The Astrophysical Journal for Jan. 10 (vol. 978 issue 2) contains:
Obase, T. Bajo, K-i. Otsuki, Y. et al. Extreme Solar Particle Ejection Event in the Last Few Million Years from Asteroid Itokawa Sample art. 142 ad9919
The solar wind contains hydrogen and helium. Meteorites (and therefore, asteroids that spawn meteorites) contain trapped solar wind. Therefore, asteroids/meteorites contain hydrogen and helium. The hydrogen implants into silicates (SiO4-based crystals) to form water, and the inert helium is trapped in crystal lattices, unreacted. Therefore therefore, measuring such helium tells us something of the solar wind.
The asteroid (25143) Itokawa contains surface gravel and pebbles (“regolith”). These fines are formed by the gradual (eons and eons) wearing of larger rocks. Over those eons and eons, rock surfaces sit exposed to space. Therefore, Itokawa has a large exposed surface area. Therefore therefore, the Itokawa surface acts as a solar wind collector.
Obase et al., in studying Itokawa particles returned by the Hayabusa mission, tell us of those particles, and their record of the solar wind. Obase et al. claim that, going by the fossilized record of the solar wind trapped in Itokawa grains, there was a very, VERY large solar flare. Since the Itokawa record only extends as far as Itokawa has lasted (rocks came from somewhere, but will wear to nothing, given enough time), that constrains such a mega-flare to the history of Itokawa itself. Obase et al. put that as a few-million-year lifetime.
Asteroids: not just a record of the early Solar System, but a cold-storage record of the continuing Solar System.