This is an interesting idea, but to see why it's a problem to implement we need to look at a little physics.
An object with no net force acting on it goes at a constant speed. This seems counterintuitive in situations on earth, but when the speed of something changes there is always a force that makes that change happen. For example, a rolled ball will slow down, but that's due to forces like friction and wind resistance.
To get something into space, you need to apply a force to overcome the force of gravity, and that force is provided by rocket engines.
In principle one could go all the way into orbit at a constant speed by getting up to that speed and then providing just the right force to counteract the pull of the earth.
Now how about the return trip? If you just drop like a stone, two forces are relevant. One is the force of gravity, pulling you down. The other is air resistance that counteracts this partially and slows you down.
Air resistance is really just a way of saying "friction with the air," and you get hot just as you would if you rubbed against anything else. The air acts as a passive brake.
You can actively slow your descent, but that needs a rocket to slow you down.
What sort of a rocket do you need to make your speed constant? Well, it would have to do pretty much what the rocket did to get you up into orbit in the first place.
Recall that to go at constant speed all we need to do is get the forces in question to cancel. That is to say, it would take a similar amount of fuel to bring you down at a slow, constant speed as it did to get you up in the first place. (Don't forget that you'd be lighter on the way down and heavier on the way up since you need to carry the fuel for your return).
Heat shields work out to be cheaper.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
