[pdthomas23 (Peter)]

Hello All,

Back in Sep/Oct 2013, 2013 TX68 was passing Earth's orbit on its way in to perihelion. It passed "under" the Earth and would still have been visible for southern observers up until Oct 12 and 13 but no-one caught it after Magdalena Ridge last saw it on Oct 9. The asteroid was then at low solar elongation and un observable. It passed back out again in Jan 2014 but was only mag 27 and then spent 2014 and 2015 travelling out to aphelion and back. When it crossed back in again in Dec 2015 it was still only mag 27. Now 2013 TX68 is heading back out again but this time both Earth and the asteroid will be near the crossing point at the same time.
The orbits don't precisely cross. 2013 TX68 passes up through Earth's orbital plane a bit before it crosses Earth's path within that plane. The uncertainty region is a long thin tube about 20 million km long but only about 200 km across stretched out along its orbit. The closest perpendicular distance from Earth to the uncertainty tube is about 20,000 km but since the tube is only 200 km across there is absolutely zero chance of impact. A close approach of 20,000 km would only occur if 2013 TX68 passes the crossing point on March 5 at the same time as the Earth is there. If it passes earlier or later the distance will be much greater.
When the JPL NEO office put out its original press release on Feb 2 the nominal time was on March 5 and the nominal distance was 1.3 lunar distances. The addition of the 4 PAN-STARRS observations from 2013-09-29 and 2013-10-04 that were reported on Feb 11 has shifted the uncertainty envelope back in time so that now the nominal pass is on March 8 at 13 lunar distances. The 1-sigma uncertainty is still 2 days so the pass could happen anywhere from March 2 to March 14 (out to 3 sigma) and thus a really close pass on March 5 is still possible, although unlikely.

Peter Thomas
Oakleigh