Observers with small telescopes in Japan, Eurasia and
Africa will have the best chance to spot MESSENGER as it makes
its closest approach to Earth - 1,458 miles (2,347 kilometers) above Mongolia
at 3:13 p.m. EDT (19:13:08 UT) on Aug. 2. The latitude and
longitude of the point on Earth's surface directly below MESSENGER
at closest approach is 47.09 degrees North, 252.54 degrees West - a point
59 miles (95 kilometers) south-southeast of Mongolia's capitol city, Ulaanbaatar.
The spacecraft will be shining at 9th magnitude, or perhaps
brighter, but not likely visible to the unaided human eye.
A few hours later, as MESSENGER speeds away from Earth over South America, observers with large telescopes in North America might see it low in the south after sunset, at perhaps 14th magnitude.
Observers can obtain details of the path in the sky computed for their location at the Jet Propulsion Laboratory's Horizons Web site, http://ssd.jpl.nasa.gov/cgi-bin/eph. Select "MESSENGER spacecraft" for the target body.
Ground track view of MESSENGER Earth flyby
Details of the orbit at closest approach are provided here