  Dynamically cold stellar streams are ideal probes of the gravitational field
of the Milky Way. This paper re-examines the question of how such streams might
be used to test for the presence of "missing satellites" -the many thousands of
dark-matter subhalos with masses 10^5-10^7Msolar which are seen to orbit within
Galactic-scale dark-matter halos in simulations of structure formation in LCDM
cosmologies. Analytical estimates of the frequency and energy scales of stream
encounters indicate that these missing satellites should have a negligible
effect on hot debris structures, such as the tails from the Sagittarius dwarf
galaxy. However, long cold streams, such as the structure known as GD-1 or
those from the globular cluster Palomar 5 (Pal 5) are expected to suffer many
tens of direct impacts from missing satellites during their lifetimes.
Numerical experiments confirm that these impacts create gaps in the debris'
orbital energy distribution, which will evolve into degree- and sub-degree-
scale fluctuations in surface density over the age of the debris. Maps of Pal
5's own stream contain surface density fluctuations on these scales. The
presence and frequency of these inhomogeneities suggests the existence of a
population of missing satellites in numbers predicted in the standard LCDM
cosmologies.
