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问题描述：

[单选] 使火箭进入绕地球轨道所需的最低的速度是多少？
A．6.9公里／秒 B．7.9公里／秒 C．9.8公里／秒
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上一篇：我们平时肉眼所见到的星中,除太阳和月亮之外，给我们感觉最亮的是哪一颗星？ 下一篇：地球到太阳的距离约为多少公里？

随机题目

太阳的表面温度就有摄氏６０００度，中心温度更高，可达摄氏( )度左右．
太阳离我们地球很远，大约有( )公里．
来自一颗5等星的光子每秒落在冥王星表面1cm2上的数量为10000个,那么如果使用口径6米的BTA望远镜,在地面上每半小时能够接收到来自一颗20等恒星的多少个光子 On 1 cm2 of Pluto"s surface fall approximately 10,000 photons per second from a star of the fifth magnitude. How many photons would fall on a detector from a star of 20m during half an hour, if BTA at the Earth is used (the diameter of the main mirror is 6m)
太阳的视差为πs = 8". 8 ,具有相同绝对星等的另一颗恒星的视差为π* = 0" .022 ,在晚上能否用肉眼看到这颗星？The sun has a parallax of πs = 8 " .8 , and a star with the same absolute brightness π* =0 " .022 . Is it possible to observe the star at night sky visually
口径10米的凯克望远镜已经在夏维夷的莫纳克亚投入运行,那里的星像直径可以小到0."3,请你估算一下用凯克望远镜进行目视观测的极限星等？Recently the Ten-meter Keck telescope began to operate on Mauna Kea (Hawaii),where the diameter of stellar images may be as small as 0".3. Can you evaluate the limiting stellar magnitude for visual observation with this telescope
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