L240
Giant rocks from space are falling from the sky more than they used to, but don't worry.
For the past 290 million years, large asteroids have been crashing into Earth more than twice as often as they did in the previous 700 million years, according to a new study in Thursday's journal Science.
But no need to cast a wary glance up. Asteroids still only smack Earth on average every million or few million years, even with the increased crash rate. NASA's list of potential big space rock crashes shows no pending major threats. The biggest known risk is a 4,200-foot (1.3-km) wide asteroid with a 99.988 percent chance that it will miss Earth when it whizzes very near here in 861 years.
Tell that to the dinosaurs. Most scientists think dinosaurs and a lot of other species went extinct after a huge space rock crashed into Central America about 65 million years ago.
"It's just a game of probabilities," said study lead author Sara Mazrouei, a University of Toronto planetary scientist. "These events are still rare and far between that I'm not too worried about it."
Mazrouei and colleagues in the United Kingdom and United States compiled a list of impact craters on Earth and the moon that were larger than 12 miles (20 km) wide and came up with the dates of them. It takes a space rock that's half a mile (800 meters) wide to create holes that big.
The team counted 29 craters that were no older than 290 million years and nine between 291 million years and 650 million years old.
But we can see relatively few big craters on Earth because the planet is more than 70 percent ocean and past glaciers smoothed out some holes, said University of Toronto planetary scientist Rebecca Ghent, a study co-author.
Extrapolating for what can't be seen brings the total to about 260 space crashes on Earth in the last 290 million years. Adding in other factors, the science team determined that the current space crash rate is 2.6 times more than the previous 700 million years.
Craters older than 650 million years are mostly wiped off on Earth by glacial forces so the scientists used impact craters on the nearby moon as a stand-in for holes between 650 million and 1 billion years old. The moon is a good guide for estimating Earth crashes, because it is close enough to be in the same bombardment path and its craters last longer.
"Perhaps an asteroid family was broken up in the asteroid belt," Mazrouei speculated. The space rocks then headed toward the Earth and moon, and the planet got slightly more because it is a bigger target and it has higher gravity, Ghent said.
Outside scientists are split about the research. Jay Melosh at Purdue said he found the number of craters too small to come to a reasonable conclusion, but Harvard's Avi Loeb said the case was convincing.
Humans might not have emerged without mass extinctions from space rocks about 250 million and 65 million years ago, Loeb said in an email, adding, "but this enhanced impact rate poses a threat for the next mass extinction event, which we should watch for and attempt to avoid with the aid of technology."
"This demonstrates how arbitrary and fragile human life is," Loeb wrote.
L240的更多相关文章
- 你必须收藏的Github技巧
一秒钟把Github项目变成前端网站 GitHub Pages大家可能都知道,常用的做法,是建立一个gh-pages的分支,通过setting里的设置的GitHub Pages模块可以自动创建该项目的 ...
- github使用技巧
转自:一秒钟把Github项目变成前端网站 后面加了一些自己的东西 一秒钟把Github项目变成前端网站 GitHub Pages大家可能都知道,常用的做法,是建立一个gh-pages的分支,通过se ...
- Redhat Linux内核升级全记录(转)
http://www.sina.com.cn 2001/06/15 15:38 中国电脑教育报 李红 Redhat Linux因为比较容易上手,所以用户很多.它系统配置完善,预装了丰富的应 ...
- Linux进程调度器的设计--Linux进程的管理与调度(十七)
1 前景回顾 1.1 进程调度 内存中保存了对每个进程的唯一描述, 并通过若干结构与其他进程连接起来. 调度器面对的情形就是这样, 其任务是在程序之间共享CPU时间, 创造并行执行的错觉, 该任务分为 ...
- C#如何在VS2015 2017版本中编写WPF UI界面引入第三方SVG图形
原文:C#如何在VS2015 2017版本中编写WPF UI界面引入第三方SVG图形 在VS2015 2017版本中编写WPF UI界面引入第三方SVG图形 最近在写WPF界面的时候遇到一个情 ...
随机推荐
- IIS中添加ftp站点
1.创建Windows账号 右击点击“我的电脑”,选择“管理”打开服务器管理的控制台.展开“服务器管理器”,一路展开“配置”.“本地用户和组”,点“用户”项.然后在右边空白处点右键,选择“新用户”将打 ...
- Apache Kylin1.5.2.1之订单案例详细构建流程
转:http://blog.itpub.net/30089851/viewspace-2122586/ 一.Hive订单数据仓库构建1. 创建事实表并插入数据 DROP TABLE IF EXISTS ...
- 操作构造字符串化宏#define STRINGIZE(x) #x
c++test工程单元测试中报错 “STRINGIZE” 未定义错误 解决方案:在头文件定义宏STRINGIZE #符号把一个符号直接转换为字符串,例如:#define STRINGIZE(x) #x ...
- OpFlex
参考: OpFlex Main OpFlex: Building and Running OpFlex Building mkdir -p ~/work pushd work git clone ht ...
- Ubuntu 上 执行命令 java -version 显示 没有那个文件或目录
解决方法 执行 which java 发现默认java目录:/usr/bin/java . 查看 JAVA_HOME 路径:$JAVA_HOME,得到 /usr/local/java/jdk1.7.0 ...
- spring +servlet 与 spring MVC
spring serlvetSpring框架中context-param与servlet中init-param的contextConfigLocation的区别https://blog.csdn.ne ...
- python 函数返回函数
def hi(name="yasoob"): def greet(): return "now you are in the greet() function" ...
- 如何给wpf的按钮添加背景图片
1:简单实用 <Button Height="143" HorizontalAlignment="Left" Margin="30,34,0,0 ...
- 3.3 x86指令简介
计算机组成 3 指令系统体系结构 3.3 x86指令简介 x86指令种类繁多,数量庞大.在这一节我们将会学习x86指令的分类,并分析其中最为基础的一部分指令. 通常一个指令系统主要包括这几类指令.运算 ...
- 5-13 Rspec实际; validates处理Errors, TDD, 单元测试和验收测试,capybara
validates处理验证错误:详见ActiveModel::Errors文档 一,errors ActiveModel::Errors的实例包含所有的❌.每个错误:key是每个属性的name, va ...