Adam Hibberd
The 2024 YR4 of the title is an asteroid which was discovered in December of 2024 with a really cool Christmas present for humanity: the prospect of a collision with Earth only 8 years down the line (in 2032) and this with an associated high level of probability.
The result was initially a high Torino impact hazard score, which as is so often the way, initially escalated and then collapsed to zero after further more detailed observations and measurements of its orbit.
Attention has now changed from an Earth impact to a Moon impact with currently a probability of 4%, and this could potentially have dire consequences, particularly if we have men on the Moon by this time.
I have written a paper on the subject of missions to 2024 YR4 which can be found here. Its next encounter with the Earth/Moon system will be in December 2028, and this is known with an excellent level of confidence as a miss of both bodies.
Note that for a flyby mission to this asteroid on its next close approach in 2028, there are two possible arrival windows, the first in October of that year and the second in December.
It turns out that for purely astrodynamical reasons, the second arrival window (in December) yields a marignally lower Earth ‘Characteristic Energy’ for the launch vehicle when the flyby probe is launched to its destination. In other words the demands on launch vehicle performance are not so challenging, meaning a more massive probe can be accommodated and delivered by the launch vehicle to the target, which is clearly advantageous.
There is a caveat though. If one examines the ‘phase’ of 2024 YR4 as seen by the probe at the encounter (where phase is a measure of the fraction of the asteroid lit by the Sun with phase = 0 degs meaning fully lit and phase = 180 degs corresponding to completely dark), then the arrival in October 2028 would actually be preferable. Look at the Figure below which reveals all.
Referring to this plot we find that optimal routes launched in October 2027 to January 2028 arrive in October 2028 (the blue solid line and left vertical axis) and the orange dashed line (right vertical axis) shows the encounter phase of 2024 YR4 will be as low as around 20 degs. Thus 160 degs of the asteroid will be lit up by the Sun for this arrival date.
Launches subsequent to January 2028 arrive optimally in December 2028 and involve an encounter phase of around 155 degs, indicating only 25 degs of the asteroid will be lit.
To summarise all this, it means the shorter and more challenging launch window from October 2027 to January 2028 would actually be preferable to maximise science return from the probe.