Which method has NOT yet been used to study the material of the interstellar medium?

Question

Quizplus · Accepted Answer

Collection of dust and gas by spacecraft has not yet been used to study the material of the interstellar medium, as there have been no spacecraft missions that have been specifically designed for this purpose. The other methods mentioned - scattering of starlight, emission of electromagnetic radiation by atoms and molecules, and absorption of light from more distant stars - have all been used to study the properties and composition of the interstellar medium.

Which Method Has NOT Yet Been Used to Study the Material