Although hypothesized in the carbon phase diagram, a general criterion to explain the formation of sp bonded carbon chains (carbynes) is lacking. We propose an energetic approach to explain carbyne formation in two conceptually simple configurations in which impact events play a major role, namely, in ancient meteorite craters and in the synthesis of cluster-assembled carbon films. For both configurations the calculated energies per particle equal each other. In one case pressure and temperature values match the carbyne region in the carbon phase diagram. A threshold range of 130-180 eV particle(-1) is suggested as an energetic condition for carbyne formation. (C) 2003 Elsevier B.V. All rights reserved.