Growing demand for cost-effective satellites has renewed interest in silicon (Si) cells for space missions. However, these cells experience significant radiation-induced performance loss, which can be mitigated using ultra-thin wafers. Recently, ultra-thin Si heterojunction (SHJ) cells have emerged as strong candidates for low-cost, lightweight satellites. Their behaviour under space-relevant temperatures and air mass zero conditions, before and after electron irradiation, is therefore essential to understand. This study examines the temperature-dependent performance of ultra-thin (50 μm) SHJ cells under such conditions and compares their behaviour to 180-μm SHJ cells and cell structures without heterojunctions. We find that the performance of SHJ cells drops sharply at low temperatures regardless of wafer thickness, dominated by reduced fill factor, whereas structures without heterojunctions show linear improvement as temperature decreases. Notably, irradiated ultra-thin SHJ cells show a self-curing capability after annealing at 80 °C, enabling partial performance recovery even during electron irradiation in space. Additionally, their specific power surpasses that of the other structures across −20 °C to 80 °C. The established models reproduce the experimental trends, offering deeper insight into their low-temperature behaviour. These findings reveal a low-temperature performance threshold for SHJ cells and underscore their importance for evaluating and optimising them in space applications.