The impact of long-term trends on the space debris population Dr Hugh Lewis Astronautics Research Group, Faculty of Engineering & the Environment

2 In the movies…

3 In the news…

4 Image courtesy of ESA 18 cm 1.2 cm Impact at 6.8 km/s The space debris hazard… 4

5 Space debris sources

6 Top “10” worst fragmentations

7 Space debris population Softball size or larger (  10 cm) ~22,000~500,000~100,000,000 Total mass: 6,300 tonnes (2,700 tonnes in LEO) Marble size or larger (  1 cm) Ball-point pen tip (  1 mm)

8 Active satellites (Aug 2012) Union of Concerned Scientists Satellite Database seen in DAMAGE 999 active satellites

9 10 cm population (May 2009) ESA MASTER 2009 population seen in DAMAGE 29,370 objects ≥ 10 cm (2 catastrophic collisions)

10 UN space debris mitigation guidelines 1.Limit debris released during normal operations 2.Minimize the potential for break-ups during operational phases 3.Limit the probability of accidental collision in orbit 4.Avoid intentional destruction and other harmful activities 5.Minimize potential for post-mission break-ups resulting from stored energy 6.Limit the long-term presence of spacecraft and launch vehicle orbital stages in the low Earth orbit (LEO) region after the end of their mission 7.Limit the long-term interference of spacecraft and launch vehicle orbital stages with the geosynchronous (GEO) region after the end of their mission

11 Remediation Even with good compliance with the commonly adopted mitigation guidelines, the space debris population is likely to grow: –Active Debris Removal About 50 removals needed to prevent one collision $1 – $3 billion per year

12 LEO mitigation & remediation 30% compliance 90% compliance 90% compliance with 5 removals p.a.

Use Monte Carlo simulation to generate “reliable” statistics 13 Evolutionary modelling 30% compliance

Show the probability of a particular population outcome: 14 Evolutionary modelling 30% compliance “Population plume:”

15 LEO mitigation & remediation 30% compliance 90% compliance 90% compliance & 5 removals p.a.

16 LEO mitigation & remediation 30% compliance 90% compliance 90% compliance with 5 removals p.a. 81% of MC runs see an increase in the LEO population after 100 years 49% of MC runs see an increase in the LEO population after 100 years 99% of MC runs see an increase in the LEO population after 100 years

17 The “known unknowns” Predictions of the future are inherently difficult: –Launch traffic Launch rate Technology development Small satellites Programmes & missions Human spaceflight –Explosions –Security Anti-satellite tests Cybersecurity Conflict –Mitigation & remediation –Solar activity –Modelling capabilities

“Many futures” approach 18 Solar activity: Launch rate: Also: explosion rate, compliance with mitigation measures

19 LEO mitigation Traditional approach: 90% compliance “Many futures” approach: 90% compliance

20 LEO mitigation Traditional approach: 90% compliance “Many futures” approach: 90% compliance

LEO mitigation & remediation 21 Traditional approach: 90% compliance & 5 removals p.a. “Many futures” approach: 90% compliance & 5 removals p.a.

22 LEO mitigation & remediation Traditional approach: 90% compliance & 5 removals p.a. “Many futures” approach: 90% compliance & 5 removals p.a.

Long-term changes… Solar activity: –Sun enters an extended period of low activity after 10% of Grand Solar Maxima Thermospheric mass density: –secular decrease in thermospheric mass density in the range -2% to -5% per decade identified from TLE data (e.g. Emmert et al., Saunders et al.) 23

24 DAMAGE: thermospheric density trend Derived from work by Saunders et al., (JGR, 2010). Density multiplier Decades since 1970 h = 300 km

25 Effects on satellite decay Debris lifetimes extended by up to 25% ( Lewis et al., 2005) –Debris population increases at a faster rate –Collision risk increases Expected lifetime: 25 years Actual lifetime: 27 years

26 90% PMD and no removals No TREND With TREND

27 90% PMD and no removals 8,082 objects No TREND With TREND

28 90% PMD and no ADR

29 90% PMD and 5 removals p.a. No TREND With TREND

30 90% PMD and 5 removals p.a. 6,437 objects No TREND With TREND

31 90% PMD and 5 removals p.a.

Solar activity assumptions Future F10.7 cm solar flux Cycle 24 Cycle 25Cycle 29 32

Long-term decline in solar activity 33 5 removals p.a.

Long-term decline in solar activity 34 No TREND With TREND

Long-term decline in solar activity 35

36 Summary Many factors/mechanisms influence the evolution of the debris population and are difficult to predict –Large uncertainties in the predicted space debris population: –Active debris removal may not be necessary or may not be sufficient –Significant reduction in the debris population is difficult to achieve (“floor effect”) Evidence for long-term changes in solar activity and thermospheric mass density but these are not routinely modelled: –Can lead to rapid (and potentially sustained) population growth –Extended lifetimes counter the benefits of mitigation & remediation –Add to uncertainties “Unknown unknowns”: –Other sources of uncertainty? –Extrapolation of density trend into the future –Anomalous densities

Acknowledgements: Financial and technical support from: Holger Krag and Heiner Klinkrad (ESA), Richard Crowther (UK Space Agency), Adam White, Aleksander Lidtke & the Project SHARP Team (University of Southampton) Contact me: Ten Stories the World Should Hear More About: Space Debris

Back-up slides

Space debris population 39

40 30% PMD and no removals No TREND With TREND

41 30% PMD and no removals 12,948 objects No TREND With TREND

42 30% PMD and no removals