def decrease_spacecrafts_weight():
    affected_spacecrafts = (Spacecraft.objects
                            .filter(mission__status="Planned", weight__gte=200.0)
                            .distinct())

    if not affected_spacecrafts.exists():
        return "No changes in weight."

    num_of_spacecrafts_affected = affected_spacecrafts.count()

    affected_spacecrafts.update(weight=F('weight') - 200.0)

    avg_weight = Spacecraft.objects.aggregate(avg_weight=Avg('weight'))['avg_weight']
    if avg_weight is None:
        avg_weight = 0.0
    avg_weight = round(avg_weight, 1)

    return (f"The weight of {num_of_spacecrafts_affected} spacecrafts has been decreased. "
            f"The new average weight of all spacecrafts is {avg_weight}kg.")