You have got very nice rocket, running at the speed of eighteen thousand miles or twenty-five thousand miles per hour. But you may go. If you don't take . . . if you don't get shelter in some planet—just as they're going in the moon planet, but do not get shelter and come back—similarly, supposing they are getting shelter, but that shelter is not very secure; they again come back. So without shelter, how long you can fly in the sky? The sky is unlimited. You can fly on your sputnik or capsule, but if you don't get the shelter in some planet, then again you come back to your place.
So this is . . . the same example is given in the Bhāgavata: āruhya kṛcchreṇa, with great scientific method you can go up. But if you don't get a shelter, then you come back again on this planet. Similarly, the nondevotees, the impersonalist, they undergo severe penances and austerities undoubtedly, and they rise up to the brahma-jyotir. Āruhya kṛcchreṇa paraṁ padaṁ tataḥ patanty adhaḥ (SB 10.2.32). From there, he again falls down. Same example. Why? Anādhṛta-yuṣmad-aṅghrayaḥ. Because they did not care for the shelter under the lotus feet of Kṛṣṇa, therefore they have to come back again to this material world.
That is practically experienced. We have seen many, many great sannyāsīs, they give up this world and take sannyāsa for merging into Brahman, but later on they come back again to the material activities for opening hospital, schools, and philanthropic work. Why? Because they could not get there. In their so-called Brahman realization, they could not get any pleasure. Therefore they come down again to the material activities. You have left this material world as brahma satyaṁ jagan mithyā. Then why you come back again to this mithyā world, to open hospitals and schools?