The Right Hand (RH) SRB's Lower Strut, which carries lateral forces only (no thrust). I've always been fascinated by this location because it was where the o-ring failed during Challenger's last flight. I was a senior in high school when it happened, had always wanted to work with the Shuttle, but never really dreamed I'd one day be here taking this photograph. The o-ring failed in the field joint just above the Strut, which is at (approximately) the 320 deg location circumferentially. The plume burned through the Strut, which allowed the SRB to rotate out and impact the top of the External Tank (ET), which started the process of total vehicle breakup. If the failure had occurred at a different circumferential location outside the SRB, there is a chance the vehicle may have not been destroyed.