Interstellar objects passing through the Solar System

3 problems

• Do interstellar comets differ in composition from Solar System comets?

  Open

  2I/Borisov carried an unusually large share of carbon monoxide and 3I/ATLAS showed a coma dominated by carbon dioxide. Whether such differences reflect where they formed or billions of years of cosmic-ray exposure in space is unknown.

• How many interstellar objects are there, and of what sizes?

  Open

  Each detection gives an estimate of how many such bodies fill space between the stars. This tells how much material planetary systems throw out while forming.

• What caused the non-gravitational acceleration of 1I/'Oumuamua?

  Contested

  'Oumuamua sped up slightly as it left the Sun, as comets do when gas jets off them, yet no gas or dust was seen. The push must come from something invisible to the instruments used.