Target Earth?

Comets and asteroids are posing threats to Earth this summer in the theaters. The much acclaimed Deep Impact opens on May 8th. They may be the summer's biggest movie boxoffice hits, but the real threat of an asteroid or comet hitting the Earth, though minimal,  is no flight of fantasy.

The Earth is not an isolated body moving around the Sun in its orbit with the Moon. Though the planets and moons move about in a predictable way in their orbits, there are other smaller bodies moving around in the solar system which are not so predictable. Many have not even been discovered, and others have orbits subject to change as they are affected by gravitational influences. Some are asteroids, some are comets and some are even old spacecraft debris. They become important to us when they move close to, or cross the Earth's orbit because then there could be a chance of a collision.

These bodies which come close to, or cross Earth's orbit are called NEOs (Near Earth Objects.) They can be a hazard because they are usually quite small (usually less than 1 km) and their orbits may not be stable. As these bodies are discovered they are reported to the Bureau of Astronomical Telegrams at Harvard. Brian Marsden is in charge of this organization. When discoveries are made, Marsden confirms them, announces them to other astronomers and as more observational data is returned, the orbit of the object is plotted.

The reason we need to identify these objects and know their orbits is because sometimes, collisions do happen. Earth has it's own scars from celestial mishaps . The most widely known is Barringer crater in Arizona. It is a large crater, nearly a mile in diameter (about the size of lower Manhattan.) The impact was formed about 25,000 years ago by a meteorite roughly 50 meters across. The crater is 600 feet deep with a outer rim about 150 feet above ground level. The object that made Barringer Crater hit traveling at 25,000 miles per hour, creating an explosive force equal to 1000 Hiroshima bombs. And it was a celestial attack which ended the reign of the dinosaurs 65 million years ago.Over 200 impact sites are known on Earth, and more are being discovered. Just last year planetologists (those who study extraterrestrial bodies) have discovered an impact crater quite close to us, in the Chesapeake Bay area. The center of the impact is just off the coast of Cape Charles on the Delmarva Peninsula. Indeed, Cape Charles itself sits overtop the crater!

 A chilling example of how close asteroids can come, is Near-Earth Asteroid 1995CR. The asteroid was discovered during the course of a dedicated detection program for Near Earth asteroids at the Steward Observatory 0.9-meter telescope on Kitt Peak mountain.  1995 CR crosses the orbits of four inner planets, Mercury, Venus, Earth and Mars, and is on a
highly chaotic and unstable path. Astronomers believe the asteroid will eventually collide with one of these planets or with the Sun, or may be ejected from the Solar System by a "sling-shot" effect. The asteroid, which is only about 183 meters in diameter, passed within 9 million km in 1995. While 9 million km may sound far, it is only a fraction of the distance to the Sun..

If an object with the size and orbit of 1995CR were to hit the Earth, it would have an impact velocity of more than 69,000 miles per hour. Current impact models suggest this would make a crater about two and a half miles across, releasing energy equivalent to a one thousand megaton bomb. While it would not cause global destruction, it would certainly cause regional devastation.

More recently, asteroid 1996JA1 passed within 453,000 km from us, just a little farther than the moon. This asteroid was even larger, at 500 meters across, over 10 times the size of the object which made Barringer Crater! But there are smaller asteroids which have passed much closer to Earth. In 1994 asteroid 1994 XM passed only 100,000 km from Earth, less than 1/3 the distance to the Moon!

 Astronomers have known for a long time that these objects come so close to Earth, and that they sometimes impact on the planet. Recently, the government has begun to take notice and several committees were formed to assess the hazard. In the US, the Morrison committee reported: "The greatest risk from cosmic impacts is associated with objects large enough to disturb the Earth's climate on a global scale by injecting large quantities of dust into the atmosphere. Such an event would depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several billions of tons. The corresponding threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km. Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat."

 Several groups have now been formed around the world are dedicated to searching for and tracking NEOs. Most of the objects identified are absolutely no hazard and do not bear watching once they have been identified. Scientists cannot overstress the importance of these groups. The Spaceguard, survey was proposed by NASA, but is considered to be a long term project, and must be funded as such. Most of the programs are running on very low funding and may find it difficult to survive for more than a few years.

 NASA is planning on starting a new program (as yet unnamed) which will concentrate on the NEO hazard problem. 3 million dollars is allotted for the program which will concentrate on coordinating data from spacecraft and ground based observations

 Retired NASA JPL astronomer Fred Peters explained why it is so important to make many observations. "You compute the orbits from observational data, then you have to project your data when you want to know where the object will be in the future. The problem is that the margin for error grows the farther into the future you go. Accuracy is directly related to actual observation, not projection." This is especially true for objects which may have their orbits altered by gravitational influences of other objects. You could plot an orbit for 3 years and then find the object passes close enough to Mars that its orbit is altered slightly. The NEOs cannot simply be identified, continuing observational data must be gathered.

 So are we in danger, how much do we really need to worry about unwanted celestial visits? Currently there are no NEOs on a direct collision course for Earth. The trouble could come from an object which has yet to be discovered, or from objects whose orbits need more data to make them accurate. Brian Marsden, head of the Central Bureau of Astronomical Telegrams, and to whom all new discoveries are reported, feels that the most important thing we can do is gather more data. While he does not believe that we need to panic, he says : "on the other hand, that one should immediately endeavor to find observations at other oppositions, e.g., on old plates. Most observatories, and especially NASA have old photographic plates of the sky. These could be used to identify NEOs so that additional observational data is gathered." The late astronomer Carl Sagan once said that "estimating an impact on any given day would produce huge odds, but given  enough time, it becomes an inevitability."

Take heart though, if you are the type to worry. Some scientists have theorized that if we discovered an object on a collision course, we could take atomic weapons into space to use to divert the object. In theory, it would work, but let's hope we don't have to put it to the test anytime soon!