Orbital Debris: An Energy Management Problem

1. Orbital Debris: An Energy Management Problem by Don Kessler Retired NASA Senior Scientist for Orbital Debris Research Asheville NC

2. Summary • Current spacecraft operations have created an unstable orbital debris environment in Earth orbit • The instability in LEO will increasingly affect the design of spacecraft in LEO over a period of 10’s of years • At higher altitudes, the instability only becomes important over a period of 100’s of years. • Both the problem and the solution are related to how we manage the energy that is associated with objects in Earth orbit

3. The Solar System:Circular Orbits confined to a plane(stable system)

4. …Except for Comets and Asteroids:Asteroid Inclinations up to 20o(unstable system)

5. Space Shuttle Launch:Fuel converted to Kinetic Energy • 2,000,000 kg: Fuel • 110,000 kg: Shuttle (including payload) • Fuel / Shuttle mass ratio = 18 • At 7.5 km/sec Shuttle kinetic energy = 14 times its mass in TNT

6. Orbital Debris in LEO:Inclinations up to 145o(very unstable system)

7. Iridium 33 - Cosmos 2251 CollisionFebruary 10, 2009 Altitude 790 km

8. Iridium-Cosmos Collision Cosmos 2251 Debris Iridium 33 Debris One year after the Iridium/Cosmos collision, about 2000 fragments cataloged, as longitudes of nodes randomize

9. Number of Cataloged Objects in Earth Orbit Anti-satellite Test plus the Iridium/Cosmos Collision doubled fragment count Iridium/Cosmos China Anti-satellite Year

10. Mass and Energy in Low Earth Orbit • 2.5 Million kilograms of mass in LEO • 5000 upper stage rockets and payloads in orbit ….mostly non-operational. • Average collision velocity about 10 km/sec • Collision kinetic energy = 25 X debris mass in TNT • Total kinetic energy available in LEO = 60 kilotons of TNT • Comparable to nuclear bomb • Fraction released with every collision

11. Building an Earth Orbital Debris Model • Model sources of small particles -Collisional fragmentation between artificial Satellites -Release of small objects by artificial satellites • Model orbit changes of small particles -Atmospheric drag -Solar radiation pressure -Collisions • Convert orbital elements into flux via spatial density • Test model with observations

12. Rate of Debris GenerationWhen will collisions become dominant debris source? Rate of Debris Generation Amount of small debris proportional to Number Number of Objects in Orbit

13. Rate of Debris GenerationWhen will collisions become dominant debris source? Rate of Debris Generation Amount of small debris proportional to Number Rate of collision fragmentation varies as square of Number Number of Objects in Orbit

14. 1978 Predicted Collision RateAssuming various growth rates in the catalogue

15. Predicted Number of Fragments from a Collision between Cataloged Objects Sampled Population Experimental sampling began in 1984 Systematic sampling in 1990 Cataloged population (only data prior to 1983) 1978 model

16. Cataloged Objects DoD Goals for past 50 years: • Maintain a catalogue of man-made objects • No independent requirement for orbit accuracy • No requirement to increase sensor sensitivity • No requirement to keep track of satellite fragmentation events • Currently reevaluating requirements to meet need for collision avoidance warnings Eglin radar in Florida Telescopes in Maui

17. Sampling Returned Spacecraft Surfacesby counting objects hitting surface Orbital Debris impacts on returned spacecraft surfaces exceed the number of meteoroid impacts. Materials melted into the craters include aluminum, titanium, paint, copper, silicone, circuit board, sodium-potassium STS-118 Radiator panel Puncture 2 mm titanium-rich debris Entry hole 7 mm Exit hole 14 mm

18. Sampling using Radarsby counting objects passing thru beam MIT Haystack, HAX > 0.5 cm JPL Goldstone > 0.2 cm

19. Haystack Measurements Iridium 33 Debris Haystack data Cumulative Number 1998 Model Catalog data Debris Characteristic Length, meters

20. Haystack and Goldstone Measurements Cosmos 2251Debris Goldstone data Cumulative Number Haystack data 1998 Model Catalog data Debris Characteristic Length, meters

21. Orbital Debris Activities over the last 30 years • Obtained large amounts of new data • Developed more complex models • Operational support for NASA and other agencies • Spacecraft design support • Established international organization (IADC) • UN acceptance of Debris Mitigation Guidelines • Conclusion reached that current debris environment exceeds a “critical density” • Current National Space Policy expands debris activities

22. Orbital Debris Mitigation Strategy:Prevent Explosions in Orbit • Deplete the on-board energy source in spacecraft and upper stage rockets at end of operations • Unused fuel • Battery charge • Began in US in 1981 with Delta 2nd stage • Began in Europe after 1986 as a result of Ariane 3d stage explosion • USSR, Japan, China discussions followed

23. First Explosion in Orbit:Ablestar upper stage, June 29, 1961 2011: 50 yrs after explosion 1965: 3 yrs after explosion John Gabbard’s original plot

24. Orbital Debris Mitigation Strategy:Reduce Collision Frequency • Reduce orbital energy to decrease orbital decay time to less than 25 years • Plan final propulsion burn to drop perigee • Drag augmentation devices • Dilute population density in popular orbits with lower density graveyard orbits • Reduces collision rate • Short-term solution • May cause long-term problem

25. Critical Density • Question asked by various researchers: At what threshold will the number of objects in Earth Orbit reach a “tipping point” so that debris is generated from collisions at a rate faster than natural forces can remove the debris? • All have concluded using the most recent models that we have already exceeded that threshold.

26. Source vs. SinkCritical Density Analysis • Data from 1985 USAF P-78 Anti-Satellite & Transit laboratory hypervelocity tests • Defined two levels of Critical Density for constant number of intact objects. -Unstable threshold: Number of fragments increases with time until an equilibrium is reached -Runaway threshold: Number of fragments continue to increase for as long as the population density of intact objects is maintained

27. P-78 Breakup Fragments January, 1986: 68 Fragments larger than 1/1250 the mass of P-78 (243 cataloged fragments) 10-8 10-9 10-10 10-11 Source: Total of 90 fragments per collision with mass large enough to catastrophically break up another intact object Sink: Speed these objects decay from orbit Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

28. P-78 Breakup Fragments January, 1987: 66 Fragments larger than 1/1250 the mass of P-78 (189 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

29. P-78 Breakup Fragments January, 1988: 59 Fragments larger than 1/1250 the mass of P-78 (137 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

30. P-78 Breakup Fragments January, 1989: 41 Fragments larger than 1/1250 the mass of P-78 (57 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

31. P-78 Breakup Fragments January, 1990: 18 Fragments larger than 1/1250 the mass of P-78 (18 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

32. P-78 Breakup Fragments January, 1991: 16 Fragments larger than 1/1250 the mass of P-78 (16 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

33. P-78 Breakup Fragments January, 1992: 11 Fragments larger than 1/1250 the mass of P-78 (11 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

34. P-78 Breakup Fragments January, 1993: 9 Fragments larger than 1/1250 the mass of P-78 (9 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

35. P-78 Breakup Fragments January, 1994: 9 Fragments larger than 1/1250 the mass of P-78 (9 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

36. P-78 Breakup Fragments January, 1995: 9 Fragments larger than 1/1250 the mass of P-78 (9 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

37. P-78 Breakup Fragments January, 1996: 9 Fragments larger than 1/1250 the mass of P-78 (9 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

38. P-78 Breakup Fragments January, 1997: 8 Fragments larger than 1/1250 the mass of P-78 (8 cataloged fragments) 10-8 10-9 10-10 10-11 Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

39. P-78 Breakup Fragments January, 1998: 8 Fragments larger than 1/1250 the mass of P-78 (8 cataloged fragments) 10-8 10-9 10-10 10-11 Results can be scaled to other altitudes. Example: If P-78 test had occurred at 950 km, each frame would represent 100 years. This frame would represent the year 3285 with the concentration of fragments near 850 km. Spatial Density, No./km3 300 400 500 600 700 800 Altitude, km

40. Intact Rocket Bodies and Payloads:Regions of Instability in 1999 10-7 10-8 10-9 10-10 Unstable Runaway Runaway 1999 Catalog of intact objects Spatial Density, Number/km3 0 500 1000 1500 2000 Altitude, Km 0 500 1000 1500 2000 Altitude, Km

41. Fragment Population between 900 and 1000 km Assumes maintaining current intact population and eliminating explosions Intact population =400 Initial fragments =200 2000 1000 0 Number of fragments capable of a catastrophic collision with an intact object Initially, intact-intact collision dominate. Later, intact-fragment collision provide positive feed-back, increasing collision rate. Fragment population increases for as long as intact population is 400. 02004006008001000 Time, years

42. Fragment Population between 900 and 1000 km Assumes maintaining current intact population and eliminating explosions Runaway Intact population =400 initial fragments =200 2000 1000 0 Number of fragments capable of a catastrophic collision with an intact object Unstable At lower altitudes atmospheric drag would remove fragments quickly enough that an equilibrium can be eventually reached. Stable 02004006008001000 Time, years

43. NASA’s LEO-to-GEO Environment Debris (LEGEND) Model • Area/Mass distribution from experimental data -Orbital decay of fragments -Hypervelocity tests • Experiment-based breakup models • Includes non-fragmentation sources • Historical and future traffic models • Monte Carlo approach to collisions & explosions • Predicts future environment under various assumed conditions

44. LEGEND Predicted Collisions in LEOCompared to observed collisions(Average of 100 MC runs) Historical Business as Usual Post-Mission Disposal No Future Launches Data (excludes Cerise) Iridium 33 & Cosmos 2251 Thor-Burner upper stage Cosmos 1934

45. LEGEND Predicted Objects Between 900km and 1000 kmAssuming no launches after 2005 Average of 100 MC runs Collision Fragments Intacts + mission related

46. LEGEND Predicted Objects in LEOWith Post Mission Disposal (PMD)and Active Debris Removal (ADR) Average of 100 MC runs Removal Criteria: Probability of collision x mass Removal criteria: Ri(t) = Pi(t) mi

47. Active Collision Avoidanceas a Debris Mitigation Technique • Collision avoidance has limitations -Existing debris cannot maneuver -Operational spacecraft may chose not to maneuver, given the uncertainty in collision predictions • May be part of mitigation strategy -May reduce the number of required removals -Optimized to only prevent collisions of most likely and massive debris sources -Current population contains too few spacecraft with sufficient collision avoidance capability to be of benefit

48. Orbital Debris Remediation • 2010 National Space Policy: Pursue research and development of technologies and techniques …. to mitigate and remove on-orbit debris… • Search for the most energy efficient technique to remove on-orbit debris

49. Applying Past Technology for Remediation • NASA study concludes removing between 5 and 10 massive objects per year (for 100 years) is sufficient • UK study predicts needing twice the NASA number • Estimate 15% to 30% increase in cost to international space activities using past technology and robotics

50. Applying Advanced Technology: Debris Sweeper • To eliminate debris that happens to pass within 20 km of a spot in Earth orbit -Nature’s technique: A 40 km diameter natural Earth moon to sweep debris -Technology: Space or ground based laser to reduce the orbital kinetic energy, increasing debris re-entry rate • Negative issues: • Lasers could be an unintended hazard to other spacecraft • Reentry risk on the ground