Two distinct populations of dark comets delineated by orbits and sizes.

Small bodies are capable of delivering essential prerequisites for the development of life, such as volatiles and organics, to the terrestrial planets. For example, empirical evidence suggests that water was delivered to the Earth by hydrated planetesimals from distant regions of the Solar System. Recently, several morphologically inactive near-Earth objects were reported to experience significant nongravitational accelerations inconsistent with radiation-based effects, and possibly explained by volatile-driven outgassing.

Non-gravitational acceleration in the trajectory of 1I/2017 U1 ('Oumuamua).

'Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state and that the physical properties of its surface resemble those of cometary nuclei, even though it showed no evidence of cometary activity. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing.

A brief visit from a red and extremely elongated interstellar asteroid.

None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space. The high predicted number density of icy interstellar objects (2.4 × 10 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen.

The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission.

We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of the mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft and a way to directly compare 67P with other comets.

Setting the scene: what did we know before Rosetta?

This paper provides an overview of our state of knowledge about comets prior to the Rosetta mission encounter. Starting with the historical perspective, this paper discusses the development of comet science up to the modern era of space exploration. The extent to which comets are tracers of solar system formation processes or preserve pristine interstellar material has been investigated for over four decades.