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A 140 meter diameter round asteroid has a surface area of πD² or 61575 m² ( 6.16 hectares, 15.2 acres ). If the bulk density is 2 g/cm³ (2000 kg/m³), the mass M is πρD³/6 or 2.87 million tonnes, and the surface gravity is 4GM/D² = ⅔πGρD = 3.9e5 m/s² = 4μ gee. A 1 acre round asteroid is 36 meters in diameter. 

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MissionToAnAsteroid 

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.p29 Minor Planet Center Cambridge MA processes 50,000 observations per day .p29 to p33: 2008 TC3 3m object over Sudan .p48 Kodak TMax 400 film for 6 to 10 minute exposures, stereo microscope .p54 CCD survey NEAT ( NEA Tracking), Glo Helin .p55 now Spacewatch, Catalina (near Tucson) Sky Survey, PanSTARRS, LINEAR, 95% NASA funded .p72 2014 HQ124 300 m class 1.2M km closest approach .p74 bistatic radar, transmit from Goldstone and receive from Aricebo .p78 1994 [[ http://used.addall.com/SuperRare/submitRare.cgi?title=Hazards+Due+to+Comets+and+Asteroids  ''Hazards Due to Comets and Asteroids'' ed Tom Gehrels ]] . 
. p29 Minor Planet Center Cambridge MA processes 50,000 observations per day . p29 to p33: 2008 TC3 3m object over Sudan . p48 Kodak TMax 400 film for 6 to 10 minute exposures, stereo microscope . p54 CCD survey NEAT ( NEA Tracking), Glo Helin . p55 now Spacewatch, Catalina (near Tucson) Sky Survey, PanSTARRS, LINEAR, 95% NASA funded . p72 2014 HQ124 300 m class 1.2M km closest approach . p74 bistatic radar, transmit from Goldstone and receive from Aricebo . p78 1994 [[ http://used.addall.com/SuperRare/submitRare.cgi?title=Hazards+Due+to+Comets+and+Asteroids  ''Hazards Due to Comets and Asteroids'' ed Tom Gehrels ]] . p80 1994 ShoemakerLevy 9 into Jupiter 200,000 km/h (56 km/s) 24,000C fireball . p82 1998 movies ''Armageddon'' and ''Deep Impact'' . p87 NASA Planetary Defense Coordination Office in DC, Lindley Johnson Planetary Defense Officer . p89 Deep Impact Probe, 372 kg at 37,000 kph (10.3 km/s) into 7.6km × 4.9km [[ https://en.wikipedia.org/wiki/Tempel_1  9P/Tempel ]] 3e14 kg? 
Asteroids
Near Earth Resources with Low Cost Delivery
As of August 2016, the NASA Near Earth Object Program has estimated Orbital Elements and absolute magnitudes for almost 15,000 objects that cross the 1 AU spherical shell that contains Earth's orbit. The rate of discovery is rapid. From those magnitudes, we can estimate size and volume, more than 1E14 cubic meters.
Over a very long time, we arrange impacts between the near earth objects to break them into 50 meter or smaller chunks, and we tag them with milligramweight laser retroreflectors for high precision tracking. We also use lasers for ablative thrust. We line these up for delivery orbits to the Moon.
Meteorites are about 5% nickeliron. Assuming a density of 8000 kg/m^{3}, that's more than 4e16 kilogram supply  20,000 years worth at current global consumption rates; which MUST diminish over time, as we have depleted everything but lower grade iron ores. Heating iron from cold solid to a hot liquid requires about 1 MJ/kg.
We construct a 30 kilometer geodesic grid of 20,000 "smelting tunnels" across the back side of the Moon, deep holes with 100 meter throats and slight bends partway down. Over a very long time, each object is arranged to fall straight down a tunnel at high speed, to impact a melting chamber at the bottom, while a lid is fired over the top to contain impact ejecta. The objects will impact at greater than 2.6 kilometers per second (the squared sum of lunar escape plus orbital velocity), and have at least 3.4 MJ/kg; after an elastic collision into lunar rock, enough energy should remain in the object to melt it.
This would a very long term project, because arranging the impacts and planarizing the material, then delivering it at a steady rate, may require thousands of orbits of the Sun  thousands of years on average. However, the assumption of continued exponential growth into the far future is contrary to every observed system in nature. All growth (at best) follows an S curve, or worst case an bell curve impulse followed by extinction.
NEO predictability
Asteroid orbits are chaotic in the long term. Newton's Clock by Ivar Peterson (1993) says on page 189: "Nearcircular orbits can suddenly stretch after a few hundred thousand years of placid behavior, becoming so elliptical that they cross the paths of Mars and Earth." See work by Jack Wisdom when he was a grad student at Caltech in the 1970's (thesis adviser Peter Goldreich). 1982 paper in The Astromomical Journal, March 1982, 599593 and Nature 315 (27 June 1985): 731733.
Later recent references suggest some Marsinfluenced NEOs have Lyapunov times (errors increase by e) of 100 years or less. Steering them into orbits predictable enough to hit the Moon 1000 years from now requires calculating their position, velocity, and future trajectory to better than 100 meter precision now. On the other hand, an intentional centimeterpersecond velocity change today can evolve into the EarthMoon distance in 1000 years. So, if we can measure these objects to optical wavelength precision now (5e7 meters), and model and nudge them accurately enough (BIG if), then a 50 year Lyupanov time over 1000 years multiplies that optical error into a 0.25 kilometer error, which can in turn be corrected with another centimeterpersecond velocity change 7 hours earlier.
Deflecting useful potential impactors into the Moon (or Mars or Venus for the potential impactors deemed "useless") will require much continuous measurement and correction for a very long time, but the cost of protection is eternal vigilance.
A 140 meter diameter round asteroid has a surface area of πD² or 61575 m² ( 6.16 hectares, 15.2 acres ). If the bulk density is 2 g/cm³ (2000 kg/m³), the mass M is πρD³/6 or 2.87 million tonnes, and the surface gravity is 4GM/D² = ⅔πGρD = 3.9e5 m/s² = 4μ gee.
A 1 acre round asteroid is 36 meters in diameter.
Meteorites
We can estimate the task with the poor man's space probe  meteorites.
Asteroid Hunters
Carrie Nugent Tigard TEDbooks 2017 523.44 NUG
Caltech/IPAC., NEOWISE thermal infrared (earth orbit)
p03 by 2011, found 90% of >1km NEA, next target 90% of 140m NEA
p09 NEA = perhelion < 1.3 AU, 14,445 NEA discovered at time of writing
 p11 90,000 kg/day of dust and small rocks hit Earth
 p15 2013/02/15 Chelyabinsk 20 meters, pancaked and exploded at 38 km altitude, 53 km away, flash of light, shockwave broke windows
 p19 shockwaves circled globe twice in three days, registered on 20 of 283 UN infrasound monitor stations
 came from sunside where telescopes can't be used
 every 100 years or so
 p24 diagram/map of northern hemisphere observatories at night
 p29 Minor Planet Center Cambridge MA processes 50,000 observations per day
 p29 to p33: 2008 TC3 3m object over Sudan
 p48 Kodak TMax 400 film for 6 to 10 minute exposures, stereo microscope
 p54 CCD survey NEAT ( NEA Tracking), Glo Helin
 p55 now Spacewatch, Catalina (near Tucson) Sky Survey, PanSTARRS, LINEAR, 95% NASA funded
 p72 2014 HQ124 300 m class 1.2M km closest approach
 p74 bistatic radar, transmit from Goldstone and receive from Aricebo
p78 1994 ''Hazards Due to Comets and Asteroids'' ed Tom Gehrels
 p80 1994 ShoemakerLevy 9 into Jupiter 200,000 km/h (56 km/s) 24,000C fireball
p82 1998 movies Armageddon and Deep Impact
 p87 NASA Planetary Defense Coordination Office in DC, Lindley Johnson Planetary Defense Officer
p89 Deep Impact Probe, 372 kg at 37,000 kph (10.3 km/s) into 7.6km × 4.9km 9P/Tempel 3e14 kg?