The relevance of the bulk density as a physical parameter characterizing interplanetary dust grains is discussed. The various measurements which lead to a determination of this parameter are reviewed. The specific case of the collected interplanetary dust grains is considered.

The bulk density of interplanetary dust grains has been and is still a matter of controversy. This quantity cannot, in general, be directly measured; it is used to relate the mass and the size of a grain. This duality stems from physics itself as there are interactions sensitive to the mass (e.g., gravitational forces) while others are sensitive to the size or the cross-section (e.g., light scattering, radiation pressure, gas and plasma interactions). The measuring technics of the grains reflect this duality as, for instance, impact sensors are generally sensitive to the kinetic energy and thus to the mass, while optical sensors are sensitive to the cross-section. One sees that the density is not strictly speaking the relevant parameter, but what is needed is a relationship between mass and average cross-section.