Compare the investigations of Mercury and Venus

Mariner 10 used a Venus encounter to redirect its trajectory toward Mercury and made three Mercury flybys in 1974–1975. It supplied close views and measurements without remaining in Mercury orbit. The encounter geometry limited which portions of the planet were imaged.

MESSENGER later entered Mercury orbit in 2011, allowing repeated investigation of the surface and environment from changing positions. Reaching orbit near the Sun requires managing trajectory energy and thermal conditions. Being closer to the Sun does not make the engineering problem simply a shorter version of a Mars flight.

Venus’s thick clouds obstruct ordinary visible-light views of its surface from space. Magellan used radar to map surface structure from orbit. A radar map is a processed measurement of returned radio signals, not a photograph of the ground through a clear atmosphere.

The Soviet Venera effort included probes and landers that measured atmospheric and surface conditions. Venera 7 transmitted from the surface in 1970. Those short-lived local measurements addressed conditions that an orbital image alone could not directly establish; the severe environment shaped the hardware and available operating time.

For a fictional comparison, a bright radar patch could reflect roughness, slope or material properties under the viewing geometry. Calling it a pale-colored rock from brightness alone would confuse radar response with visible appearance. Additional observations help test the alternatives.

A historical sequence therefore tracks both improved access and changing questions. Flybys, repeated orbital measurements and local probes each have strengths and blind spots. Compare how the missions reduced particular uncertainties rather than ranking them only by arrival date or distance from Earth.