  We estimate the likelihood of direct injection of supernova ejecta into
protoplanetary disks using a model in which the number of stars with disks
decreases linearly with time, and clusters expand linearly with time such that
their surface density is independent of stellar number. The similarity of disk
dissipation and main sequence lifetimes implies that the typical supernova
progenitor is very massive, ~ 75-100 Msun. Such massive stars are found only in
clusters with > 10^4 members. Moreover, there is only a small region around a
supernova within which disks can survive the blast yet be enriched to the level
observed in the Solar System. These two factors limit the overall likelihood of
supernova enrichment of a protoplanetary disk to < 1%. If the presence of short
lived radionucleides in meteorites is to be explained in this way, however, the
Solar System most likely formed in one of the largest clusters in the Galaxy,
more than two orders of magnitude greater than Orion, where multiple supernovae
impacted many disks in a short period of time.
