A lunar landing with 1202 and 1201 alarms

@Mark Ollig

The Block II Apollo Guidance Computer, or AGC, operated on all crewed Apollo flights from Apollo 7 in 1968 through Apollo 17 in 1972.

The AGC was manufactured at Raytheon, a major US defense contractor, and developed at the Massachusetts Institute of Technology (MIT) Instrumentation Laboratory, founded and directed by Charles Stark Draper.

The AGC’s central processor contained about 5,600 NOR gates. A NOR gate, short for ‘NOT OR,’ is a basic logic circuit that produces a binary “1” only when all its inputs are “0.”

Made of transistors and resistors, these Micrologic NOR circuits powered the computing functions of the Apollo command and lunar modules.

For Block II, engineers used dual three-input NOR integrated circuits.

The AGC’s processor was specifically designed for Apollo’s navigation and control needs. Single-chip microprocessors didn’t become available until 1971, when Intel launched the 4004.

The AGC’s main clock operated at about two million cycles per second. In practice, this lets the computer carry out around 85,000 simple instructions each second.

More complex operations, such as addition, ran slower, about 43,000 per second.

The Block II Apollo Guidance Computers stored their programs in two kinds of memory.

They had about 72 kilobytes of fixed “core rope” memory, which was roughly 36,000 slots, each holding a single instruction or number that could not be changed.

The AGC also contained about 4 kilobytes of erasable magnetic-core memory, with roughly 2,000 slots that astronauts or onboard programs could update during the mission.

Producing core rope memory often took six to eight weeks to complete a single module and cost about $15,000 in the late 1960s.

Each AGC used 36 modules, or about $540,000 in 1969 dollars, which equals roughly $4.75 million today.

Skilled technicians carefully hand-wove delicate copper wires through and around compact, doughnut-shaped ferrite cores.

Threading a wire through a core represented a binary “1,” while bypassing the core signified a “0.”

Data was permanently stored by how wires were woven through or around the cores, resulting in a durable and reliable form of non-volatile read-only memory that remains effective even in harsh space environments.

The AGC was accessed through the Display and Keyboard, or DSKY, pronounced “disk-key.”

The DSKY acted as both the control pad and data display, allowing astronauts to communicate with the AGC.

By entering command codes like Verb 16 Noun 68, they could instantly access range, velocity, and time-to-go, simplifying tasks such as system checks and mission alarm responses.

As director of the software engineering division at MIT’s Instrumentation Laboratory, Margaret Hamilton led the team that developed the pioneering onboard AGC flight software for the Apollo command and lunar modules.

She popularized the term “software engineering” to confirm that software was developed with the same precision as was used with the spacecraft’s hardware.

Hamilton and her team built proactive error detection and dynamic safeguards into the software, inspired in part by an incident in which her young daughter, playing with a DSKY prototype, pressed random key combinations and caused unexpected input commands.

Those safeguards were active during Apollo 11, guiding the Lunar Module Eagle as it descended toward the lunar surface July 20, 1969.

Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin had kept the Eagle’s rendezvous radar system in AUTO mode during lunar descent so it could quickly reacquire the command module Columbia in lunar orbit after landing.

Unbeknownst to them, the AUTO mode triggered “cycle steals,” or processing delays that slowed the AGC’s scheduler and caused program alarms 1201 and 1202.

When Armstrong and Aldrin reported the alarms to Mission Control in Houston, guidance officer Steve Bales and computer specialist Jack Garman, working from their consoles, quickly reviewed the AGC’s behavior.

They confirmed that the AGC continued to manage all essential landing and navigation tasks and that the Eagle’s engine and trajectory were unaffected.

Bales relayed his assessment to flight director Gene Kranz, who authorized capsule communicator Charlie Duke to say, “We’re go on that alarm,” giving Armstrong and Aldrin official clearance to land.

According to NASA’s technical transcript, the first 1202 alarm occurred at 102:38:26 ground elapsed time, counted from Apollo 11’s launch at 9:32 a.m. Eastern time (8:32 a.m. Minnesota time) July 16, 1969.

Excerpts from the NASA transcript of the alarm exchanges between Aldrin, Armstrong, and Duke:

102:38:26 Armstrong: “Program alarm.”

102:38:30 Armstrong: “It’s a 1202.”

102:38:32 Aldrin: “1202.”

102:38:48 Armstrong: “Give us a reading on the 1202 program alarm.”

102:38:53 Duke: “Roger. We got you . . . we’re go on that alarm.”

102:42:17 Aldrin: “Roger. Understand. Go for landing. Three thousand feet. Program alarm.”

102:42:22 Aldrin: “1201.”

102:42:24 Armstrong: “1201.”

102:42:25 Duke: “Roger. 1201 alarm. We’re go. Same type. We’re go.”

During the final descent, Armstrong saw that the Eagle was headed into West Crater and its hazardous boulder field, which lay directly in the path of the preprogrammed landing site.

Using the Attitude Controller Assembly (a grip hand control), he manually flew the Eagle away from the crater while still relying on AGC stabilization data.

The hand controllers for both the Apollo Command and Lunar Modules were designed and manufactured by Honeywell Inc., with final assembly completed at its Aerospace Division facilities in the Twin Cities; yes, from our state of Minnesota.

The Eagle carrying Armstrong and Aldrin touched down safely in the Sea of Tranquility.

Buzz Aldrin later recalled in 2016, “We touched down . . . we probably had about 15 seconds of fuel left.”

Margaret Hamilton, whose work helped land humans on the moon, celebrated her 89th birthday Aug. 17 this year.

A telephone office visit ‘ringing’ with nostalgia

@ Mark Ollig

Upon entering the Winsted Telephone Company office a few weeks ago, now operated by TDS Telecom, I was greeted by an atmosphere of stillness and calm.

Established in 1917 and incorporated in 1920, the Winsted Telephone Company has been associated with my family since 1927, when my great-uncle, Loren J. Ollig, acquired its stock.

Ownership then passed to my great-grandfather, Wallace N. King, in 1929, and then to my grandparents, Mathew L. and Marie Antoinette (nee King) Ollig, in 1931.

In 1948, at the corner of Second St. S. and McLeod Ave. W., my grandparents had the company’s brick office building built, which still stands today.

My earliest memories there date back to 1965, when I attended Winsted’s first kindergarten.

Kindergarten ran from noon to about 3 p.m.

Afterwards, I walked half a block south to the telephone office, where my dad managed the company and would drive me home when he finished work.

I remember the knotty-pine paneled walls, the small glass-blocked window facing the street, and the front counter.

A payphone was on the north wall, just to the left of the entrance.

Above my dad’s desk hung a framed painting of his father, Mathew L. Ollig, who passed away Aug. 1, 1957.

I clearly recall the “dial room” with the sound of relays clicking from the Leich (pronounced “like”) electromechanical rotary-dial telephone switching system.

While at the office, I sometimes sharpened pencils, took out trash, or swept the floor in the dial room.

Some of the people who worked there during this time included my father’s brother Jim, my mother’s brother David, Frank Roufs, and Kenny Norman.

During my recent visit, I saw the main distribution frame (MDF) I helped install in early 1986 for the Nortel Digital Multiplex System-10 (DMS-10) digital voice-processing switch.

Some of the nearly 40-year-old copper wiring pairs are still neatly wire-wrapped around the 88-Series terminal block pins.

At the back of the MDF, ten vertical rows of surge protector blocks are mounted and wire-wrapped to the copper pairs of various cables.

These cables extend ten feet upward into an overhead metal racking system that is bolted to the ceiling.

The racking runs about 20 feet west and then 30 feet south along the wall.

There, the cables are spliced to the two main 900-pair copper cables used to deliver telephone service to local subscribers.

These two cables extended into underground conduits leading to above-ground pedestal enclosures on the south side of the telephone office.

There, they were spliced into smaller distribution cables throughout the Winsted exchange.

I learned that some of this copper wiring and cable is still in use today.

The DMS-10 digital switch itself, however, was removed many years ago.

Today, Winsted telephone subscribers’ calls use advanced optical technology and a fiber-optic transport cable to the TDS Telecom office in Monticello.

Telephone call processing at this location is managed by a MetaSwitch Voice over Internet Protocol (VoIP) softswitch platform.

During my recent visit, I walked through a large office area, which once housed an environmentally-controlled room.

This room contained rows of DMS-10 cabinet bays, ancillary systems, and other devices I remembered working with.

I recalled the whirring of the cooling fans, which prevented the components on the printed wiring cards from overheating.

My mind’s eye sees the two Cook 9-track magnetic tape units (one active and one standby), which were housed in a dedicated cabinet bay and cabled directly into the DMS-10.

They recorded long-distance billing data onto half-inch-wide tape spooled onto 10.5-inch tape reels.

When a tape was full, we would send it to a processing center to extract the data and print the subscribers’ long-distance statements.

There was also a DECwriter dot-matrix teletype terminal for printing the DMS-10 system logs and maintenance messages.

A VT-100 (video display terminal) was used for provisioning the digital DMS-10 switch.

The teletype terminal and VT-100 were connected to the DMS-10’s RS-232C ports, a standard protocol for serial data communication.

In 1988, Winsted Telephone Company installed a fiber-optic cable to the US West tandem office in Buffalo.

The tandem office served as a central switching center, routing calls between Winsted and the Public Switched Telephone Network used for voice communication.

The fiber-optic cable was connected to the NEC RC-28D digital multiplexer located in the same room as the DMS-10 switch.

The RC-28D provided the interface between the fiber and the digital signals transporting long-distance calls to and from Winsted.

It combined multiple T1 circuits (Transmission System 1), each carrying a DS1 signal, into a single higher-capacity DS3 (Digital Signal Level 3) pathway.

Each T1 carried 24 digital voice channels operating at 64 kbps each, for a total data rate of 1.544 mbps.

When 28 T1 circuits were combined, they were multiplexed into a DS3 (Digital Signal Level 3) operating at 44.736 mbps.

The simple sum of 28 T1s is 43.232 mbps (28 × 1.544), but the DS3 speed is higher because extra framing and justification bits are added to keep the digital signals synchronized.

One DS3 carried 672 voice channels.

Yes, I still remember this stuff.

DS3 was the standard transport format used across fiber routes for long-distance traffic during the 1980s.

Some fiber-optic networks still use it today, though modern systems usually rely on SONET (Synchronous Optical Network) in North America or IP (Internet Protocol)-based transport.

My brother Mike and I received training on the RC-28D digital multiplexer at the Nippon Electric Company facility in Herndon, VA.

A SensaPhone device we installed on the wall monitored the office’s environmental and equipment alarms.

If it detected high or low temperatures or a DMS-10 alarm, it would automatically call four preprogrammed numbers using its distinct electronic voice.

We could also call the SensaPhone to check the current room temperature and listen to 15 seconds of the room’s background sounds.

All of this equipment is gone now.

I noted the battery room has been upgraded with new batteries and a new rectifier (converts AC to DC) for powering the existing telecommunications equipment.

The north side of the original brick building is adjacent to what was once Herb’s Bakery.

In 1971, the telephone company purchased the bakery along with the neighboring property to the east, where a warehouse for cable, equipment, and company vehicles was built.

The old bakery building was entirely renovated with a reception area entry off of Second St. S., a supply room, a break room, my father’s office, and a hallway connecting the original telephone office.

As I stood alone in the now-closed reception area, memories of years gone by came flooding back.

It had once been alive with ringing phones, office staff helping customers, and people stopping by for coffee and bakery treats.

Each December, we set up a Christmas tree in the reception area, its glowing lights could be seen from the street after dark.

The room that was once my father’s office feels vastly different now.

The Italian mahogany wood paneling, which once filled the office with a deep warmth that evoked both professionalism and calming reassurance, is now hidden under a coat of white paint.

Among the items stored there is my father’s glass-topped wooden desk, where he spent countless hours.

After he passed away in 1982, my mother used his desk until her retirement in 1995.

I started working part-time at the Winsted Telephone Company on weekends and summers from 1973 to 1976, before beginning full-time in 1977.

During those and the ensuing years, my family was heavily involved at the telephone company, with my father, mother, two brothers, and sister all contributing to its operations.

Also, many talented and dedicated individuals from the local community worked there, providing reliable customer service to meet the community’s growing telecommunications needs.

After TDS Telecom acquired the Winsted Telephone Company Nov. 13, 1993, I continued working at the local office for another year, and later out of the TDS Telecom office in Monticello.

I began working at home for TDS Telecom in 2018 until my retirement in October 2023.

My sincere gratitude to Paul at the TDS Telecom office in Winsted for allowing me to visit and step back into yesteryear.

It brought back many nostalgic memories.

Looking at Winsted’s early telephone network

@ Mark Ollig

Established in 1871, the Minnesota Railroad and Warehouse Commission initially regulated railroads, set rates, oversaw grain warehouses, and investigated accidents.

In 1915, the Minnesota Legislature ratified “an act to regulate telephone companies and to place them under the control and jurisdiction of the Railroad and Warehouse Commission” (Chapter 152 – House File No. 21).

This act recognized telecommunications as a public utility, requiring companies to submit rates, file annual reports, and maintain an office in Minnesota.

In the early 1910s, Main Avenue West in Winsted was lined with telephone poles fitted with multiple horizontal crossarms.

Each eight-foot crossarm was bolted to the pole with metal hardware and supported by wooden (or metal) braces/brackets and typically carried about ten closely spaced glass insulators, each set on a wooden pin.

Galvanized iron wires, often installed in pairs and running parallel, were supported by these insulators.

To anchor the wire, a 12-inch length of insulated stranded wire was wrapped around the galvanized iron wire and pressed into the one-eighth-inch-deep, three-eighths-inch-wide groove of the glass insulator, securing it firmly in place.

The galvanized iron wire then continued on to the next insulator connection on the following pole, and so on down the line.

This arrangement formed the backbone of Winsted’s early open-wire outside telephone network, with single-wire “drops” to each telephone location.

These early drops ran on the “single wire and earth” principle, using a single conductor and the ground as the return path, until metallic (two-wire) circuits became the standard for improved performance and to reduce interference from grounded power lines.

In the early days of telephony, wooden magneto phones transmitted audio voice using an earth-ground connection as the return path to complete the circuit.

This transmission relied on dry batteries such as zinc-carbon cells, which typically provided around 1.5 volts.

Usually, two or three of these cells were connected in series to power the carbon transmitter (microphone).

The magneto generator was hand-cranked to provide a ringing signal.

The Winsted Telephone Company was established in 1917 and incorporated on April 10, 1920.

Loren J. Ollig, my great-uncle, acquired the stock of the Winsted Telephone Company and became its president on Aug. 31, 1927.

In 1931, my great-grandfather, Wallace N. King of Waverly, purchased the company, with ownership passing to my grandfather, Mathew L. Ollig, and my grandmother, Marie Antoinette (née King) Ollig, in 1932.

The 1930 census reported 482 residents in Winsted.

From the Winsted Telephone Company’s 1931 annual report, “For the Year Ended Dec. 31, 1931,” page eight, under “Plant Data,” details are given on external telephone poles, galvanized iron, open wire, and cable in use.

Three miles of local telephone pole lines and 18 miles of pole lines used for toll services were recorded.

According to the 1931 annual report, the Winsted Telephone Company had 78 local metallic circuits (two-wire balanced pair), eight rural iron wire grounded circuits, and three leased metallic toll circuits used for long-distance or inter-exchange calls.

A total of 233.15 miles of galvanized iron wire strands in 10, 12, and 14-gauge sizes were used within the Winsted exchange; the high number is due to multiple iron wires strung along each telephone pole.

The 1931 annual report recorded that 50 miles of leased 10 and 12-gauge iron wire were used for toll circuits connecting to other telephone exchanges.

The company’s underground cable totaled 4,818 feet and consisted of 19 and 22-gauge wiring.

In 1931, the Winsted Telephone Company used a Monarch legacy model switchboard, which connected subscribers to local businesses and other residents.

A caller would crank the hand generator on their magneto phone, creating an electrical ringing current that traveled to the local switchboard and alerted the operator via a spring-loaded metal drop annunciator.

The annunciator, visible above the line connector jack, would fall open when signaled, and an audible click could be heard as the relay activated on the switchboard panel.

Local subscribers could also call other towns through the switchboard by using leased telephone exchange toll circuits to nearby telephone exchanges.

These dedicated lines, rented from other exchanges, allowed local callers to reach operators or subscribers outside the immediate Winsted exchange.

The operator would plug into the toll circuit and route the call through the connected nearby exchange.

At that time, Winsted’s switchboard operator was the local community “news node” and would be called whenever the fire siren sounded or church bells rang, as residents wanted to know where the fire was or who had died.

Local business owners would have their calls “forwarded” by the switchboard operator to wherever they were whenever they left their store.

The Winsted switchboard was located less than a block directly north of the present-day Winsted Telephone (TDS Telecom) office, in what would later become the Klip and Kurl Salon building, which stood on the north side of the Pantry Cafe and is now a vacant lot.

In 1937, Winsted Telephone Company reported a total of 180 stations (telephones), comprising 35 business and 61 residential stations in town, and 84 in the rural area.

The total length of underground cable for both 19 and 22-gauge wiring was recorded at 4,968 feet.

The 1915 Minnesota Chapter 152—H.F. No. 21, “an act to regulate telephone companies and to place them under the control and jurisdiction of the Railroad and Warehouse Commission,” can be read at https://bit.ly/4fzgqLd.

Next week, your columnist continues his nostalgic journey with a visit to the present-day Winsted Telephone Company, now TDS Telecom.

Main Street East in Winsted, circa 1910s.
Sturdy telephone poles with crossarms, glass insulators, and wire line
the south side of the street, forming the backbone of Winsted’s early
 telecommunications network. “The Examiner,” Winsted’s local newspaper,
established in 1910, appears on the south (right) side (looks to be where the
present-day post office is?).

Mark Ollig’s grandmother, Marie Antoinette (née King) Ollig, operates
 the Winsted Telephone Company’s Monarch legacy model switchboard in the
 early 1930s (likely around 1932). The movable “lollipop”-style microphone, attached
 to a mounting arm bracket, was standard operator equipment of that era.
This is combined with a close-up of the Monarch legacy model switchboard taken in 1941. 
Photos courtesy Mark Ollig

Harvard Mark I: the dawn of modern computing

@Mark Ollig

In 1937, Harvard University physicist Howard H. Aiken envisioned a powerful electromechanical calculating machine to solve complex scientific mathematical problems.

Inspired by Charles Babbage’s Analytical Engine, Aiken aimed to bring his vision to life with the support of Harvard faculty members Professor Emory Leon Chaffee and Harlow Shapley.

In November 1937, Aiken officially submitted his design proposal to IBM, seeking their expertise and resources to construct the automatic calculating device.

After a thorough review, IBM President Thomas J. Watson Sr. approved the project in early 1939, providing complete engineering support and funding.

Construction started that year at IBM’s Endicott, NY, laboratories, led by chief engineer Clair D. Lake, Frank E. Hamilton’s mechanical creativity, Benjamin M. Durfee’s precision assembly skills, and James W. Bryce’s expertise in relay circuitry.

Aiken, Harvard, and IBM established the official name for the device: IBM Automatic Sequence Controlled Calculator (ASCC).

The IBM ASCC spanned 51 feet in length, stood 8 feet tall, and weighed nearly 10,000 pounds.

Enclosed in a stainless-steel and glass frame, the IBM ASCC contained about 750,000 parts, including numerous gears, switches, 3,304 relays, and counters, all working together in coordinated unison, connected by 500 miles of wiring.

The IBM ASCC’s internal computation logic, relays, and control circuits were powered by a 50-volt DC generator, as described in Harvard’s 1946 operations manual, which includes the electrical schematics.

Driving the gears, rotating shafts, and other moving parts was a four-horsepower AC motor, ensuring the machine’s mechanical assemblies remained synchronized.

The IBM ASCC provided a total of 132 storage locations: 72 standard storage registers (accumulators) and 60 constant-register switches (constant dials).

Every register or dial on this machine could hold a decimal number that was as long as 23 digits, and it could show whether the number was positive or negative.

In total, the machine could store 3,036 digits, which was impressive for a computer built before electronic technology was widely used.

This gigantic calculating machine contained six-foot aisles so operators could move alongside it and access its backside cabling and parts.

Programming the IBM ASCC required collaboration among coders, mathematicians, and engineers to convert mathematical problems into machine instructions.

Operators, typically Navy personnel or technicians, loaded program tapes, managed data, and supervised calculations.

The IBM ASCC program instructions were encoded as specific hole patterns on long paper tape, using up to 24 columns per row.

Unlike modern binary codes, the holes in paper tape represented decimal values, register numbers, or machine functions.

Each position on the tape corresponded to a specific operation or register within the IBM ASCC.

The pattern of holes in each row of the tape defined an instruction, specifying which operation to perform and which registers or constants (fixed values set in the IBM ASCC) to use repeatedly during calculations.

The finished rolls of paper tape served as “programs” for the IBM ASCC, which read each row of instructions in sequence, line by line.

Setting up a new calculation required more than just punching holes in paper tape.

Coders, along with operators, would configure physical patch cords and adjust ten-pole switches controlling various circuits, ensuring the data was routed correctly within the computing machine.

Input values were fed into the IBM ASCC using punched cards, while built-in IBM electric typewriters printed data output on paper.

For its time, calculations run using the IBM ASCC were solved remarkably fast.

Addition and subtraction operations took about 0.3 seconds to complete, while a multiplication problem took between three and six seconds.

Calculating division on the IBM ASCC took about 15.3 seconds, nearly four times longer than multiplication, due to the increased number of mechanical cycles required, which put greater demands on the machine’s moving parts.

The IBM ASCC could automatically solve complex scientific problems, including systems of linear equations, by executing lengthy arithmetic sequences.

It performed single sine or logarithm calculations in about a minute, while desk calculators of that era were unable to handle these functions, relying instead on printed tables, slide rules, and approximations.

The completed IBM Automatic Sequence Controlled Calculator was delivered in early 1944 to Harvard University in Cambridge, MA, and installed in the Cruft Physics Lab, where it became operational in May.

Harvard University President James B. Conant formally accepted the IBM ASCC from IBM President Thomas J. Watson Sr. during a public dedication ceremony Aug. 8, 1944.

Soon after, the IBM ASCC became known as the Harvard Mark I.

During WWII, the US Navy Bureau of Ships used the Harvard Mark I for generating gunnery tables, processing reports, and performing military engineering calculations.

The Harvard Mark I played a key role in the Manhattan Project by solving complex mathematical problems related to shock waves and detonation timing for atomic bomb design.

Its precise calculations were used by the engineering teams developing the atomic bombs dropped on Japan in 1945.

“A Manual of Operation for the Automatic Sequence Controlled Calculator,” written in 1946, was edited by Lt. Grace Hopper, who is credited with extending, revising, and writing several chapters.

The manual states that she “more than any other person is responsible for the completion of the book.” It is available on the Internet Archive at https://bit.ly/47bnIT5.

The Harvard Mark I’s moving relays, counters, gears, switches, drive shafts, and sprocket drums advancing the program paper tape generated a rhythmic mechanical symphony, until it was decommissioned in 1959.

You can see and hear the Harvard Mark I, which helped launch the dawn of modern computing, at:https://bit.ly/40Nz384.

Surveyor 5’s ‘giant leap’ for Apollo

@Mark Ollig

The mission of the Surveyor 5 lunar spacecraft began at 3:57 a.m. EDT Sept. 8, 1967.

It lifted off from Cape Canaveral’s Launch Complex 36B aboard an Atlas-Centaur two-stage rocket.

The Surveyor 5 lander weighed 2,218 pounds and carried instruments to test lunar soft-landing technology, collect surface data, and support safe human landings.

It was one of several unmanned spacecraft built to prove precision soft-landing technology essential for Apollo.

The lander stood about 10 feet tall on a lightweight, three-legged aluminum frame with crushable footpads built to absorb impact.

A central mast supported a solar array of 792 cells, supplying 85 watts of electrical power.

Near the mast were a high-gain antenna for detailed data and images, and an omnidirectional antenna for continuous Earth communication.

As it approached the moon, Surveyor 5 began its landing sequence, which was a two-stage automated process.

First, a large solid-fuel retrorocket fired at approximately 39,000 feet above the moon’s surface to slow the spacecraft’s descent.

Once jettisoned, three gimbaled vernier engines (small, steerable thrusters) handled the final approach.

Guided by a radar altimeter, the engines enabled a controlled descent toward the surface.

However, an unexpected helium regulator valve leak forced NASA flight controllers to shut the engines off early, causing the spacecraft to free-fall the final 42 feet to the lunar surface at a speed of 7.4 miles per hour.

Surveyor 5 safely landed Sept. 10 at 8:46 p.m. EDT in Mare Tranquillitatis (the Sea of Tranquility) which is a broad, flat plain on the moon formed by ancient volcanic lava flows.

The spacecraft touched down inside a small, rimless crater approximately 30 by 40 feet in diameter and 4 feet deep, situated at a 20-degree slope at coordinates 1.4551°N, 23.1943°E.

One of its footpads landed near the crater rim, while the other two settled lower on the slope.

Surveyor 5’s vidicon television camera recorded the lander’s touchdown and its effect on the lunar surface.

This provided NASA engineers with data used to develop the descent and lift-off procedures for the Apollo Lunar Module’s engines.

After landing, Surveyor 5 slid slightly downhill; an event captured in images showing furrows in the soft regolith, aka loose, rocky material covering the lunar surface.

The panoramic view provided by the Surveyor 5 lander allowed NASA to assess the area around the crater and to study the lunar surface in detail.

Developed by NASA’s Jet Propulsion Laboratory and Caltech, the onboard alpha-scattering instrument was activated about 11.5 hours after landing via commands sent from NASA’s Deep Space Network station near Madrid, Spain.

These commands lowered the alpha-scattering instrument about 30 inches to make contact with the lunar surface on a nylon line.

The instrument’s gold-and-white sensor head, measuring 7-by-6-by-5 inches and weighing five pounds, contained six Curium-242 sources that bombarded the moon’s soil with alpha particles.

Built-in detectors then measured the energy of the scattered particles to identify the chemical makeup of the regolith.

The results showed that the soil was rich in elements such as silicon, aluminum, iron, magnesium, calcium, and titanium.

These elements confirmed that the dark, flat Mare Tranquillitatis region was formed by ancient volcanic activity.

Surveyor 5’s radar reflectivity measurements, which processed the lunar surface texture and composition data at the landing site, validated Mare Tranquillitatis as a safe region for the first crewed landing, which would be the Apollo 11 mission.

About 53 hours after landing, on Sept. 13, 1967, NASA controllers briefly fired one of Surveyor 5’s vernier engines for half a second to observe how the thrust would disturb nearby lunar soil.

NASA announced that the firing created no new cratering and did not generate any significant dust cloud.

Thermal data collected by Surveyor 5 recorded surface temperatures ranging from 234 degrees Fahrenheit during the lunar day to minus 274 degrees at night.

Its television camera transmitted 19,118 photographs to Earth during its mission, which concluded with its final data transmission at 11:30 p.m. EST Dec. 16, 1967.

Surveyor 5 became the fifth spacecraft from Earth to make a successful soft landing on the moon, following Luna 9 (1966), Surveyor 1 (1966), Luna 13 (1966), and Surveyor 3 (1967).

Surveyor 4 was launched July 14, 1967, at 7:53 a.m. EST from Cape Canaveral’s Launch Complex 36A.

As it approached the moon July 17, all radio contact was lost during its descent; just 2.5 minutes before landing. NASA concluded that the spacecraft likely exploded or suffered catastrophic failure, possibly due to the explosion of the solid-fuel retrorocket.

The Apollo 11 lunar module, Eagle, touched down in the southwestern region of the Sea of Tranquility July 20, 1969, at coordinates 0.67416°N, 23.47314°E.

Eagle’s landing site is approximately 15.5 miles southeast from where Surveyor 5 touched down.

The data received from Surveyor 5 played a significant role in Apollo 11’s historic landing in the Sea of Tranquility and its “giant leap” into the history books.