def flux_density_gruen(m):
    """
    Flux density as given by Gruen (1985). May eventually be moved to a util file.

    :param float m: mass in grams
    :return: Number of objects per metre squared per second
    """
    n = (2200.0 * m ** 0.306 + 15.0) ** -4.38 + 1.3e-09 * (m + 
        100000000000.0 * m ** 2 + 1e+27 * m ** 4) ** -0.36 + 1.3e-16 * (m +
        1000000.0 * m ** 2) ** -0.85
    return n
