  Recent studies suggest that only three of the twelve brightest satellites of
the Milky Way (MW) inhabit dark matter halos with maximum circular velocity,
V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM
simulations of the Aquarius Project, which suggest that MW-sized halos should
have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites
in such massive subhalos is thus puzzling and may present a challenge to the
LCDM paradigm. We note, however, that the number of massive subhalos depends
sensitively on the (poorly-known) virial mass of the Milky Way, and that their
scarcity makes estimates of their abundance from a small simulation set like
Aquarius uncertain. We use the Millennium Simulation series and the invariance
of the scaled subhalo velocity function (i.e., the number of subhalos as a
function of \nu, the ratio of subhalo V_max to host halo virial velocity,
V_200) to secure improved estimates of the abundance of rare massive
subsystems. In the range 0.1<\nu<0.5, N_sub(>\nu) is approximately
Poisson-distributed about an average given by <N_sub>=10.2x(\nu/0.15)^(-3.11).
This is slightly lower than in Aquarius halos, but consistent with recent
results from the Phoenix Project. The probability that a LCDM halo has 3 or
fewer subhalos with V_max above some threshold value, V_th, is then
straightforward to compute. It decreases steeply both with decreasing V_th and
with increasing halo mass. For V_th=30km/s, ~40% of M_halo=10^12 M_sun halos
pass the test; fewer than 5% do so for M_halo>= 2x10^12 M_sun; and the
probability effectively vanishes for M_halo>= 3x 10^12 M_sun. Rather than a
failure of LCDM, the absence of massive subhalos might simply indicate that the
Milky Way is less massive than is commonly thought.
