19/08/2026
Korolev Ordered the Moon to Be Considered Solid
On October 28, 1964, Sergei Korolev, the founding father and Chief Designer of the Soviet space program, wrote a short technical note addressing one of the fundamental questions facing the lunar program: what kind of surface should a lunar landing vehicle be designed to land on?
In this note, Korolev set out specific engineering conditions for a future landing. Today, the story is often reduced to the famous phrase, “Consider the Moon solid.” But the actual document, preserved at the Museum of Cosmonautics in Moscow, is far more interesting.
A Real Uncertainty
At the beginning of the Space Age, no one had yet made a soft landing on the Moon or examined its surface directly. Scientists and engineers needed to understand not only the chemical composition of lunar soil, but above all its physical and mechanical properties: could the surface support a spacecraft, or would its landing gear sink into a loose surface layer?
One of the best-known hypotheses of the time was proposed by American astrophysicist Thomas Gold, who suggested that the Moon might be covered by a substantial layer of extremely loose dust. If so, a landing vehicle could potentially sink deep into the surface.
In the Soviet Union, the question was also the subject of serious scientific debate. Different models of the structure and physical properties of the Moon’s uppermost layer were investigated. Heinrich Steinberg, Vsevolod Troitsky, and other Soviet researchers attempted to determine the properties of the lunar surface from the astronomical and radio-astronomical observations available at the time. Gradually, evidence emerged supporting the view that the surface had sufficient bearing capacity to support a landing vehicle.
A Problem for Engineers
This uncertainty created a very practical problem for engineers. A landing system could not be designed simultaneously for every conceivable type of surface. Engineers even discussed ways of reproducing a hypothetical deep, loose layer on Earth. One proposed test concept involved creating a layer several meters deep of unhulled millet, whose mechanical properties were considered a possible analogue for extremely loose lunar soil.
But the designers needed specific assumptions to work with. On October 28, 1964, Korolev set them down in his own hand:
“Note
The landing of the LK [lunar spacecraft] should
be calculated for
sufficiently firm
pumice-like ground.
Vertical velocity ≈ 0 m/s
during descent at h = 1 m ±
Lateral velocity should be
‘practically,’ at the specified value,
≈ 0 m/s
28/X-64 Korolev”
The actual note is therefore considerably more precise than the famous later retelling, “Consider the Moon solid.” Korolev did not simply declare the Moon to be “solid”: for landing calculations, the designers were to assume “sufficiently firm, pumice-like ground,” while the note also specified requirements for the spacecraft’s vertical and lateral velocities.
The Answer from the Moon Itself
A little more than a year later, the Moon itself began to provide the answer.
On February 3, 1966, the Soviet robotic spacecraft Luna 9 achieved the first successful soft landing on the surface of another celestial body. The spacecraft did not sink into a deep layer of dust and transmitted the first images directly from the lunar surface. The landing demonstrated experimentally that the surface at the landing site was capable of supporting a spacecraft.
Korolev did not live to see it. He died on January 14, 1966 — just 20 days before Luna 9 landed.
Why Does This Matter Today?
Today, we have lunar samples, decades of remote-sensing data, and a vastly different understanding of regolith. Yet for new lunar missions, surface properties are once again not only a subject of planetary science but a concrete engineering parameter — for landing, mobility, and the operation of equipment on the Moon.
Image: Sergei Korolev alongside his original note, dated October 28, 1964, which is preserved at the Museum of Cosmonautics in Moscow.
