What is the story of Indra stealing the wheel of Surya and killing Śuṣṇa- the demon?

 

Rig veda is all about various spiritual aspects and thus, there are no physical gods.

Rig Veda mentions esoterically the one and only one DIVINITY, ie., formless energy/BRAHMAN, in various epithets, viz., AGNI, INDRA, SURYA, VISHNU, ADITI, etc.

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Hence, Indra stealing the wheel of Surya should be understood  in esoteric sense only. This incident was mentioned by many Ṛṣis throughout Rig Veda.

For example:

Rig Veda 6.31.3 - Ṛṣi (seer): suhotra

त्वं कुत्सेनाभि शुष्णमिन्द्राशुषं युध्य कुयवं गविष्टौ । दश प्रपित्वे अध सूर्यस्य मुषायश्चक्रमविवे रपांसि ॥
tvaṃ kutsenābhi śuṣṇam indrāśuṣaṃ yudhya kuyavaṃ gaviṣṭau | daśa prapitve adha sūryasya muṣāyaś cakram avive rapāṃsi ||

English translation:

You, Indra—fight along with Kutsa against the insatiable Śuṣṇa, the demon of bad harvest, in the quest for cattle, and against his ten before the (ritual) mealtime. Then you steal the wheel of the Surya. You have toiled at your labors.

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If we study carefully the above narration, we can find that Indra stole the wheel of the Surya and saved Kutsa against the demon Śuṣṇa, by killing the latter.

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Now, we have to understand the underlying concept in this Rik.

The three (3) levels in spiritual levels are governed by 3 divinities, viz., the base level governed by AGNI, mid-space or antariksha by Indra and finally the Heaven or final illumination by Surya.

In spiritual realm, the practitioner is supposed to adhere strictly to ṛta (ऋतम्) or TRUTH , ie., in thoughts, in speech and in action. This is sine qua non for any spiritual practitioner, as the BRAHMAN or the formless ENERGY resides in ṛta (ऋतम्).

 Rig Veda 6.16.35

गर्भे मातुः पितुष्पिता विदिद्युतानो अक्षरे । सीदन्नृतस्य योनिमा ॥
garbhe mātuḥ pituṣ pitā vididyutāno akṣare | sīdann ṛtasya yonim ā ||

English translation:

In the womb of (DIVINE) mother (ADITI), as the father of the father, having illuminated forth at the imperishable (place), sitting on the birthplace of ṛta (ऋतम्)/TRUTH.

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This ṛta (ऋतम्) is called the wheel of the BRAHMAN/Surya.

The obstacles faced in the spiritual path were mentioned esoterically in Rig Veda, as Śuṣṇa/Bṛsaya/Vṛtrá/Paṇis, etc.

Hence, adherence strictly to ṛta (ऋतम्) or TRUTH will eliminate all obstacles faced in the spiritual path.

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Thus, Indra - the governing divinity in the mid-space or antariksha will assist the spiritual practioner, who adhered to ṛta (ऋतम्) or TRUTH in progressing in spiritual practices, by eliminating the obstacles.

That is the story of Indra stealing the wheel of Surya and killing Śuṣṇa, the demon.