Asteroid Impact Mitigation: Why? How? When? Simon Green Planetary and Space Sciences School of Physical Sciences, The Open University Milton Keynes, UK.
Why. should we care?
Impacts everywhere? Physical evolution of Solar System bodies is driven by impacts of comets and asteroids NASA (Apollo 16) NASA/JPL-Caltech/Space Science Institute 3
Impacts on Earth Barringer crater 1.2 km ~50 000 y Popigai ~100 km 36 My Manicouagan ~100 km 214 My NASA S.F. Green Vredefort Dome ~300 km ~2 By NASA Wolf Creek 875 m < 0.3 My NASA LPI 4 LPI, V.I. Sharpton
Mass Extinctions Species loss: 85% 75% 95% 75% 75% LPI, David Kring 5
Iridium Layer Nature.ca, David Kring Shocked quartz Lawrence Berkeley Laboratory USGS 6
Chicxulub ~ 300 km multi-ring crater [Athena Review Vol. 2, no.1] ~ 30 km deep ~12 km diameter impactor Age 66 Ma ~50 000 km 3 of ejecta 7
1908: Tunguska Airburst at ~5-10 km Over 2000 km 2 damage ~ 50 m asteroid CIA/B.D. Bryant Leonid Kulik expedition 1929 8
15 February 2013 Near miss predicted 2012 DA 14 ~30 m Impact unpredicted: Chelyabinsk ~17 m asteroid Reuters P Chodas (NASA JPL) E. Guido/ N. Howes/ Remanzacco Observatory Константин Кудинов Kyodo/AP 9
How often do they hit? Interval (y) Chelyabinsk Megaton TNT 1 Estimated casualties 0 Tunguska 100 5 thousand Barringer 10 thousand 500 thousand 1 million 1 billion Mass extinction event 100 million 6 billion 10
How... can we deflect an asteroid?
Not like this 12
Find the next Impactor Potentially Hazardous Asteroids (PHAs) Minimum orbit Intersect Distance (MOID) < 0.05 AU (7.5 million km) Diameter > 100 m A. Fitzsimmons GOOD NEWS Many search programmes Under a 1000 NEAs with D > 1 km Almost all found BAD NEWS Estimated ~ 100 000 NEAs with D > 100 m and ~5000 PHAs Only around 25% of PHAs known We haven t tested any mitigation method PanSTARRS1 13
First observed impactor Catalina Sky Survey 1.5m Mt. Lemmon, Arizona 2008 TC 3 2008 TC 3 Catalina Sky Survey, University of Arizona 19 hours later Richard Kowalsky & Ed Beshore A. Fizsimmons, H. Hsieh, S. Duddy Image courtesy: EUMETSAT Peter Jenniskens (SETI institute/nasa Ames) 14
Impact Hazard Number < H Impact interval / years Diameter (km) Greatest risk from ~100-400 m objects Discovered objects 15
Relevant asteroid properties Orbit Size, shape Mass, density Structure Monolith or rubble pile? Spin properties Periods: seconds-many days Composition Observations required: Remote Spacecraft?? (sample return) 16
Space missions 1 Ceres 950 km Dawn 2015 1000 km 4 Vesta 530 km Dawn 2011 17
Space missions Asteroid Missions 100 km Ceres 950 km Dawn 2015 4179 Toutatis 4.8 2.4 2.0 km Chang e 2 2012 Vesta 530 km Dawn 2011 18 18
Space missions 10 km 9969 Braille Deep Space 1, 1999 Dactyl (243 Ida I) Galileo, 1993 4179 Toutatis Chang e 2, 2003 Itokawa Hyabusa, 2005 2867 Steins Rosetta, 2008 5535 Annefrank Stardust, 2002 19
Slow push: Gravity Tractor Pros No physical interaction with asteroid High degree of control Technology ready Astrium/NEOShield project Cons Requires decades warning time Only suitable for small (<~300 m) targets Station keeping in complex gravitational field 20
Impulse: Kinetic Impactor Pros Simple concept Impact targeting already tested Cons Requires ~decade warning time Not suitable for larger impactors Uncertain response dependent on asteroid properties 21
Kinetic Impactor: Momentum transfer Conservation of momentum M V = m v impact m M v sc 22 V
Kinetic Impactor: Momentum transfer Change in Momentum of asteroid M V = m v sc + m ejecta v ejecta M V = b m v sc v ejecta m ejecta M b is momentum multiplication factor Values from 1 (no ejecta) to >5, depend on v V
Some simple calculations 100 m asteroid, mass 1 million tonnes Require orbit position change of 10 000 km to safely miss Earth Gravity Tractor: Spacecraft mass: 5 tonnes Hover distance of 200 m from asteroid centre Ion engine with specific impulse of 10 000 N s kg -1 Gravitational force: F = G M m/r 2 = 0.0083 N Momentum change required: M DV = F t 10 9 x (10 7 /t) = 0.0083 t Time required t = >35 years DV = 9 mm s -1 Fuel required 0.9 tonnes 24
Some simple calculations 100 m asteroid, mass 1 million tonnes Require orbit position change of 10 000 km to safely miss Earth Kinetic impactor: Spacecraft mass: 5 tonnes Impact speed: 5 km s -1 Momentum enhancement: b = 2 Speed change DV = b (m/m) v sc = 2 (5000/10 9 ) 5000 = 5 cm s -1 Time required t = 6.3 years 25
Blast Deflection Buried/surface or stand-off Pros Suitable for shorter warning times Suitable for large impactors Cons Politically sensitive Cannot be tested under current space law Uncertain response dependent on asteroid properties Neoshield project 26
Credit: Tim Warchocki (adapted from National Research Council Final Report: Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies ). 27
Other methods Ion Beam Shepherd Laser Ablation Mirror bees University of Strathclyde Bryan Christie Design Astrium/NEOShield project Surface thrusters Mass Driver ejection of mass Electrostatic charge deflection Light Tug light absorption Yarkovsky effect 28
When... do we need to try?
30
Apophys Discovered: 19 June 2004 during close approach to Earth Diameter ~350 m Close approach in 2029 Impact probability rose to 1 in 37 (Torino scale level 4) Then fell to zero Possible impact in 2036 Torino scale level 1 Then also fell to zero Possible impact in 2068, probability 1 in 256 000 Why? 31
Impact probabilities View from asteroid along its trajectory Low probability of impact Position uncertainty ellipse In b-plane Impact probability 1 0 Time 32
Impact probabilities View from asteroid along its trajectory Increasing probability of impact Position uncertainty ellipse In b-plane Impact probability 1 0 Time 33
Impact probabilities View from asteroid along its trajectory Increasing probability of impact Position uncertainty ellipse In b-plane Impact probability 1 0 Time 34
Impact probabilities View from asteroid along its trajectory High probability of impact Position uncertainty ellipse In b-plane Impact probability 1 0 Time 35
Impact probabilities View from asteroid along its trajectory Certain impact Position uncertainty ellipse In b-plane Impact probability 1 0 Time 36
Impact probabilities View from asteroid along its trajectory Position uncertainty ellipse In b-plane Impact probability 1 Miss 0 Time 37
Impact probabilities View from asteroid along its trajectory keyholes Position uncertainty ellipse In b-plane Impact probability 1 Miss 0 Time 38
Lots of studies A Global Approach to Near-Earth Object Impact Threat Mitigation HAMMER: Hypervelocity Asteroid Mitigation Mission for Emergency Response 39
AIDA Asteroid Intercept and Deflection Assessment AIDA = DART + AIM 2016 40
AIDA target (65803) Didymos 2003 November 20-24 Phase P orbit = 11.9 h 0 0.2 0.4 0.6 0.8 1 Arecibo, November 2003 Primary rotational lightcurve (time-axis stretched) Range (15 m/pixel) Doppler frequency (0.3 Hz/pixel) Phase P orbit = 11.9 h 41
AIDA target (65803) Didymos Didymain D = 800 m 11.9 h orbit Didymoon 2.26 h Tidally locked? D = 160 m 1.18 km 42
Comparison with Deep Impact Comet 9P/Tempel vs. Didymos 160 m 800 m ~ 6 km/s DART ~ 350 kg 10 km/s Deep Impact 370 kg 6.5 km Predicted Didymoon orbit period change of 4 minutes 43
NASA DART mission continues Launch: Dec 2020 May 2021 Arrival: 7 October 2022 Deflection test on Didymoon (to 10%) Imaging before impact Ground-based observation support Status: In Phase B 44
ESA Hera mission proposed Launch: October 2023, backup Oct 2024 Arrival: Jul-Sep 2026 Didymoon mass Crater properties Binary asteroid science Interplanetary Cubesats Small JAXA Impactor? scaling laws MASCOT+ lander? Status: Funded for Phase B Decision for continuation Oct. 2019 45
Coming soon NEA Sample Return missions to primitive asteroids: Hayabusa 2 Arrival at (162173) Ryugu (~800 m) July 2018 OSIRIS-Rex Arrival at (101955) Bennu (~500 m) August 2018 Bennu shape model Michael C Nolan/Arecibo Observatory 46
(not ) The End 47
Effects of large Impacts K/T boundary type event: >1000 km radius immediate ignition radius Ballistic ejecta ignites vegetation over entire globe If ocean impact - global scale tsunami Global storms and high speed winds Dust loading: Nuclear winter for months or years If ocean impact water loading of atmosphere heats atmosphere Acid rains turn upper 100m of ocean acid enough to dissolve shell Ozone layer depletion and mutagens in atmosphere 48