First discovered by the Compton Gamma Ray Observatory in 1994 (Zhang et al. 1994), GRO J1655-40 is one of only six Galactic superluminal sources observed so far. The binary system parameters have been calculated with great precision, owing to knowledge of the previous sources. Most other Low-Mass X-ray Binaries (LMXBs) reside within the plane of the Galaxy and are therefore obscured by gas, dust and debris, thereby blocking views of their companion stars.

Depending upon the attitude of the black hole’s orbital plane, it’s speed may actually be much faster than the reported 400,000 km/h, considering its calculated orbital speed of < a href= http://apod.gsfc.nasa.gov/apod/ap010508.html >approx. 450 times a second, a portion of which would be added to the binary’s trajectory speed.

When will it be at its closest? I could find no ready info on the topic, but the current distance is estimated at 6,000 light-years and, according to your posted article, its closest approach to Earth is estimated to come to about 1,000 light years (roughly 236 times further than Sol’s nearest stellar neighbor). Barring perturbation of material in the path of GRO J1655-40, which could be flung in the direction of the Solar system, the passage will be completely inconsequential to life on Earth.

Traveling at approx. 3,506,400,000 km/year (400,000 km/h [roughly 111 km/s]) with 1 light year being 9,434,828,404,210 kilometers, and covering a distance of 47,174,142,021,050,000 kilometers (5,000 light years), I would personally estimate the date for the beast's arrival (from 6,000 light years to 1,000 light years) to be between the years 13501000 AD, and 13499000 AD (approx. 13,500,000 years from now).

Not a mathematician, so I could be wrong,
Dan
--ps--
* Iso-delay surfaces in an X-ray binary (animation)