https://readcomiconline.li/Comic/The-Official-Handbook-of-the-Invincible-Universe/Issue-1?id=108226&s=s2#42
Changes in mass create changes in inertia. Invincible appears able to mentally control this on a bodily level, so that (for example) all his atoms can be given a net gain in inertia in just one direction. This allows him to move his entire body and fly at very great speed simply by willing it. Invincible has been observed to travel 8000 miles in less than 25 seconds, from a standing start, beginning and ending in earth's atmosphere. In such journeys, like all Viltrumites, Invincible subconsciously creates a force field (see Force Fields) to protect his clothing and any load from friction and the heat of re-entry. Freed of the constraints of flying in the Earths atmosphere, Invincible once flew to Mars in four days. The average smallest distance between Mars and the Earth is 78 million kilometers or 48 million miles. Accordingly, he averaged speeds of 800,000 kilometers per hour to reach his destination.

Changes in mass create changes in inertia. Invincible appears able to mentally control this on a bodily level, so that (for example) all his atoms can be given a net gain in inertia in just one direction. This allows him to move his entire body and fly at very great speed simply by willing it. Invincible has been observed to travel 8000 miles in less than 25 seconds, from a standing start, beginning and ending in earth's atmosphere. In such journeys, like all Viltrumites, Invincible subconsciously creates a force field (see Force Fields) to protect his clothing and any load from friction and the heat of re-entry. Freed of the constraints of flying in the Earths atmosphere, Invincible once flew to Mars in four days. The average smallest distance between Mars and the Earth is 78 million kilometers or 48 million miles. Accordingly, he averaged speeds of 800,000 kilometers per hour to reach his destination.