Complex sentence: Another very distant body is Sedna, which was announced in 2004, and although it has an over 10000 year orbit will be slowly come closer over the next 70 years to its perhelion.
Split into simpler sentences: Another very distant body is Sedna, that was announced in 2004. Although it has an over 10000 year orbit, it will be slowly come closer over the next 70 years to its perhelion.