def return_period_earth(energy_mttnt):
    """
    Return period of Asteroid/Comet of a given energy (Megatons TNT) in Years.
    :param energy_mttnt: Energy in Megatons TNT
    :returns: Return period of given energy level in years
    :Reference: EarthImpactEffect.pdf, Equation 3*
    """

    return 109 * (energy_mttnt ** 0.78)