Solution to the Sagan Dilemma via Surface Albedo Modification Technique

Shen Ge1, Virgiliu Pop2

1Shen Ge, Graduate Research Assistant, Texas A&M University, shenge86@gmail.com2Virgiliu Pop, Researcher, Romanian Space Agency, virgiliu.pop@gmail.com

Introduction to NEOs and PHAs

• Near-Earth object (NEO) is a solar system object whose orbit intersects with that of Earth.

• All NEOs have a perihelion distance less than 1.3 AU

• Potentially hazardous objects (PHAs) are NEOs that can be less than 0.05 AU from Earth and is at least 150 m

• Many studies have gone into deflection of such bodies away from Earth

Flyby of asteroid 2004 FH, closest miss ever noticed

Sagan Dilemma

• In early 1990s, Carl Sagan and other authors posed this challenge:

• “if one can deflect an asteroid away from a collision, one can also deflect an asteroid toward a collision”

Offensive Deflection

Consider a system capable of deflecting an asteroid in a time interval tr where

RE = deflection distanceΔV = velocity incrementP = asteroid orbital period

Below one orbital period, deflect along-track. Above one orbital period, deflect across-track

Finding the approximate phase space is just doing the same thing in reverse.

Note AE is the cross-sectional area of the Earth allowing gravitational focusing of bodies approaching 10 km/s

Ratio Q is the probability that an asteroid can be misdirected among asteroid that need to be mitigated!

Harris, A., Canavan, G., Sagan, C., Ostro, S. “Deflection Dilemma: Use vs Misuse of Technologies for Avoiding Interplanetary Collision Hazards.” 5 Nov 1993.

Impulsive Techniques

• Impulsive techniques create instantaneous large velocity changes.

• They include techniques such as– Stand-off nuclear detonation– Kinetic impactors

Impulsive Technique Deflection Distance

Sanchez, Cuartielles, J.P. “Asteroid Hazard Mitigation: Deflection Models and Mission Analysis.” PhD Thesis. 2009.

Sanchez has studied the ΔV requirements for deflecting an asteroid by 1 Earth radius given sufficient warning years

Q Ratio for Impulsive Technique

• Looking at the asteroid Apophis, we obtain 4.6% harmful deflection probability if we assume we can deflect across-track and– tr = P = 0.8865 years

– RE = 6371 km– ΔV = 0.1 m/s

Yarkovsky Effect

-Yarkovsky effect is the result of anisotropic heating of a celestial object.-As the sunlit side of the asteroid rotates away from the sun, the warmer dusk side radiates more energy than on the cooler dawn side. -The resulting net force acts in a direction that is determined by the asteroid’s spin axis, rotation rate, and orbital period.

Earlier Deflection -> Smaller ΔV

• The Yarkovsky effect can be modified by painting the surface to change the thermal distribution

• Whereas in the past, Yarkovsky effect modification will never be considered for an asteroid as large as Apophis unless given decades of forewarning, studies at JPL and elsewhere have shown its efficacy if enough surface modification is applied before the close approach near Earth in 2029 using the concept of keyholes.

Background on Keyholes

• Keyholes are cross-sectional regions in space which may intersect an asteroid’s orbit. If the asteroid passes through this keyhole, then upon some future cyclic return near Earth, the asteroid will have a definite chance of impact.

• For Apophis, the 2036 impact will only occur if it passes through a ~610 m keyhole in 2029

Deflection Time for Apophis-Studies done at Texas A&M has sown that mitigation efforts can start in 2023 for estimation error to match- Any malevolent intent for this mission can also only begin at this time to have enough certainty of striking Earth-It also cannot start too late since the deflection distance will be too little!

Q Ratio for Surface Albedo Modifier

• Looking at the asteroid Apophis, we obtain O(10-5)% harmful deflection probability if we assume we can deflect along-track and– tr = 6 years– P = 0.8865 years– RE = 610 m– ΔV = O(10-7) m/s

• Minuscule compared to 4.6%

Legal Implication: Nuclear Technique

• Nuclear explosive device used for peaceful purposes has no distinction from a nuclear explosive device that can be used as a weapon

• Article IV of Outer Space Treaty prohibits installation of weapons of mass destruction (WMDs) on celestial bodies and in Earth orbit

Legal Implication: Nuclear Technique

• Article II.2.d. of Peaceful Nuclear Explosions (PNE) Treaty forbids any explosion except in compliance with provisions of 1963 Moscow Treat

• Therefore, use of PNE says deflection of asteroids by nuclear explosions are illegal!

Legal Implication: Impactors

• Bilateral USA-USSR 1972 Anti-ballistic Missile (ABM) Treaty prohibits in Art. V the development, testing, and deployment of ABM systems, including space-based ones

• Asteroid deflection systems using missiles (nuclear or not), etc. may be accused of having real ABM capabilities.

Conclusion

• Modifying the Yarkovsky effect offers several advantages:– Minuscule ΔV dramatically lowers probability of

impact misusage– Long-term mitigation timeline negates tampering

with system at last minute– Answers legal obstacles encountered by other

more aggressive techniques

Questions?