Our Irregular Satellites Survey has as its main objective the exploration and characterization of the irregular satellites of the planets. This survey has discovered 63 of the 97 known irregular satellites, all within the last few years.

The irregular satellites of the planets are thought to have been captured from independent orbits around the Sun. Their scientific significance lies in the fact that such captures are effectively impossible in the modern-day Solar System and, therefore, must have occured at a much earlier time. Study of the irregular satellites therefore constitutes a window on an early epoch. Whether capture occurred in association with the planet formation epoch or later, from a clearing phase in the Kuiper Belt, is unknown.

A popular-level overview of this hot subject is linked here

A slightly more technical review is linked here

A more technical discussion of the underlying science is here