The odds are good that eventually our planet will be hit by an asteroid or a comet, as it has many times before. Engineers and scientists here at NASA?s Langley Research Center are trying to spot these hazards before they sit. One good place to set up an astronomical alarm system is on the Moon.

A large number of asteroids larger than a half-mile across are in orbits that bring them close to Earth. If any of them were to hit our planet, it would cause social and environmental upheaval. Locating the whereabouts of these objects is the major job of several Earth-based telescopic searches.

A large number of near-Earth asteroids are around 300 feet across or smaller. Millions of these sized objects may exist, says Dan Mazanek, head of the Comet/Asteroid Protection System (CAPS) study. ?What we?re undertaking with CAPS is finding out what it takes to expand the range of detectable objects both in size and distance, to try and cover the entire hazard.? They hope to come up with a plan to scan the entire celestial sky on a regular basis, in order to create a continuous warning system, and to implement this system within the next 20 to 40 years. They hope to detect the objects with enough warning time to destroy or deflect them before they hit the Earth.

Smaller-sized asteroids cause localized damage. However, an impact near an urban area or coastline could result in considerable loss of life, extensive damage, and economic disruption, so even a small asteroid hit could rack up trillions of dollars worth of destruction. In all probability, Mazanek says, the next object that does hit us will be in the 165 feet to 330 feet size. ?We hope it hits somewhere without ramifications,? he says.

Then there are the long period comets. They do not regularly come into near-Earth space, because they have orbital periods that can measure many millions of years. They can arrive with little or no warning from conventional ground-based telescopes. ?In all likelihood, the ground-based telescopes are not going to have the sensitivity to pick up these objects with enough time,? says Mazanek. The group has the goal of being able to spot a small comet at 5 to 7 Astronomical Units (AU) from Earth. One AU is roughly 93 million miles. ?Depending on its orbital trajectory, that would give you somewhere around a year?s worth of warning time,? he says.

Warning times for long period comets might be only weeks or months. Only 10 percent of known long period comets have been discovered more than 100 days before passing by the Sun. For smaller asteroids, potentially no warning time is available if they have not been catalogued.

Then there are the objects with orbits that are a matter of chance. A lot of these objects have orbits that, over a period of time, will change. ?Being able to pick those out would be beneficial,? Mazanek says.

There are drawbacks to depending solely on ground-based equipment instead of using space-based detection systems. Viewing small and or dim objects by ground-based telescopes is significantly limited due to atmospheric turbulence, the telescope?s location, the day and night cycle of Earth, poor weather and the interference of the Moon. CAPS wants to put detection hardware in Earth orbit and even on the Moon?s surface.

The placement of CAPS in space will be easier to do in the future, when space has become an area of high activity, with a busy space station, lots of reusable in-space transportation, command and data handling services, lunar gateways, and crew and cargo transfer vehicles. ?That?s a big issue,? says Mazanek. ?Maintaining, servicing, and upgrading CAPS depends on what your vision is for the next 40 years.?

But nearly 30 years has passed since the last Apollo mission in 1972, and astronauts no longer expect to see a permanent human presence on the Moon. That may change in the future, Mazanek says. ?I think there?s a lot of strong arguments for the Moon being a very ideal astronomical location for these type of observations. It has a natural, 28-day rotation period.? If you have CAPS on the Moon, your only real worry is power, as long as you can protect equipment against the lunar environment.? The moon itself is a natural asteroid deflector, with many craters to prove it.

It may be possible to make use of asteroids or comets, such as mining them. ?There?s a huge amount of natural resources available in these objects. If we ever do go out and colonize the solar system, it?s pretty obvious that we?re probably going to use resources available in space and not bring everything from Earth,? Mazanek says. ?We have to come up with ways to live off the land.?

Part of the CAPS project is to investigate technologies and techniques that can deflect potentially hazardous near Earth objects. Mazanek calls it ?orbit modification.? It may even be possible for us to someday have a stockpile of small asteroids under our orbital control. ?There may come an object that?s so large you can?t deflect it. You could use your stockpile of small objects as a mechanism to fight the larger one,? says Mazanek. ?It is like a game of cosmic billiards, being able to use smaller objects to deflect larger ones. There?s an irony there that has a lot of appeal.?

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