Bob,
Is it safe to say then that if we detected a large comet inbound and just passing the orbit of Jupiter, and through observation it is determined that it will likely hit the Earth, there is NOTHING we can do to stop it? Can't use nukes because you just break up one large problem into many little problems.
If your answer is yes, and I think it will be, then let me ask the following:
Is the current scientific discussion on monitoring Near Earth Objects (NEO) just a sham and a waste of tax payer money? I realize its not a waste if you are a scientist who would receive grant money, but in the end, lets say you discover an Asteroid that is nearly certain to impact the Earth; there really isn't anything that can be done accept write your will and pray to your maker?
Jonathan
The answer to the first part is yes. You can't do anything.
https://www.cv.nrao.edu/~pmurphy/patsl9.html describes the the time-line of the shattered comet Shoemaker-Levy 9 collision with Jupiter July 1994. The comet was first identified on 24 March 1993. Before the end of March 1993 it was realized that the comet had made a very close approach to Jupiter in the summer of 1992. At the beginning of April 1993, after sufficient observations had been made to determine the orbit more reliably, it was found that the comet was in orbit around Jupiter. By late May 1993 it appeared that the comet was likely to impact Jupiter in 1994. Searches of archival photographs identified pre-discovery images of the comet from earlier in March 1993 but searches for even earlier images were unsuccessful.
So how much warning time might one have for an unknown interstellar comet? Comets travel at ~ 51 km/s.
https://www.lpl.arizona.edu/impacteffects/ The minimum and maximum separation distances from the earth to Jupiter are approximately 600 million km to 900 million km so we would have about a 5 to 7 month period from discovery in the plane of Jupiter until a close encounter with earth.
To intercept the comet, a rocket launched from earth must reach a velocity of at least 11.2 km/s or faster
https://en.wikipedia.org/wiki/Escape_velocity and the fastest velocity ever obtained by a spacecraft is 17 km/s
https://en.wikipedia.org/wiki/Voyager_1 Under the most favorable circumstances if an interceptor traveling at 17 km/s was launched within a month of discovery, the intercept would occur within 150,000 km of earth. Such a fast intercept is unlikely and a velocity of 12 km/s more likely and that would make the delay time even longer and the distance from earth even closer. As you get closer to earth you need to have a more powerful explosion to increase the debris dispersion (angular debris spread).
The problems are several.
1.) It only took a month to determine that the comet would hit Jupiter, but Jupiter has a cross-section 100 times than of earth and a mass 320 times greater than earth, so the predictions don't have to be anywhere as accurate. Bottom line. It would take a longer time to determine if the comet would hit the earth or just provide a close encounter. You don't want to blast apart something that will miss you, because you will increase the probability of a fragment hit.
2.) An interceptor rocket does not exist. Could one be designed, built, programed and launched less than a 5-7 months? Absolutely not.
3.) Effects of fragment impact.
https://www.space.com/scienceastronomy/mystery_monday_040412.html
https://www.lpl.arizona.edu/impacteffects/
https://www.armageddononline.org/asteroid.php
https://en.wikipedia.org/wiki/List_of_U.S._states_by_population_density
If a single 4 km fragment of an icy comet hit land, it would basically wipe out ~ 960,000 square miles. (by fire and overpressure) A land impact of this magnitude happens once every 80,000,000 years. If this happened in the US, you would expect 96,000,000 deaths.
If a single 2 km fragment of an icy comet hit land, it would basically wipe out ~ 240,000 square miles. (by fire and overpressure) A land impact of this magnitude happens once every 15,000,000 years. If this happened in the US, you would expect 24,000,000 deaths.
If a single 1 km fragment of an icy comet hit land, it would basically wipe out ~ 60,000 square miles. (by fire and overpressure) A land impact of this magnitude happens once every 3,000,000 years. If this happened in the US, you would expect 6,000,000 deaths.
If a single 0.5 km fragment of an icy comet hit land, it would basically wipe out ~ 15,000 square miles. (by fire and overpressure) A land impact of this magnitude happens once every 750,000 years. If this happened in the US, you would expect 1,500,000 deaths.
While the results of a larger than 0.5 kilometers comet impacting the US would be devastating, the US represents only about 5% of the world's land area. The probability of a 0.5 km diameter impacting the US is once in 15,000,000 years. The probable death rate is ~3x10-11 per year by a 0.5 km diameter comet impact rising to 2x10-10 per year for a 4 km diameter comet. The probability of dying from heart disease in 2x10-3 per year or 2x10-7 per year by getting killed by lightning.
I'm not worried.
Bob
