| < Should we be concerned? > |
| Astronomers went through 4.5 years of collected photometric measurements of the Hubble space telescope and reported, in 2009, the discovery in a distance of 45 Astronomical Units (AUs) of an icy fragment measuring about 1.000 m across (Schlichting et al 2009). Lying beyond Neptune's orbit, it represents the smallest object that we know at such a distance. However, it was not discovered by direct observation, but by painstaking evaluation of tons of recorded material. |
| Most Kuiper belt objects, surrounding our sun at distances of 30-50 AUs, have been discovered by direct observation and measure at least 100 km across. Fast and reliable detection of smaller objects at that distance is still beyond our technical potential. We do not see anything smaller than 100 km at that distance. Should we worry about this gap in our knowledge? Maybe we should. |
| The object that caused the mass extinction at the end of the Cretaceous period 66 million years ago is estimated to a size of 10 km. Since 1998 the NASA collects data on asteroids larger than 1 km; scientists are optimistic that this catalogue is nearly complete now - for objects circulating in the main asteroid belt at distances of 2 - 3.4 AUs around the sun. Possible trajectories of some of the larger objects have been calculated. |
| 2007 VK184 is a near-Earth asteroid estimated to be about 130 meters in diameter. The asteroid has a 1 in 1820 chance (0.055%) of impacting Earth on June 3, 2048. The asteroid would impact the ground with the equivalent of 40 megatons of TNT and create a 2.1 km impact crater. If this is the most threatening event predicted for the next 35 years, we really need not suffer from restless sleep. But unfortunately, the cosmos is much larger than our tiny solar system. |
| On July 28, 2006, the Russian astrophysicist Viktor L. Afanasiev didn't believe his eyes: By chance he recorded the illuminating trace of a meteor hitting the earthen atmosphere at 300 km/s. Fortunately, the size of this object was below 1 millimeter; it evaporated quickly along its path. But for an average meteor, it was too fast (by a factor 4 - 10). The authors note that its velocity would even exceed the rotation velocity of our galaxy (220 km/s) and rather match the velocity of the milky way moving towards the mass center of our Local Group (316 km/s). |
| Any object arriving at the far away Kuiper belt mentioned above and aiming towards our Earth with such a velocity, would reach us within about one year. This would not leave too much time to react to such a threat. And it would make no difference, if the object was moving towards us, or we instead towards the object, resting itself peacefully in the empty space. The result would be the same, depending only on the mass of the bolide. |
| In the last 500 million years there have been five major mass extinctions that on average extinguished half of all species. For most of them large meteorite impacts are deemed responsible. The frequency of cosmic impacts on planets and their moons follows rather predictable statistics, and we have no reason to expect that this should be different in the future. Also our Earth will suffer further impacts, including major ones. |
| Careful observation of the outer space around us is therefore one of the more vital missions of astronomy. Presently, nothing really menacing seems to loom on the horizon. But as explained above, this could change quickly. While we could launch sufficiently detailed strategies to deal with a possible impact in 2048 or later, nature will probably have much shorter notice for us. A colony in space, on the moon or on Mars might allow a small group of humans to survive even the most devastating disaster and to return to Earth, when the dust has settled. |
| MB 7/13 |
| Schlichting
HE, Ofek EO, Wenz M, Sari R, Gal-Yam A, Livio M, Nelan E, Zucker S
(2009) A single sub-kilometer Kuiper belt object from a stellar
occultation in archival data. Nature 462: 895>97 Afanasiev VL, Kalenichenko VV, Karachentsev ID (2007) Detection of an intergalactic meteor particle with the 6-m telescope. Astrophys Bull 62 |
