Srila Prabhupada said
Now the scientists are trying to reach the moon planet, but it is very difficult for them to gain entrance, because they do not have a suitable body. But if we enter into the higher planets by this yoga system, then we will get a body suitable for those planets. For every planet there is a suitable body. Otherwise, you cannot enter. For example, although we cannot live in the water with this body, we can live in the water with oxygen tanks--for fifteen or sixteen hours. But the fish, the aquatic animals, have a suitable body--they are living their whole life underwater. And of course, if you take the fish out of the water and put them on the land, they'll die instantly. So you see, even on this planet you have to have a suitable kind of body to live in a particular place. Similarly, if you want to enter into another planet, you have to prepare yourself by getting a particular type of body.
The process for entering into the spiritual sky is also there. By constant engagement in devotional service through the nine process, one's spiritual body is easily attained. The nine process are simple and easily done.
Hearing the name and glories of the Supreme Personality of Godhead
Chanting His glories
Remembering the Lord
Serving the Lord’s feet
Worshiping the Deity
Offering obeisances to the Lord