Has a comet already passed by the Earth that would pose a danger to the Earth in the future?

Answer

Dear Gilles,

The short answer to your question is no. There is currently no known comet that poses a threat to Earth in the future. There are a number of asteroids known that have a (very small) chance of ever colliding with the earth. Some of these have already (considered on an astronomical scale) passed close to Earth or will do so in the future, before the possible collision encounter.

Arguably the best known of these objects is asteroid 99942 Apophis, which has been in the news a while back. On April 13, 2029, this object will come very close to Earth (but the probability of a collision will then be zero). However, the encounter will cause the object’s trajectory to be altered due to the gravitational influences, which could lead to a collision on a later pass. Currently, according to NASA, there is a one in 45,000 chance that asteroid 99942 Apophis will collide with Earth on April 13, 2036. About a year later, the thing comes close again, but the chance of impact is much smaller. These estimates are based on more than 700 observations of the asteroid over about two years. The longer an object is observed, the more accurately one can determine its trajectory, and therefore the better one can say how great the probability of a collision is. The current values ​​put Apophis on the lowest rung of the collision risk scale. This means that the probability of a collision with Apophis is no greater than the probability that we are more likely to collide with an undiscovered object. Apophis (then still known under the provisional name 2004 MN4) made the news at the end of December 2004 when, based on the few observations available at that time, a collision probability of one in 37 was obtained (for April 2029). This put the asteroid higher on the risk scale than any object ever. However, the following days and weeks allowed more observations to predict the orbit more accurately, and the probability of impact decreased. It is not unlikely that it will fall even further in the coming years, perhaps even to zero. But what if there were to be a collision? Apophis is an asteroid with a diameter of about 250 meters and a mass of about 21 million tons. If a collision were to occur, it would occur at a speed of approximately 45,000 km/h, which would generate an energy of 400 megatons. This is more than 25,000 Hiroshima atomic bombs. The consequences on a regional scale would therefore be enormous, but there is no danger to humanity as a whole.

A second known object that could eventually pose a threat to Earth is asteroid 1950 DA. It approached in 2001 to a distance 21 times further than the moon. Since the object (as the name indicates) was discovered more than half a century ago, it is possible to make very accurate predictions well into the future. Currently, there is a maximum chance of one in 300 that this asteroid will collide with Earth on March 16, 2880. And since 1950 DA is believed to be 1.1 km in diameter, this would certainly be a global catastrophe. However, since there are still more than 871 years to go before that happens, there is a very good chance that by then either the possibility of a collision will be ruled out, or a technique will be developed to defuse or deviate the object. The link on the right refers to an article that NASA has devoted to 1950 DA, which is still regularly updated.