Chapter 2 of 5

Three feet down

Automatic braking, radar-controlled descent and touchdown

Told in Surveyor 1’s imagined voice · the events are real · source

NASA Surveyor 1 spacecraft sitting silently on Oceanus Procellarum, the first US spacecraft to land on another planet on June 2, 1966 in this image taken by NASA Lunar Reconnaissance Orbiter.
Surveyor 1Near the Moon, slowing down became my biggest job.
NASA/GSFC/Arizona State University
01An altitude-marking radar began the automatic landing sequence. The main solid retrorocket fired after a delay and removed most of my approach speed. A radar mark and the later rocket ignition were two separate events.
The big rocket finished its job before my feet reached the ground.
02After the main braking phase, its spent casing was discarded. My smaller vernier engines continued the descent. The radar altimeter and Doppler velocity sensor supplied information about height and motion to the control system.
Image of Surveyor 1 shadow against the lunar surface in the late lunar afternoon, with the horizon at the upper right.
Surveyor 1Measurements helped the engines adjust the descent.
NASA/JPL
03The controller used radar feedback to regulate vernier thrust as I approached the surface. Very near the ground, the engines cut off, leaving a short final free fall. This was an automatic sequence, not a person steering every instant with a joystick.
On June 2, my three feet met the Moon.
04The footpads contacted the surface within 19 milliseconds of one another. Shock absorbers and crushable components helped absorb the landing energy. I had made the first US soft landing on the Moon—and could now investigate the place where I stood.

The big idea

Landing brings sensing, control, propulsion and impact absorption together. A successful system needs each stage to hand over to the next.

Think about it

Why does a landing controller need measurements as well as an engine?

Fact file

Landing date
2 June 1966
NASA’s summary rounds touchdown to 06:17:36 UT; Part I gives 06:17:37 GMT. June 1 appears in Pacific-time reports.
Source ↗
Footpad contact span
Within 19 milliseconds
Documented in the landing report; not a reconstructed force curve or perfectly simultaneous contact.
Source ↗
Final descent
Radar feedback → vernier control → short free fall
Accounts differ on exact cutoff height. The sequence is retained without the draft’s asserted 4.3 m hover.
Source ↗

In this chapter: Altitude-marking radar, solid retrorocket, three vernier engines, RADVS and shock-absorbing legs

Sources: Main record · Surveyor I Mission Report, Part I · JPL TR 32-1023 · Surveyor 1 · NSSDCA Master Catalog

Machine illustration: AI-made. Photos: NASA Image and Video Library, credited on each picture.

Chapter 2 of 5

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