the orbit model. This is important for two reasons: for imminent impacts it helps to make a better prediction of where the impact will occur and whether there is any danger of hitting a populated area. for asteroids that will miss Earth this time round, the more accurate the orbit model is, the further into the future its position can be predicted. This allows recovery of the asteroid on its subsequent approaches, and impacts to be predicted years in advance. Estimating size and impact severity Assessing the size of the asteroid is important for predicting the severity of the impact, and therefore the actions that need to be taken (if any). With just observations of reflected visible light by a conventional telescope, the object could be anything from 50% to 200% of the estimated diameter, and therefore anything from one-eighth to eight times the estimated volume and mass. Because of this, one key follow up observation is to measure the asteroid in the thermal infrared spectrum (long-wavelength infrared), using an infrared telescope. The amount of thermal radiation given off by an asteroid together with the amount of reflected visible light allows a much more accurate assessment of its size than just how bright it appears in the visible spectrum. Jointly using thermal infrared and visible measurements, a thermal model of the asteroid can estimate its size to within about 10% of the true size. One example of such a follow up observation was for 3671 Dionysus by UKIRT, the world's largest infrared telescope at the time (1997). A second example was the 2013 ESA Herschel Space Observatory follow up observations of 99942 Apophis, which showed it was 20% larger and 75% more massive than previously estimated. However such follow-ups are rare. The size estimates of most near-Earth asteroids are based on visible light only. If the object was discovered by an infrared survey telescope initially, then an accurate size estimate will already be available, and infrared follow up will not be needed. However none of the ground-based survey telescopes listed above operate at thermal infrared wavelengths. The NEOWISE satellite had two thermal infrared sensors but they stopped working when the cryogen ran out. There are therefore currently no active thermal infrared sky surveys which are focused on discovering near-Earth objects. There are plans for a new space based thermal infrared survey telescope, Near-Earth Object Surveillance Mission, due to launch in