HERITAGE CHAPTER · NASA AMES, 1968 TO THE MID-1970S · Educational
The Learjet that looked at the universe.
In the late 1960s NASA’s Ames Research Center bolted a telescope to the cabin wall of a Learjet, let it look out through an opening in the left side of the fuselage, and flew astronomers to 45,000 feet, above most of the water vapor that hides the infrared sky from the ground.
A separate airplane. This chapter is about NASA’s own Learjet observatory, described in NASA’s 1974 report as a Learjet 24B. Lloyd’s Lear 24XR, N24SA, never carried a telescope, and nothing here depicts one aboard it. The diagram below shows a schematic Learjet front view, not any particular airframe.
01 / WHY ALTITUDE
Why the sky is clearer from 45,000 feet.
Water vapor in the lower atmosphere absorbs much of the infrared light arriving from space, so ground-based infrared telescopes see only through a few gaps. NASA’s history of airborne astronomy puts the remedy simply: flying high “puts telescopes above the water vapor in Earth’s atmosphere that blocks certain types of light, like infrared.”
The 1974 Ames report describes its Learjet telescope as a facility for observing celestial objects at infrared wavelengths “where ground-based observations are difficult or impossible” because of absorption in the atmosphere, and states what the airplane bought: typically 70 minutes of observing per flight at altitudes above 13 km (45,000 ft).
How much vapor remains above an aircraft depends on the day and the place, but it falls away quickly with height. The diagram below shows the idea, not a measurement.
Sources: NASA, “History of Airborne Astronomy at NASA”, Learjet paragraph · NASA TM X-62,389, “Lear Jet Telescope System” (September 1974), Preface p. iv.
02 / INTERACTIVE DIAGRAM · ILLUSTRATIVE
Above the vapor.
Move the airplane up. The bar shows, qualitatively, how much of the water-vapor column is still above it; the line from the port is the telescope’s line of sight at its mean elevation of 20°.
Arrow keys move 500 ft; Page Up and Page Down move 5,000 ft.
nearly all of the columnIllustrative, not a calibrated transmission spectrum; the bar shrinks on a square-root scale so the top of the climb stays readable.
- 0–10,000 ftLower troposphere: humid and hazy; most of the water vapor.
- 10,000–25,000 ftMiddle troposphere: clouds and weather; the vapor thins with height.
- 25,000–40,000 ftUpper troposphere: cold and much drier.
- 40,000–50,000 ftObserving altitude: above 45,000 ft (13 km) the report’s Learjet gave about 70 minutes of infrared observing per flight.
03 / THE TELESCOPE
A twelve-inch telescope
in the cabin.
The Ames instrument was patterned after the original “Flying Infrared Telescope” of F. J. Low, H. H. Aumann and C. M. Gillespie, and built as a facility other investigators could bring their detectors to. Everything below comes from the 1974 report, with its printed page numbers.
- Optics
- A Dall-Kirkham Cassegrain: an ellipsoidal primary mirror made of Cervit, coated with aluminium and an overcoat of silicon monoxide, and a spherical secondary. The primary’s maximum useful diameter is 30 cm (12 in.).§I.A, p. 1
- The chopping secondary
- The secondary mirror oscillates about the telescope’s axis, driven by a pair of solenoids, so an on-axis detector sees two adjacent patches of sky, the “right beam” and the “left beam”, at up to about 100 Hz. The driver circuit offers about 15, 30, 48, 70 and 95 Hz; the throw is adjustable to about 15 arc minutes either side of the axis.§I.A, p. 1; figures 12 and 16
- Where it sat
- Attached to the Lear Jet fuselage just aft of the entrance door, on the left side of the cabin, looking out at a mean elevation of 20° above the horizon. A variable-angle adapter allowed 13° to 28°.§II.A, p. 9; figure 3, p. 4; §I.A, p. 1
- How it moved
- ±3° of gyro-stabilised, gimballed motion in roll (elevation) and yaw (azimuth); up to about 2° more in roll by flying with a wing down or up, and more azimuth by turning the airplane.§I.A, pp. 1–2
- The open port
- An air seal near the front of the telescope leaked cabin air when the aircraft was pressurised, so the telescope could move freely in its gimbals: the tube was open to the outside air, sealed only around its moving body, with no window in the light path.§I.A, pp. 1–2; figure 2, p. 3 (gimbal air seal)
- Guiding
- An auxiliary guide scope with a reticle, mounted on the right side of the tube, gave the observer something to steer by; a joystick box was optional. The balance was trimmed in flight by removing about a kilogram.§I.A, p. 2; §II.A, p. 9; §II.D, p. 14; checklist p. 39
- One observation
- The object appeared high in roll if setting or low if rising; the pilot then flew a predetermined plan, keeping the yaw axis in mid-range, while the object crossed the roll axis from 28° to 12°. When it left the field of view the observation was over.§I.A, p. 2
- A mission, by the clock
- Fuel the aircraft and fill the oxygen tanks at T−6 hours; final fill with liquid helium at T−3 hours; oxygen masks and headsets on at T−15 minutes; at takeoff, “hang on to telescope to prevent bumping”; on the climb, avoid pointing the telescope at the Sun and record the cabin pressure differential.§V.B, checklist, pp. 37–39
04 / WHAT IT OBSERVED, AND WHEN
Late 1960s to the mid-1970s.
The report calls the telescope a facility “for observations of celestial objects at infrared wavelengths”. The sources read for this chapter do not list the objects it observed, so none are named here rather than guessed at.
Source: NASA TM X-62,389, Preface p. iv.
- 1968
Scientist Carl Gillespie uses a 12-inch infrared telescope aboard a Learjet 23 at 50,000 feet.
NASA, “History of Airborne Astronomy at NASA”, image caption.
- Late 1960s to early 1970s
The original 12-inch “Flying Infrared Telescope”, built by Dr Frank Low at Rice University, flies several times with operations managed by Ames, and becomes the prototype for the Kuiper Airborne Observatory’s telescopes. It is now in the Smithsonian.
Smithsonian NASM, “Airborne Infrared Telescope” (A19830086000), object description · NASA TM X-62,389, Preface p. iv.
- Early 1970s
The Learjet Observatory, a Learjet 24B, is photographed over California with the telescope just in front of the wing.
NASA, “History of Airborne Astronomy at NASA”, image caption.
- September 1974
Ames publishes “Lear Jet Telescope System”, the report this chapter is built on: a multi-user facility managed by the Airborne Science Office, giving typically 70 minutes of observing per flight above 13 km.
NASA TM X-62,389, Preface p. iv (Erickson, Goorvitch, Dix and Hitchman).
- 1965 to 2014
The wider lineage: NASA Ames flew the Convair 990 “Galileo” for solar-eclipse work in 1965, the Kuiper Airborne Observatory (a C-141 with a 36-inch telescope) from 1975 to 1995, and SOFIA (a Boeing 747SP with a 106-inch telescope) from 2014 at about 45,000 feet. NASA Glenn also flew Learjets from 1963 to test solar cells under the sunlight they would meet in space; its article names no model, so those flights are not attributed to a Lear 24.
NASA, “History of Airborne Astronomy at NASA”, body · NASA Glenn, “Legacy Forged Through Decades of Flight Research”, solar-cell paragraph.
05 / WHICH AIRPLANE, EXACTLY
Which airplane, exactly?
NASA’s history page and the 1974 report both call the Ames observatory aircraft a Learjet 24B; the same page pictures a 12-inch telescope aboard a Learjet 23 in 1968, so Learjets of two models carried airborne infrared telescopes in different years. NASA also flew a Learjet 24 as an airborne test aircraft at Dryden until it was damaged beyond economical repair in a landing accident in 2001, with no injuries; the NTSB records that airplane as a Model 24A. Whether it was the same airframe as the Ames observatory rests on a commercial blog’s chain of numbers, so this chapter tells them as two stories and asserts no link.
And not this one. Lloyd’s Lear 24XR, N24SA, is a Learjet Inc. Model 24 with no NASA history in any source read. No telescope, port, NASA equipment or NASA markings were ever part of it, and none appear on its model in the Aircraft Atlas.
Sources: NASA, “History of Airborne Astronomy at NASA”, captions · NASA TM X-62,389, Preface p. iv · NASA, “Lear-24”, caption · NTSB Aviation Accident Final Report LAX01TA204, pp. 1–2 · FAA Aircraft Registry, record for N24SA.
06 / KNOWLEDGE CHALLENGE · FIVE QUESTIONS · Educational
What did the airplane make possible?
Pick an answer; each one opens the evidence behind it, with the source and where in it to look. Finishing all five records the result in My Hangar on this device.
0 of 5 answered
QUESTION 1 OF 5
What did the Learjet let the instrument on board measure?
Evidence. The report’s preface describes the telescope as a multi-user facility for observing celestial objects at infrared wavelengths, where observations from the ground are difficult or impossible because of absorption in the atmosphere. Its mission checklist has the detector’s dewar filled with liquid helium three hours before takeoff, the mark of an infrared detector.
“observations of celestial objects at infrared wavelengths, where ground-based observations are difficult or impossible”
Source: NASA TM X-62,389, “Lear Jet Telescope System” ↗, Preface p. iv; checklist §V.B p. 37 (liquid helium).
Why it matters. The airplane did not make the telescope bigger or steadier than a ground instrument; it changed what was in the way. Above most of the water vapor, infrared wavelengths that never reach a mountaintop could be measured with a 30 cm mirror.
QUESTION 2 OF 5
Why fly the telescope to 45,000 feet rather than use it on a mountain?
Evidence. NASA’s history of airborne astronomy explains that flying high puts a telescope above the water vapor that blocks infrared light from reaching ground-based telescopes; the 1974 report gives the altitude the Learjet reached for it, above 13 km (45,000 ft).
“puts telescopes above the water vapor in Earth’s atmosphere that blocks certain types of light, like infrared”
Sources: NASA, “History of Airborne Astronomy at NASA” ↗, Learjet paragraph · NASA TM X-62,389 ↗, Preface p. iv.
Why it matters. Water vapor is concentrated in the lowest miles of the atmosphere, so a modest altitude gain leaves most of it below. The diagram above is a picture of that idea; the report’s 70 minutes per flight is the price of it.
QUESTION 3 OF 5
How did the telescope see out of a pressurised cabin?
Evidence. The report says the telescope attached to the fuselage just aft of the entrance, on the left side of the cabin, and that an air seal near its front leaked cabin air when the aircraft was pressurised so the telescope could move freely in its gimbals.
“leaks cabin air when the aircraft is pressurized to permit free motion of the telescope in the gimbals”
Source: NASA TM X-62,389 ↗, §I.A pp. 1–2 (air seal); §II.A p. 9 and figure 3 p. 4 (mounting, left side).
Why it matters. A window would have absorbed the very wavelengths the flight was for, so the tube looked out through an opening and the cabin simply leaked a little around it: ±3° of gyro-stabilised motion at a mean 20° above the horizon, sealed by air rather than glass.
QUESTION 4 OF 5
How much observing did one flight typically give the astronomers?
Evidence. The preface states what the Learjet permitted: typically 70 minutes of observing per flight at altitudes above 13 km. Each object had to be flown across the telescope’s field, the pilot keeping the yaw axis in mid-range while it crossed from 28° to 12° of elevation.
“typically 70 min. of observing per flight at altitudes in excess of 13 km (45,000 ft)”
Source: NASA TM X-62,389 ↗, Preface p. iv; §I.A p. 2 (the flight pattern); §V.B pp. 37–39 (the mission checklist from T−6 hours).
Why it matters. An hour and ten minutes above the vapor, bought with a six-hour countdown, oxygen masks and a helium fill, is the whole bargain of airborne astronomy in one number; the later Kuiper Airborne Observatory and SOFIA bought longer legs with bigger airplanes.
QUESTION 5 OF 5
Which airplane carried the Ames telescope described in the 1974 report?
Evidence. The report’s preface places the telescope in a Lear Jet model 24B; NASA’s history page captions the early-1970s photograph “The Learjet Observatory (Learjet 24B aircraft)”. The FAA registry lists N24SA as a Learjet Inc. Model 24, serial 24-117, and no source read connects it with NASA. The Convair 990 and the C-141 were NASA’s other airborne observatories.
“The telescope is mounted in a Lear Jet model 24B”
Sources: NASA TM X-62,389 ↗, Preface p. iv · NASA, “History of Airborne Astronomy at NASA” ↗, captions and body · FAA Aircraft Registry ↗, record for N24SA.
Why it matters. Same type, different airplane. Lloyd’s Learjet 24 is the door into this story, not a character in it; keeping the two apart is what lets the rest of this chapter be trusted.
CHALLENGE COMPLETE · Educational
What the airplane made possible, in five answers.
Understanding the evidence matters more than the count.
A personal study record on this device. Not a pilot logbook, a training endorsement or a proficiency assessment.
07 / SOURCES & PROVENANCE
Sources and provenance
Sources & provenance 13 claims · 8 sources
Verified: read directly in a primary source (NASA report, NASA page, museum record). Reported: stated by a cited source that is not an authoritative record. Unverified: mentioned or contested, not established.
Spotted an error, or have a document that settles a claim? Suggest a correction
- VerifiedWhy altitude helps: above the water vapor that blocks infrared
NASA, “History of Airborne Astronomy at NASA”, Learjet paragraph (published September 24, 2018; updated July 26, 2023).
- VerifiedTelescope in a Lear Jet model 24B; typically 70 minutes above 13 km (45,000 ft); managed by Ames’s Airborne Science Office
NASA TM X-62,389, Preface p. iv.
- VerifiedOptics: Dall-Kirkham Cassegrain, Cervit primary, 30 cm useful diameter; chopping secondary to ~100 Hz, 15/30/48/70/95 Hz, ~15 arc min throw
NASA TM X-62,389, §I.A p. 1; figures 12 and 16.
- VerifiedPointing: mean elevation 20°, adapter 13°–28°, ±3° gyro-stabilised gimbals in roll and yaw, ~2° more by banking; the air seal leaks cabin air when pressurised
The report’s text layer loses its degree symbols (“200” reads as 20°, “±30” as ±3°); the figures are the printed ones.
NASA TM X-62,389, §I.A pp. 1–2.
- VerifiedMounting just aft of the entrance, left side; guide scope on the right of the tube; elevation turnbuckle to the floor
NASA TM X-62,389, §II.A p. 9; figure 3 p. 4; §II.D p. 14; figure 2 p. 3.
- VerifiedOne observation: the object crosses the roll axis from 28° to 12° while the pilot flies a predetermined plan
NASA TM X-62,389, §I.A p. 2.
- VerifiedMission checklist: oxygen, liquid helium at T−3 h, masks at T−15 min, “hang on to telescope”, avoid the Sun, cabin differential
NASA TM X-62,389, §V.B pp. 37–39.
- Verified1968: 12-inch telescope aboard a Learjet 23 at 50,000 ft; early 1970s: Learjet Observatory, a Learjet 24B, telescope just in front of the wing
NASA, “History of Airborne Astronomy at NASA”, image captions.
- VerifiedThe original Low “Flying Infrared Telescope” (Rice University), flown late 1960s to early 1970s, prototype for the KAO telescopes; the Ames instrument patterned after it
Smithsonian NASM, A19830086000, object description · NASA TM X-62,389, Preface p. iv.
- VerifiedOther NASA airborne observatories: Convair 990 (1965), KAO C-141 (1975–1995), SOFIA 747SP (2014 onward); NASA Glenn’s Learjet solar-cell flights from 1963, model unstated
NASA, “History of Airborne Astronomy at NASA”, body · NASA Glenn, “Legacy Forged Through Decades of Flight Research” (August 18, 2026), solar-cell paragraph.
- VerifiedNASA Dryden’s Lear 24 test aircraft, damaged beyond economical repair on June 7, 2001, no injuries; recorded by the NTSB as a Model 24A
NASA, “Lear-24”, caption · NTSB Aviation Accident Final Report LAX01TA204, pp. 1–2 (not linked: the hosted file name carries another registration) · Aviation Safety Network record 323277.
- UnverifiedSame airframe: that the Ames observatory Learjet later became the Dryden test aircraft
Rests on a commercial blog’s chain of registrations and tail numbers, corroborated only in part by the 1974 report’s photograph. Not asserted anywhere on this site; the two are told as separate stories.
- VerifiedNot N24SA: the registry lists Lloyd’s airplane as a Learjet Inc. Model 24, serial 24-117; no source read connects it with NASA
FAA Aircraft Registry, record for N24SA.
Imagery: no NASA photograph is reproduced on this page. The report’s photograph of the Learjet carries the NASA insignia and another registration, and NASA insignia are not public domain; the two diagrams are original schematics with NASA acknowledged as the source of the facts they illustrate. Every source was read on September 7, 2026.
ABOUT THIS CHAPTER
Educational. Written from NASA’s 1974 technical report and NASA’s own history pages, with the Smithsonian for the original instrument. Nothing here implies any NASA endorsement of this site, and no NASA insignia appear. Lloyd’s Lear 24XR is the door into the story, not a part of it.
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