Svartalfiar,
“Planets” do not exist. There are “stars” and there are “wandering stars.” God did not make any “planets.” Read Genesis 1, Genesis 7:11, and Jude.
“Stars” are IN the firmament (aka “sky”), just as the Bible says. Use an online Bible to search for the words “star” or “stars” in Genesis.
The moon is under the firmament. We can see stars through it.
Your statement, “Distances can be fairly accurately measured ...” is not correct. We can make all the calculations we want based on whatever theory we hold, but we are not capable of “measuring” astronomical distances at all. The distances we “measure” using calculations are based on an unproven theory. It is not valid to presuppose that an unproven theory is “true” and then make calculations based on such a supposition to “prove” that the theory is true. This is known as “circular reasoning.”
Your statement, “Can we travel to the other planets? if they’re within 3600 miles, wouldn’t it be easy to go visit and see if they’re actually there?” makes unfounded suppositions.
First, you have assumed that there are “other planets.” There are no “other planets.” There are stars. Read Genesis.
Second, you have assumed that we are on a “planet.” We are not. We are on Earth. Earth is not a “planet.” Earth is a realm on a plane. It does not move. Water on its surface (oceans, lakes, etc.) does not curve. It may move (waves, tides, rivers, etc.), but it does not curve. Undisturbed, it is level everywhere you observe it.
Third, you have assumed that we can travel through a vacuum using rocket engines. We cannot.
Two forces are involved in rocket motion:
1) The push against air during the ascent phase. This is a very strong force, but requires a medium (air) to make it possible. It only lasts until you reach a vacuum or near vacuum several dozens of miles up.
2) Newton’s Third Law of Motion. This is a relatively weak force associated with throwing chemical propellants behind the rocket at high velocity. It is not sufficient for sustained ascent through a vacuum or near vacuum. That vacuum or near vacuum begins at roughly 60 miles up. You still have thousands (possibly) of miles to go.
Your questions are insightful: “Since the firmament is the farthest away from Earth we can go, What happens when we get there? Is it just a wall of stopping you from going farther?”
In my opinion, we cannot pass through the firmament without God’s permission. As far as I know, we do not have such permission. The Bible describes the firmament (sky) as a physical wall of molten glass. Job 37:18
NASA and all of the other “space” agencies are lying to you.
Blessings to you and your loved ones.