Optimal Asteroid Deflection in FreeFlyer

July 30, 2021

You’ve just found out that an asteroid is headed toward the Earth, what are you going to do? Assemble a team of rag-tag oil drillers and launch them into space? If you’re in Hollywood, that’s probably your answer. But in the real world, the answer is a little more down to Earth. You’d most likely model the asteroid’s trajectory and examine deflection options. Using FreeFlyer® you can do just that!

Every two years the International Academy of Astronautics hosts the Planetary Defense Conference. This exercise brings together hundreds of world experts to discuss the threat to Earth posed by Near Earth Objects and actions that could be taken to deflect an object that may impact the Earth. As part of the conference, an exercise is conducted with a simulated Near Earth Object. While many aspects of the simulation are realistic, the object is completely simulated and does not pose a threat to the Earth. The 2021 PDC recently ended, and this year the simulated NEO was given the designation 2021 PDC (or PDC21). This simulation was used as the baseline for the Mission Plan constructed in FreeFlyer with one key difference: in the simulation that took place at the Planetary Defense Conference, the asteroid (designated PDC21) was only “discovered” six months before its potential impact date. As such, a deflection mission was not possible. To allow for a deflection mission, in our FreeFlyer analysis the discovery date was changed to six years before the date of impact.

While there are various ways to deflect an asteroid (up to and including sending an oil driller), this Mission Plan only examines a kinetic impactor launched on a ballistic trajectory. This deflection method was chosen because it’s one of the most common approaches researched for deflecting asteroids, and because it doesn’t require complex explosives modeling as would be the case with a nuclear explosive device deflection attempt. One of the resources from the Planetary Defense Conference is the Near Earth Object (NEO) Deflection App created by NASA JPL and the Aerospace Corporation. This app allows the user to examine basic deflection trajectories for a variety of near Earth objects, including the simulated PDC objects. The app also provides information about the objects, as well as some information about launch vehicles that could be used to launch a deflection mission. This information was used to set some assumptions for the Mission Plan, including the spacecraft mass/launch energy relationship for the Falcon Heavy launch vehicle.  The NEO Deflection App was also used for initial trajectory planning and as a baseline with which results found in FreeFlyer could be compared.

After finding an initial guess trajectory from the NEO Deflection App, a Lambert solver was used to find an initial guess for launch delta-v values. While the NEO Deflection App and Lambert solver produced a valid trajectory, this needed to be refined further as both of those tools used a basic two-body model. Our Mission Plan used models the gravitational effects of all the planets in the solar system as well as the gravity of the Moon, Sun, and PDC21. This meant that when the launch delta-v found by the 2-body Lambert solver was applied to the spacecraft it did not intercept the asteroid when using real-physics model propagation.  To solve this issue an optimization process was used to adjust the delta-v values and find a trajectory where the spacecraft intercepts the asteroid.

The results of this optimization process were used to seed a second optimization process to solve for the maximum Earth miss distance. The second process used a similar setup to solve for the intercept trajectory but then continued propagating PDC21’s trajectory to the asteroid’s closest approach to Earth. The optimizer then attempted to maximize the distance from PDC21 to the Earth at this point by varying the impactor’s launch epoch, intercept epoch, and launch delta-v.

FreeFlyer found a trajectory that resulted in a miss distance of 1.697 Earth Radii (~10800 km). The NEO Deflection App found a miss distance of 1.253 Earth Radii (~8000 km). Two key differences between the Mission Plan and the NEO Deflection App result in the larger miss distance found by FreeFlyer. First, as mentioned earlier the Mission Plan built in FreeFlyer used high fidelity force modeling. This results in a more accurate trajectory for the interceptor spacecraft and the deflected asteroid. The other key difference is that FreeFlyer is using an optimizer to find the maximum miss distance. Conversely, the NEO Deflection App simply finds any valid solution, so their result is just one of many possible non-optimized solutions.

So, if you ever find yourself looking at an asteroid headed towards the Earth, hold off before strapping an oil driller to a rocket, just launch FreeFlyer instead.

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