Will we see it Coming?

The biggest risk to the Earth is from a long-period comet that appears at random from the Oort Cloud . Comets have a very low albedos, making them difficult to detect optically. However, as they heat up in the inner solar system, they will begin to out-gas forming extensive clouds of dust and gas known as "comae", which makes them fairly easy to detect. Comets start to brighten markedly when they reach a heliocentric distance of 3 AU from the Sun.¹⁶ (1 Astronomical Unit (AU) = 92,900,000 miles = Mean Earth Distance from Sun). Comets travel at speeds near 100,000 miles per hour. At this rate, they could impact Earth less than 77 days after they first begin to brighten.

Many amateur and professional astronomers constantly search the sky every night. Some are driven by the zeal to spot one of these majestic bodies, to be the first to view a new comet and to have the honor of naming it. A system is in place for reporting comets, verifying sightings, and computing comet trajectories.

In addition, a more methodical approach for detecting and tracking Near Earth Objects (NEOs) has been underway for several years. Since 1984, the Spacewatch program has searched the skies continuously and has identified a number of large asteroids. Much of these search efforts have been done by a handful of volunteers. The number of people searching for threatening astronomical objects is smaller than the staff of one McDonald’s restaurant. In 1992, NASA proposed a Spaceguard Survey that would detect asteroids and short-period comets larger than 1 kilometer across.⁸ But in general, very little has been done by the government to fund these efforts and address this hazard that could end our civilization.¹³ The Spacewatch program, the Near Earth Asteroid Tracking (NEAT) program, the Lowell Observatory Near-Earth Object Survey (LONEOS) and the Lincoln Laboratory LINEAR, the Catalina Sky Survey, BAO-SCAP, ODAS and AANEAS programs have achieved limited successes. There are approximately 900 to 1,200 Near-Earth Asteroids and short period comets larger than 1 kilometer in diameter; of these, we have discovered and charted the paths of approximately 587.⁵² These programs will be ineffective at identifying long-period comets, but should they stumble into one, it may provide mankind with sufficient lead-time to counter this threat.

An approach currently in the concept stage is the use of spacecraft to search and locate NEOs including asteroids, short period comets and long period comets.⁴⁰ In this concept, approximately 5 satellites (referred to as Shield Sentries) would be placed in a heliocentric orbit around the sun at a distance of 0.7 AU. These satellites would have greater detection sensitivity than Earth based telescopes. They would also be able to detect NEOs on Earth’s blind side. The greatest advantage to this concept is that they would push out the detection window another 9 months. This advanced impact warning time is critical to achieve any possibility of success with a mitigation approach. The advanced warning time would also have the added benefit of reducing chaos from the process of shelter preparation construction and stocking.

Bottom Line: We will probably see a comet coming and the Earth will probably have several days warning.

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