This is explained in Bhagavad-gītā (BG 9.21):
- te taṁ bhuktvā svarga-lokaṁ viśālaṁ
- kṣīṇe puṇye martya-lokaṁ viśanti
- evaṁ trayī-dharmam anuprapannā
- gatāgataṁ kāma-kāmā labhante
"When those who follow the pravṛtti-mārga have enjoyed heavenly sense pleasure, they return to this mortal planet again. Thus, through the Vedic principles, they achieve only flickering happiness." Following the pravṛtti-mārga, the living entity who desires to be promoted to the higher planetary systems performs sacrifices regularly, and how he goes up and comes down again is described here in Śrīmad-Bhāgavatam, as well as in Bhagavad-gītā. It is also said, traiguṇya-viṣayā vedāḥ: "The Vedas deal mainly with the three modes of material nature." The Vedas, especially three Vedas, namely Sāma, Yajur and Ṛk, vividly describe this process of ascending to the higher planets and returning. But Kṛṣṇa advises Arjuna, traiguṇya-viṣayā vedā nistraiguṇyo bhavārjuna: one has to transcend these three modes of material nature, and then one will be released from the cycle of birth and death. Otherwise, although one may be promoted to a higher planetary system such as Candraloka, one must again come down (kṣīṇe puṇye martya-lokaṁ viśanti (BG 9.21)). After one's enjoyment due to pious activities is finished, one must return to this planet in rainfall and first take birth as a plant or creeper, which is eaten by various animals, including human beings, and turned to semen. This semen is injected into the female body, and thus the living entity takes birth. Those who return to earth in this way take birth especially in higher families like those of brāhmaṇas.
It may be remarked in this connection that even the modern so-called scientists who are going to the moon are not able to stay there, but are returning to their laboratories. Therefore, whether one goes to the moon by modern mechanical arrangements or by performing pious activities, one must return to earth. That is clearly stated in this verse and explained in Bhagavad-gītā. Even if one goes to the higher planetary systems (yānti deva-vratā devān (BG 9.25)), one's place there is not secure; one must return to martya-loka. Ābrahma-bhuvanāl lokāḥ punar āvartino 'rjuna: (BG 8.16) aside from the moon, even if one goes to Brahmaloka, one must return. Yaṁ prāpya na nivartante tad dhāma paramaṁ mama: but if one goes back home, back to Godhead, he need not return to this material world.