Once in Earth-crossing orbits, they last for time periods (e.g., 10 6 to 10 7 years) that are very short compared with the age of the solar system before encountering a planet, impacting the Sun, or being ejected from the solar system primarily by Jupiter's gravity.
(they; last; for time periods; T:Once in Earth-crossing orbits) (time periods; are; very short)
they time periods
last are
for time periods T:Once in Earth-crossing orbits
very short