Global warming imposes us to reflect on the way we carry research, embarking on the obligation to minimize the environmental impact of our research programs, with the reduction of our travel footprint being one of the easiest actions to implement, thanks to the advance of digital technology. The X-ray Integral Field Unit (X-IFU), the cryogenic spectrometer of the Athena space X-ray observatory of the European Space Agency will be developed by a large international consortium. The travel footprint associated with the development of the X-IFU is to be minimized. For that purpose, a travel footprint calculator has been developed and first released to the X-IFU consortium members. The calculator uses seven different emission factors and methods differing by up to a factor of ~5 for the same flying distance. The observed differences illustrate the lack of standards and regulations for computing the footprint of flight travels and are explained primarily, though partly, by different accountings of non-CO2 effects. The calculator enables us to compute the travel footprint of a large set of travels and can help identify a meeting place that minimizes the overall travel footprint for a large set of possible city hosts, e.g. cities with large airports. The calculator also includes the option for a minimum distance above which flying is considered the most suitable transport option ; below that chosen distance, the emission of train journeys are considered. To demonstrate its full capabilities, the calculator is first run on one of the largest scientific meetings; the fall meeting of the AGU and meetings of the IPCC for which it is used to compute the location that would minimize the travel footprint. Then the travel footprint of X-IFU is estimated to be ~500 tons of CO2-eq/yr. With this paper, the on-line travel footprint calculator is released to the science community (abridged).