The best people to study the Astral Plane are serious scientists who are often skeptics. It is a reality, yet one that can be dangerous. So a sober approach with the scientific method is important for the progress of the subject.
We actually Astral Project nightly in our dreams. It is a natural occurrence and part of our physical makeup. Dreams are necessary to stay alive and coherent.
The Astral Plane is as vast as the Universe itself. As it is a part of its existence, essentially born of it, (the universe) and in it. Also, this is one of the only things where the reverse is also true. The Astral Plane gives birth to and is a host of the Universe itself. " The aura is a magical mirror of the universe and the universe is a magical mirror of our aura." -Edward Alexander Crowley
Thought is no less important as an existing thing than something that you can touch and see, or smell. It causes major changes in the outcome of our material reality. "Imagination is more important than knowledge." -Albert Einstein
Think of things like dark matter, anti-matter, and pure energy. Even things like electricity and ether. Just because we now have a means of bringing out the tangible visibility of their existence does not mean they did not exist before hand. There are many other things that we are not able to notice through the science of our senses.
Thinking in terms of relativity is important for us to classify things and make comprehensive judgements and observations. Time is male and space is female God is above sex. Time is of no circumstance to those things discovered. They have always existed, as well as the possibility of their outcomes, long before we achieved the goal of making them known.
Same with the Astral Plane and its parts. There are some great discoveries and much uncharted territory in the field of Ethereal Plane research and practice.
Ever been in a dream where you running as fast as you can but everything is in slow motion? Or trying to hit something really hard and it comes out really soft, even gentle? There are three planes to the Astral Plane. The Higher, the Lower, and the Ethereal Plane. That slow movement is actually the Ethereal Plane.
The Ethereal Plane is the gate to the lower Astral. And as it would stand to reason the lower Astral is the gate to The Higher Astral Plane. Once you have achieved the higher Astral you should be able to visit both of the lower planes easily.