An Oort cloud comet is composed primarily of dust and ice and has spent most of its life in the far-flung distant reaches of our Solar System (2,000 au to 200,000 au from our Sun). It is eventually nudged inward towards our Sun by gravitational influences such as galactic tides or some passing massive body, like a star or rogue planet for instance.
Usually when it arrives within the region of Jupiter's orbit, the volatiles which constitute a fair fraction of its composition, sublimate and eject from its surface. This ejecta creates a coma (the atmosphere around its nucleus) and also a long spectacular tail, sometimes visible in daylight from the Earth.
Comet C/2014 UN271, otherwise known as BB after its discoverers Bernardinelli/Bernstein, is a very strange cosmic snowball indeed. Its scale is unprecedented in the Oort cloud comet family, with a huge diameter of 137 km. The distance of UN271 at discovery was an appreciable 29 au away, approximately the radius of Neptune’s orbit around our Sun.
The forthcoming ESA F-class mission known as Comet Interceptor will hang out at the Sun/Earth Lagrange 2 point and will wait until a pristine Oort cloud comet is detected, most likely by the soon-operational Vera C. Rubin Telescope. It will then be dispatched for an intercept. Had UN271 been in range of Comet Interceptor, this object would no-doubt have been a prime candidate for a target. However UN271 will remain way outside of the Comet Interceptor’s comfort zone - as UN271 approaches the Sun, its perihelion will be far, far away, at approximately the distance of Saturn’s path around the Sun.
But are there other mission architectures which would allow an intercept of UN271? Alternatively, would a rendezvous be possible? Clearly the latter option, where the spacecraft must slow down to match velocities with the target, would garner more in the way of science return than a straightforward flyby, though the downside is that it would require more on-board propellant.
Not so long ago I set myself the task of utilising my Optimum Interplanetary Trajectory Software (OITS) to investigate the feasibility of a mission to UN271 . It should be noted that fundamental to the functionality of OITS is the assumption of instantaneous application of ΔV (velocity increment) at discrete points along the interplanetary trajectory. As a consequence, a high thrust propulsion system must be assumed, the most obvious being chemical (rockets) - however this isn’t so much of a restriction because most missions do involve chemical rockets.
Another drawback with OITS is that it requires the user to specify the exact sequence of points - usually planets – to be encountered between the home planet (Earth) and the target (in this case UN271).
It is up to the user, therefore, to identify the sequence likely to minimize the overall ΔV to reach the target of interest. (The user tries to choose a combination which maximizes the degree of gravitational assist which the spacecraft receives on its way to the target.) A blunderbuss kind of approach may be required, to attempt as many different combinations of inner planets as possible, running OITS many times over, to find which has the lowest ΔV, giving eventually the fuel-optimal route.
The latter process is precisely what I exploited for my research into missions to Comet UN271 and there may be a sequence with superior performance in terms of reducing fuel-usage, because there is no guarantee with this method that you will find the globally optimal solution. However I am fairly satisfied with what I was able to achieve in my investigation using OITS – a thoroughness and attention to detail which it is possible I was not able to achieve with my work on other targets.