How to Weigh a Killer Asteroid at 22 Kilometers per Second

Estimating a mass for a potentially hazardous asteroid (PHA) is perhaps the single most important thing to understand about it, after its trajectory. Actually doing so isn’t easy though, as the mass for objects in the tens to hundreds of kilometers in size are too small to have their mass calculated by traditional radio-frequency tracking techniques. A new paper from Justin Atchison of the Johns Hopkins University Applied Physics Laboratory and his co-authors proposes a method that could find the mass of asteroids even on the smaller end of that range, but will require precise coordination.

In essence, the method is all about distance. A spacecraft will change its velocity based proportionally on the mass of the object it's approaching. However, for objects with a small enough mass, that perceived change in velocity, which can be measured, is too miniscule to be measured correctly.

To solve this problem, the authors suggest using another factor in the equation for change in velocity - the distance of the spacecraft to the asteroid. A spacecraft’s change in velocity is inversely proportional to the distance of its closest approach with an object - so smaller distances between the spacecraft and its target asteroid mean larger, and therefore more measurable, changes in velocity.

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Measuring such changes from far away is nigh on impossible, though. The authors propose a solution that is much closer to hand, by having a reconnaissance mission release a small CubeSat during its approach to a target asteroid. The Cubesat would float around 10km away from the asteroid, while the main mission itself would get within an extremely close distance. They calculated an altitude of just three times the diameter of the body itself, so for a 50m asteroid, that would only be a height of 150m.

Another influence on the spacecraft’s change in velocity is the speed with which it passes by its target. Again, this is an inverse relationship, so the faster the speed, the less the velocity changes. Ideally, a spacecraft would languidly stay at a minimum height above the asteroid’s surface for a long period of time, but due to orbital mechanics that is usually infeasible. But relatively slow relative speeds can still have a major impact on the mission’s ability to correctly estimate its target’s mass.

No matter how close, or how slow, a mission goes, though, the authors estimate that for asteroids on the smaller side (i.e. less than 140m in diameter), simple radio-frequency tracking between the Cubesat and its mothership is still insufficient. More precise measurements require more precise instrumentation, and the host spacecraft would need to be fitted with a Laser Rangefinding Instrument or High Precision Doppler Instrument to improve its sensitivity to a point where it can accurately measure even small mass objects.

One such mass measurement scenario is the current mission to Psyche. Credit - NASA Jet Propulsion Laboratory YouTube Channel

There’s still another operational bottleneck though - optical navigation. At high flyby speeds, the spacecraft’s cameras might not get a good enough look at the asteroid to calculate its precise position. Doing so is required to execute the safe, highly precise maneuver required to accurately calculate the object’s mass. While existing optical navigation systems would suffice for some of the easier scenarios, new systems would be needed for some of the faster flybys.

Speaking of faster flybys, the authors modeled some potential missions as a case in point. One of particular interest to current planetary defense efforts was a mission to asteroid 2024 YR4 - which as of the time of writing still has a 4% chance of hitting the Moon in six years, potentially causing significant damage of orbital assets around the Earth. In this scenario, the flyby for the main spacecraft would take place at a blistering 22 km/sec, even though the asteroid itself is only around 60m in diameter. That precise optical navigation system would come in handy for such a scenario - and that may very well be a realistic one sometime in the next six years.

Whether it will be necessary is still up for debate. But at some point in the future of humanity, such a mission will be necessary, to provide insight before we decide how to deflect or otherwise deal with a potentially dangerous asteroid. Using these advanced techniques could provide the necessary tools in our toolkit to deal with even the smallest objects - and if we even do have to use them, planetary defense specialists, and probably the wider public, will be glad for the work that’s gone into papers like this one.

Learn More:

J. A. Atchison et al. - Operational Mass Measurement for Flyby Reconnaissance Missions of Potentially Hazardous Asteroid

UT - Dramatically Decreasing the Time it Takes to Measure Asteroid Distances

UT - Force on Asteroids Measured for the First Time

UT - As Expected, the Threat from 2024 YR4 has Essentially Dropped to Zero