What is the optimum space junk to target for deorbiting?

Question

The danger posed by space debris depends on the debris' mass, ballistic coefficient and orbit.

For a given mass of debris, a single large mass (like an intact dead satellite) poses a much smaller risk to other spacecraft than the same mass broken into fragments. However, massive debris stays in orbit longer, before it spontaneously deorbits, than the cloud of small debris. And the longer the large mass stays in orbit, the more likely it is to hit something and turn into a cloud of small debris itself.

Debris in orbits less than 600km tend to deorbit spontaneously.

Deorbit fuel cost is proportional to debris mass. But rendezvous fuel cost is proportional to the number of fragments that make up that mass.

Vulnerable target density varies widely with orbital altitude and inclination. It makes sense to make a priority of debris in orbits of high target density.

Source

On the other hand, debris which shares a coplanar orbit with its targets has a lower relative velocity so it is less likely to be broken up by collision with other debris and less likely to do damage if it collides with a target.

Very confusing.

Deorbiting uncooperative debris will be very expensive, so efforts should be focused on low cost: benefit debris. But which debris?

What combination of mass, altitude and orbital inclination make for the best deorbit targets in terms of damage reduction ?

Answer 1

This area has provoked quite a bit of interest in recent years.

The typical approach is to try and get a handle on what objects would make the biggest difference if they could be removed. Of course that could be interpreted different ways, not least as it could become political.

The following study is notable for contributions by a broad swathe of the research community. Identifying the 50 statistically-most-concerning derelict objects in LEO

They mention a variety of parameters that are of interest:

• mass - as a collision with a large object would produce more fragments and thus drive up the secondary risk
• encounter rates
• orbital lifetime - can be thought of as modifying the former as the "lifetime encounter rate"
• proximity to operational satellites - adds a weighting to some objects

Another parameter would be the tendency of similar objects to auto-fragment, though I'm not sure if there is a large population of these at present in LEO.

Answer 2

Answer: Big junk in crowded polar orbits. Removal of 100 largest bits of junk is enough to prevent an exponential cascade of debris.

This Japanese study identifies the orbital bands which are at the highest risk of creating an exponential collision cascade and model how many large debris objects need to be removed to negate the cascade risk. https://www.jstage.jst.go.jp/article/tstj/7/ists26/7_ists26_Pr_2_7/_pdf#:~:text=Electrodynamic%20tether%20is%20promising%20as,from%20crowded%20regions%20are%20proposed.

They identify these characteristics of optimal removal targets:

1. Large size (>0.5m^2) since they are the biggest sources of collision fragments which lead to cascades.

2. Crowded orbital bands :

-Sun synchronous orbits 700-1000km altitude and 98-100 inclination

-900-1000km at 83* inclination

-1400-1500km altitude at 74*, 83* and 52* inclination

Lower orbits tend to clear themselves from atmospheric drag. GSO satellites are at relatively low risk.

There are 400 objects which meet the above criteria. If 25% of these objects were de-orbited, it would negate the risk of a collision cascade.

Since these are highly used orbits, there many planned future launches. This is bad (more targets and potentially more debris) but also a good opportunity for active deorbiting spacecraft to hitch a ride into the most problematic orbits.

Once orbited, rendezvous is relatively economical. A delta-v of 50m/sec would be adequate to use the J2 effect to change right assention by one degree every 20 days. Phase angle adjustment could then be accomplished by a delta-v of 35m/sec. In general, rendezvous in these crowded orbital bands can be achieved with propellant about 8% of the mass of the spacecraft.

The paper favors deorbiting by attaching an ElectroDynamic Tether (EDT). Modeling showed a 10 km tether could deorbit a 3,400kg satellite within a year. Thrust from the EDT is much lower than a propulsive deorbiter, so attachment should be easier.