Finally, meteoroids can be composed of either cometary or asteroidal materials. Observations of this comet led to several significant findings regarding cometary science, especially for Brahe. The Helix Nebula was the first planetary nebula discovered to contain cometary knots. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice. There are about 40,000 cometary knots in the Helix Nebula. The latter indicates that cometary nuclei are very fragile entities. Nevertheless, cometary impacts in particular may have contributed volatiles to the planet. This enables the collection of cometary particles without damaging them due to high impact velocities. The magnitude of the cometary bombardment is also reduced. The fluffy morphology of the grains suggests a cometary origin. In 2005 it was also found in cometary dust returned by the Stardust probe. However, the actual increase in net cometary collisions is thought to have been quite small. The cometary hypothesis gained a general acceptance among Soviet Tunguska investigators by the 1960s. Several otherwise unremarkable bodies in the outer belt show cometary activity. This reinforces theories that much of Earth's ocean water is of cometary origin.