It can't travel slower without burning huge amounts of fuel.Originally Posted by man on the moon
About 3/4 of the way to the moon (176,000 mi or 283,000 km from earth) is the earth/moon gravitational equipoise. Any closer to the earth, and the earth's gravity pulls a stationary object back. Any closer to the moon, and the moon's gravity will pull a stationary object to it.
Figuratively it's like a hill you must coast over. Leaving the earth at 25,000 mph, the spacecraft is constantly slowing. By the time it crosses the earth/moon "gravipoise", (top of the hill) it's only going a few thousand mph. Then the moon attracts it more strongly, and it starts accelerating "downhill" toward the moon.
On the way back to earth the same thing. Even if you cross the gravipoise point at 1 mph, afterward sliding downhill to earth for two days you'll be at about 25000 mph.
The question is what's the cheapest way to slow down? Haul massive amounts of propellant to the moon and back? Or use the earth's atmosphere. Atmospheric braking is by far the lowest cost way (at least for returning to earth).
For a lunar shuttle it's a tough choice. It must shed about 7000 mph (11,200 km/hr) to enter earth orbit. That requires a lot of fuel. However beefing up the lightweight structure to handle aerobraking and adding the aerobraking hardware would also be costly.
Technically speaking aerobraking for orbital entry just sheds some speed, the remainder is shed by rockets. A more advanced technique called aerocapture in theory can achieve orbit with little or no propellant usage: