The images of comet Halley's nucleus returned during three spacecraft encounters in March 19861,2 are challenging the theory of the release of particles from a comet's nucleus. Instead of a spherically symmetric outflow, each spacecraft observed strong jets flowing from the sunward side of the nucleus. Here we present ground based 2-23 μm photometry and 10.3 μm imaging of Halley taken within hours of the Giotto spacecraft encounter. The photometry shows a colour temperature of 360 K and silicate emission features at 10 and 20 μm, and the image shows jet activity similar to that observed by Giotto, but on a scale of thousands of km. The expected 10 μm surface brightness, based on the particle mass distribution measured by Giotto and assuming solid, spherical grains, is a factor of six lower than the observed value. We suggest that fluffy particles could remove this discrepancy.