It's difficult to definitively say which satellite is the "first" densest satellite, as there are many factors to consider and the definition of "densest" can be subjective.

However, some contenders for a historical "first" densest satellite could be:

* Sputnik 1 (1957): Though not the densest overall, it was a pioneering spacecraft, and its design likely prioritized mass and functionality over density.

* Vanguard 1 (1958): This satellite used a lightweight aluminum shell, focusing on achieving a long lifespan and was designed for scientific observations.

Modern considerations:

* Density is often determined by the materials used. For example, satellites using highly dense materials like iridium or osmium could be considered very dense.

* Purpose influences density: Satellites designed for specific tasks like imaging or communication might be denser than others.

Ultimately, there's no single answer to the "first densest satellite" question. It depends on how "densest" is defined and what aspects of the satellite are prioritized.