Śrī Nārada Muni could enter all these planets in both the material and spiritual spheres without restriction, as much as the almighty Lord is free to move personally in any part of His creation. In the material world the living beings are influenced by the three material modes of nature, namely goodness, passion and ignorance. But Śrī Nārada Muni is transcendental to all these material modes, and thus he can travel everywhere unrestricted. He is a liberated spaceman.
(Srimad Bhagavatam------1:6:31------purport).

Parvata Muni: is considered to be one of the oldest sages. He is almost always a constant companion of Nārada Muni. They are also spacemen competent to travel in the air without the help of any material vehicle. Parvata Muni is also a devarṣi, or a great sage amongst the demigods, like Nārada. He was present along with Nārada at the sacrificial ceremony of Mahārāja Janamejaya, son of Mahārāja Parīkṣit. In this sacrifice all the snakes of the world were to be killed. Parvata Muni and Nārada Muni are called Gandharvas also because they can travel in the air singing the glories of the Lord. Since they can travel in the air, they observed Draupadī's svayaṁvara ceremony (selecting of her own husband) from the air. Like Nārada Muni, Parvata Muni also used to visit the royal assembly in the heaven of King Indra. As a Gandharva, sometimes he visited the royal assembly of Kuvera, one of the important demigods. Both Nārada and Parvata were once in trouble with the daughter of Mahārāja Sṛñjaya. Mahārāja Sṛñjaya got the benediction of a son by Parvata Muni.
(Srimad Bhagavatam------1:9:6-7------purport).

Saintly persons like Vidura must be treated as well as a denizen from heaven. In those days denizens of heavenly planets used to visit homes like that of Mahārāja Yudhiṣṭhira, and sometimes persons like Arjuna and others used to visit higher planets. Nārada is a spaceman who can travel unrestrictedly, not only within the material universes but also in the spiritual universes. Even Nārada used to visit the palace of Mahārāja Yudhiṣṭhira and what to speak of other celestial demigods. It is only the spiritual culture of the people concerned that makes interplanetary travel possible, even in the present body. Mahārāja Yudhiṣṭhira therefore received Vidura in the manner of reception offered to the demigods.
(Srimad Bhagavatam------1:13:14------purport).

Śrī Nāradajī is an eternal spaceman, having been endowed with a spiritual body by the grace of the Lord. He can travel in the outer spaces of both the material and spiritual worlds without restriction and can approach any planet in unlimited space within no time. We have already discussed his previous life as the son of a maidservant. Because of his association with pure devotees, he was elevated to the position of an eternal spaceman and thus had freedom of movement. One should therefore try to follow in the footsteps of Nārada Muni and not make a futile effort to reach other planets by mechanical means. Mahārāja Yudhiṣṭhira was a pious king, and therefore he could see Nārada Muni occasionally; anyone who desires to see Nārada Muni must first be pious and follow in the footsteps of Nārada Muni.
(Srimad Bhagavatam------1:13:60------purport).

The materialistic scientist's endeavor to reach other planets by mechanical vehicles is only a futile attempt. One can, however, reach heavenly planets by virtuous activities, but one can never expect to go beyond Svarga or Janaloka by such mechanical or materialistic activities, either gross or subtle. The transcendentalists who have nothing to do with the gross material body can move anywhere within or beyond the material worlds. Within the material worlds they move in the planetary systems of the Mahar-, Janas-, Tapas- and Satya-loka, and beyond the material worlds they can move in the Vaikuṇṭhas as unrestricted spacemen. Nārada Muni is one of the examples of such spacemen, and Durvāsā Muni is one of such mystics. By the strength of devotional service, austerities, mystic powers and transcendental knowledge, everyone can move like Nārada Muni or Durvāsā Muni. It is said that Durvāsā Muni traveled throughout the entirety of material space and part of spiritual space within one year only. The speed of the transcendentalists can never be attained by the gross or subtle
materialists.
(Srimad Bhagavatam------2:2:23------purport).

The four Kumāras are described herein as siddheśvarān, which means "masters of all mystic power." One who has attained perfection in yoga practice immediately becomes master of the eight mystic perfections—to become smaller than the smallest, to become lighter than the lightest, to become bigger than the biggest, to achieve anything one desires, to control everything, etc. These four Kumāras, as siddheśvaras, had achieved all the yogic perfectional achievements, and as such they could travel in outer space without machines. While they were coming to Mahārāja Pṛthu from other planets, they did not come by airplane, but personally. In other words, these four Kumāras were also spacemen who could travel in space without machines. The residents of the planet known as Siddhaloka can travel in outer space from one planet to another without vehicles. However, the special power of the Kumāras mentioned herewith is that whatever place they went to would immediately become sinless. During the reign of Mahārāja Pṛthu, everything on the surface of this globe was sinless, and therefore the Kumāras decided to see the King. Ordinarily they do not go to any planet which is sinful.
(Srimad Bhagavatam------2:22:2------purport).

Siddhaloka and Brahmaloka are both within the same planetary system. Brahmaloka is understood to be the highest planet within this universe. Siddhaloka is considered to be one of the satellites of Brahmaloka. The inhabitants of Siddhaloka have all the powers of yogic mysticism. From this verse it appears that the great sage Nārada is an inhabitant of Siddhaloka, although he travels to all the planetary systems. All the residents of Siddhaloka are spacemen, and they can travel in space without mechanical help. The residents of Siddhaloka can go from one planet to another individually by virtue of their yogic perfection. After giving instructions to the great King Prācīnabarhi, Nārada Muni departed and also invited him to Siddhaloka.
(Srimad Bhagavatam------2:29:80------purport).