We argue that tracing star formation histories with GAIA using main sequence turn-off ( MSTO ) point dating will mainly be effective in cases of very mild interstellar extinction ( E _ { B - V } < 0.5 ) .
For higher reddenings the MSTO approach will be severely limited both in terms of traceable ages ( t < 0.5 Gyr at 8 kpc ; E _ { B - V } = 1.0 ) and/or distances ( d = 2 kpc if t \leq 15 Gyr ; E _ { B - V } = 1.0 ) , since the MSTO will be located at magnitudes too faint for GAIA .
AGB stars may alternatively provide precise population ages with GAIA for a wide range of ages and metallicities , with traceable distances of up to 250 kpc at E _ { B - V } = 0 ( 15 kpc if E _ { B - V } = 2.0 ) .
It is essential however that effective temperatures , metallicities , and reddenings of individual stars are derived with the precision of \sigma ( \log T _ { eff } ) \sim 0.01 , \sigma ( [ M / H ] ) \sim 0.2 , and \sigma ( E _ { B - V } ) \sim 0.03 , to obtain \sigma ( \log t ) \sim 0.15 .
This task is quite challenging for GAIA photometry and spectroscopy , though preliminary tests show that comparable precisions may be achieved with GAIA medium band photometry .