Because the gravity on the Moon’s surface is only a sixth of Earth’s, it takes much less energy to accelerate to that 1.5 miles per second than it does on Earth.

这是新概念4 lesson35中的一句话，加粗部分中的that是作什么作用？我的理解是that作介词to宾语，后面的1.5 miles per second是that同位语。如果是这样的话，为啥不直接让1.5 miles per second作宾语？希望专家老师帮忙解答下。