Note 1. The Palermo Technical Impact Hazard Scale was developed to enable NEO specialists to categorize and prioritize potential impact risks spanning a wide range of impact dates, energies, and probabilities. Actual scale values less than -2 reflect events for which there are no likely consequences, while Palermo Scale values between -2 and 0 indicate situations that merit careful monitoring. Potential impacts with positive Palermo Scale values generally indicate situations that merit some level of concern. The scale compares the likelihood of the detected potential impact with the average risk posed by objects of the same size or larger over the years until the date of the potential impact. This average risk from random impacts is known as the background risk. For convenience the scale is logarithmic, so, for examples, a Palermo Scale value of -2 indicates that the detected potential impact event is only 1% as likely as a random background event occurring in the intervening years, a value of zero indicates that the single event is just as threatening as the background hazard, and a value of +2 indicates an event that is 100 times more likely than a background impact by an object at least as large before the date of the potential impact in question. ("The Palermo Technical Impact Hazard Scale" (

Note 2. The power available from the Sun at the Earth's orbit above the atmosphere is 1.4 kilowatts/square meter. At normal incidence this energy is capable of raising the temperature of an ideally black (non-reflecting) surface to 396 K. Although water ice readily evaporates at this temperature, silicate minerals such as forsterite (MgSiO4) or quartz (SiO2) require temperatures in the neighborhood of 3000 K before they begin to evaporate at 1 atmosphere (atm) pressure.

Note 3. An impulse of 3.87 x 108 Newton seconds (Ns) momentum (grazing encounter) or 7.37 x 108 Ns (safe encounter) has to be applied. Because of the required enormous total impulse, more than one launch is necessary.

Note 4. Based on the estimated velocity budget and the known launch capability, the total mass of spacecraft at time of arrival at Athos is determined as 9574 kg (the specific impulse of spacecraft propulsion is assumed as 350 s). The relative velocity within Athos' direction of flight amounts to 8670 m/s. After applying the momentum enhancement factor of 3, the impact of a single spacecraft evokes a total impulse of 2.49 x108 Ns. Thus, for deflection into a safe encounter, three spacecraft are necessary, which equals a total impulse of 7.47x 108 Ns. Nevertheless, the launch of four identical spacecraft is scheduled instead. After impact of all four spacecraft the post-interaction tracking will reveal the correctness of the enhancement factor assumption. In a worst-case scenario, it would turn out that the enhancement factor is minimal (1.2). In this case, the applied impulse (3.98x 108 Ns) would still be sufficient for the deflection into a grazing encounter. On the other hand, for larger enhancement factors the encounter distance would be even larger than required for safe encounter.

Note 5. If the plane change maneuver is performed close to the apogee (1 of the 2 intersection points), the velocity of the striker is low (< 8 km/s) and the required delta-v for the plane change is of the order of 85 m/s. Such a deflection requires a solid and compact structure of the striker and an appropriate blasting strategy, probably multiple small nuclear blasts spread over several periods. It is observed that the kinetic energy required for the plane change of the striker is of the same order of magnitude as the kinetic energy required for a deflection of Artemis of 0.5 m/s. In addition, the orbital parameters of the proposed striker are far from the optimum.