16501

We request 6 mid-cycle orbits using WFC3 in 2021 June to probe the newly suspected triple nature of main-belt comet 288P. This object is unique. It has wide separation comma nearly equal-sized components comma and also displays repetitive comet-like activity. 288P is one of only a few known asteroid binaries whose components can be spatially resolved by WFC3. Archival WFC3 observations obtained two months after the Cycle 28 deadline reveal that 288P is likely triple. That is comma the light curve of one of the two main components is best represented as the sum of the lightcurves of two (spatially unresolved) components with different periods. Additional data are needed to confirm the third component. Successful identification of the third component would make 288P the first-ever detected triple asteroid system to exhibit comet-like activity comma and provide crucial constraints on asteroid break-up models and on the role of activity-causing ice sublimation. We emphasize that these measurements maximally exploit the capabilities of HST for high resolution imaging. HST is the only telescope able to spatially separate the two main nuclei (maximum separation about 0.1 arcsec) and hence to measure their rotational lightcurves separately. The proposed observations are time-critical to be carried out in 2021 June comma which is the last opportunity to spatially separate the two main components before the return of dust activity in fall 2021.