The solar wind carves out a cavity, known as the heliosphere, in the warm local interstellar cloud, which is itself embedded in a larger hot cloud. It is generally assumed that there is an overall pressure balance between these three regions. Thermal pressure and magnetic field pressure in the local interstellar cloud should therefore balance the inward pressure from the hot cloud, and determine the size of the heliosphere. Here we present direct measurements of the density and speed of interstellar hydrogen and helium ions deep inside the Solar System, from which we derive the thermal pressure in the local interstellar cloud. Combined with the fact that the magnetic field strength in the local cloud is constrained to be less than 4.3 μG (to be compatible with the fact that the Voyager 1 spacecraft has yet to encounter the heliosphere termination shock), the total pressure that we infer is insufficient to balance the inward pressure from the hot cloud. We conclude that either the magnetic field in the local cloud is inhomogeneous, or there is a significant, as yet undetected, non-thermal component to the pressure in the local cloud.
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