Introduction

Some investigations of the surface or sub-surface of near-Earth objects (NEOs) that are needed to support mitigation demand contact with the surface. The main examples are:

• Seismological methods, requiring both sources and receivers, to examine the internal structure of NEOs and look for cracks and voids that may influence the mitigation strategy and its effects.

• Surface and sub-surface mechanical properties measurements, to determine the material's response to drilling, digging, hammering, impacts, explosive detonations, etc. The type of measurements performed would depend on the mechanical interaction involved in the mitigation strategy being pursued (e.g., whether low or high strain rate).

• Measurements of sub-surface thermal properties and volatile content, with a view to using non-gravitational forces (outgassing) for mitigation.

• Emplacement of a radio beacon to help refine predictions of a NEO's future orbit.

• Radio transmission tomography, to examine the interiors of NEOs.

There are of course many other potential investigations requiring surface contact that appear to be rather less important for mitigation, being motivated wholly by science or space resources studies. For example, mitigation studies would seem to require compositional information no more detailed than that that can be determined remotely (e.g., by X-ray and infrared (IR) spectroscopy) and by comparison with meteorite analogs. An object's response to a mitigation technique is determined more directly by a set of key physical properties – particularly mechanical, thermal, and structural.