Another of the reasons for this lack of notice is that, having fallen so far through the Sun's gravitational field to get there, Oort-cloud objects are, by the time they reach the inner solar system, travelling fast.If one hit Earth it would have an impact velocity of 50-70km per second, about three times that of a nearby asteroid.And smaller rocks with such velocities (and therefore presumably of Oort-cloud origin) do enter the atmosphere from time to time.One such was observed, as Denis Vida of the University of Western Ontario told the meeting, by a programme called the Meteorite Observation and Recovery Project, which watches for meteors over Alberta, in western Canada.The object in question weighed about 2kg - too small to reach Earth's surface.Instead, it burned up in the atmosphere as a fireball.But its velocity meant it came closer to the ground than most fireballs of that size, ending its run at a height of just over 46km, whereas 70-80km is normal.However, penetration that far into the atmosphere also meant it must have been a lump of solid rock, rather than the fragile icy conglomerate that standard comets are made of.A kitten, then, of a Manx comet.Intrigued, Dr Vida searched the astronomical literature for similar fireballs.He found two others: one seen in the 1980s by the Albertan instrument's predecessor; the second noticed over the Czech Republic in the 1990s.
无法观测到无尾彗星的另一个原因是，奥尔特云中的物质从极远处穿越太阳引力场才到达太阳系内部，它们到达时，会处于一个快速移动的状态。如果无尾彗星撞上地球，它的撞击速度将达到每秒50-70公里，大约是地球附近小行星速度的三倍。这种高速的(因此推测它们可能来自奥尔特云)小块岩石确实时不时地进入大气层。正如西安大略大学的丹尼斯・维达在会上所说的那样，观测加拿大西部艾伯塔省上空的“陨石观测和回收项目”就观测到了这样的流星。该流星重约2公斤，质量太小，无法到达地球表面。因为它在大气层中就会像火球一样燃烧，烧成灰烬。但它的高速运动意味着它能比大多数类似大小的火球更接近地球表面，结束飞行的地点在略高于地面46公里的高度，而正常情况下，流星结束飞行的距离大约是距地面70-80公里。然而，如此深入大气层也说明它一定是一块坚硬的岩石，而不是和标准的彗星一样由脆弱的冰块组成。那么，这是一只马恩岛猫彗星的“小猫”。出于好奇，维达博士搜索了天文文献中出现的类似火球。他发现了两个：一个是在20世纪80年代用阿尔伯塔天文仪器的前身观测到的；另一个是在20世纪90年代在捷克共和国上空发现的。
暂无注释
