pole-Aitkin (SPA) basin (blue and violet) is the largest and oldest crater on the Moon. The SPA basin, on the far side of the Moon, measures approximately 2,500 kilometres in diameter, which is nearly one quarter of the Moon's circumference. Lunar samples suggest that most of the major basins on the moon formed around 4 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System.