  Among the current population of the 81 known trans-Neptunian binaries (TNBs),
only two are in orbits that cross the orbit of Neptune. These are (42355)
Typhon-Echidna and (65489) Ceto-Phorcys. In the present work, we focused our
analyses on the temporal evolution of the Typhon-Echidna binary system through
the outer and inner planetary systems. Using numer- ical integrations of the
N-body gravitational problem, we explored the orbital evolutions of 500 clones
of Typhon, recording the close encounters of those clones with planets. We then
analysed the effects of those encounters on the binary system. It was found
that only 22% of the encounters with the giant planets were strong enough to
disrupt the binary. This binary system has an ~3.6% probability of reaching the
terrestrial planetary region over a time scale of approximately 5.4 Myr. Close
encounters of Typhon-Echidna with Earth and Venus were also registered, but the
probabilities of such events occurring are low (~0.4%). The orbital evolution
of the system in the past was also investigated. It was found that in the last
100 Myr, Typhon might have spent most of its time as a TNB crossing the orbit
of Neptune. Therefore, our study of the Typhon-Echidna orbital evolution
illustrates the possibility of large cometary bodies (radii of 76 km for Typhon
and 42 km for Echidna) coming from a remote region of the outer Solar System
and that might enter the terrestrial planetary region preserving its binarity
throughout the journey.
