This is kind of embarrasing to admit, but I had a long-time misconception about the delta-V requirements for reaching NEOs. A long time ago, I read some figure for delta-V requirements for earth-crossing asteroids. The figure was ridiculously low, something like 60m/s. At the time I read it, I didn’t really have a lot of experience with orbital dynamics, so I just filed the number away. I had assumed from what I read that that was the delta-V required to reach some near-earth asteroids. Unfortunately, while I wish I was the only one dumb enough to have made that mistake, there’s a good chance I wasn’t.
Anyhow, I probably would’ve figured it out a little quicker if I had been more interested in NEOs. I’ve always been a planetary chauvanist, and a Moon Firster at that. I always just waived away the much easier access to NEOs (which turns out not to have been as much easier as I thought) with the argument that while the transportation delta-V requirements were less, the trip times were a lot longer, and the difficulty of operating that far from home would likely drive the costs up a lot higher than just shear delta-V numbers alone would indicate.
So this misconception sat uninvestigated (and fortunately mostly harmless) for several years until earlier this week I was running some numbers regarding the so-called “Flexible Path” approach that was discussed by the Augustine Committee. To my surprise when I actually looked up the numbers, the closest and easiest to reach NEOs all required delta-Vs from Low Earth Orbit of greater than 3.8km/s (which is approximately delta-V needed to reach Earth-Moon L-1 or one of the Earth-Sun L-points). In fact some required over 10km/s of delta-V just for rendezvous! After thinking it through, it actually made plenty of sense. NEOs aren’t orbiting earth, they’re orbiting the Sun. So it makes sense that you would need to do an earth escape maneuver first (3.2km/s right there) plus some more to change your orbit to intersect there, and a final burn to match their orbits and rendezvous.
So where the heck did the 60m/s number come from? It turns out that the 60m/s number is the delta-V needed to depart the closest of earth-crossing NEOs in a trajectory that intersects with earth’s atmosphere. If you actually wanted to bring the returning vessel into LEO, unless you have a really good aerobrake you’re talking about at least 3.2km/s just to decelerate from an escape trajectory, and honestly it’s probably the same amount of delta-V to return from an NEO into LEO as it is to depart LEO and rendezvous with the NEO–as it typically is in orbital mechanics.
What this means to me is that the round-trip delta-V’s needed for NEOs, especially for missions that don’t just go directly to reentry, are actually a lot more demanding than I had ever suspected. Without extensive aerobraking, for a round-trip you’re looking at at least 7.6km/s of delta-V, ie nearly SSTO levels of delta-V. Even with aerobraking and in-situ propellant production at the NEO, you’re still talking at ~4km/s of delta-V on the outbound leg–which means that with a LOX/LH2 system, only about 1/3 of your LEO mass will even reach the asteroid. This also makes a propellant depot/transportation node at one of the Earth-Moon L-points look a lot more interesting for missions to NEOs. The delta-V from L-2 to most of those locations is around 1-2km/s, which means that most of your mass that leaves L-2 will arrive at the destination (about 65-80% for a LOX/LH2 stage, depending on your target).
In summary, I still think that NEOs have their place, and I still think that they do have some transportation advantages compared to going down into the lunar gravity well. But now that I’ve cleared up my misconception, it looks like actual near-term commercial exploitation of NEOs is not likely going to be any easier than commercial exploitation of the Moon.
Latest posts by Jonathan Goff (see all)
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- AAS Paper Review: RAAN Agnostic 3-Burn Departure Methodology for Deep Space Missions from LEO Depots (Part 2 of 2) - September 17, 2018