TI 2013


Hubble Telescope Mirror Failure report

Executive summary (excuse the formatting it is copied from a pdf)

Hubble Mirror Failure Report
Executive Summary

The Hubble Space Telescope (HST) was launched aboard the Space Shuttle
Discovery on April 24, 1990. During checkout on orbit, it was discovered that the
telescope could not be properly focused because of a flaw in the optics. The HST
Project Manager announced this failure on June 21, 1990. Both of the high resolution
imaging cameras (the Wide Field planetary Camera and the Faint Object
Camera) showed the same characteristic distortion, called spherical aberration, that
must have originated in the primary mirror, the secondary mirror, or both.
The National Aeronautics and Space Administration (NASA) Associate
Administrator for the Office of Space Science and Applications then formed the
Hubble Space Telescope Optical Systems Board of Investigation on July 2, 1990, to
determine the cause of the flaw in the telescope, how it occurred, and why it was
not detected before launch. The Board conducted its investigation to include
interviews with personnel involved in the fabrication and test of the telescope,
review of documentation, and analysis and test of the equipment used in the
. fabrication of the telescope's mirrors. The information in this report is based
exclusively on the analyses and tests requested by the Board, the testimony given
to the Board, and the documentation found during this investigation.
Continued analysis of images transmitted from the telescope indicated that
most, if not all, of the problem lies in the primary mirror. The Board's
investigation of the manufacture of the mirror proved that the mirror was made in
the wrong shape, being too much flattened away from the mirror's center (a.
0.4-wave rms wavefront error at 632.8 nm). The error is ten times larger than the
specified tolerance.
The primary mirror is a disc of glass 2.4 m in diameter, whose polished front
surface is coated with a very thin layer of aluminum. When glass is polished,
small amounts of material are worn away, so by selectively polishing different
parts of a mirror, the shape is altered. During the manufacture of all telescope
mirrors there are many repetitive cycles in which the surface is tested by reflecting
light from it; the surface is then selectively polished to correct any errors in its
shape. The error in the HST's mirror occurred because the optical test used in this
process was not set up correctly; thus the surface was polished into the wrong
The primary mirror was manufactured by the Perkin-Elmer Corporation, now
Hughes Danbury Optical Systems, Inc., which was the contractor for the Optical
Telescope Assembly. The critical optics used as a template in shaping the mirror,
the reflective null corrector (RNC), consisted of two small mirrors and a lens.
The RNC was designed and built by the Perkin-Elmer Corporation for the HST Project.
This unit had been preserved by the manufacturer exactly as it was during the
manufacture of the mirror. When the Board measured the RNC, the lens was
incorrectly spaced from the mirrors. Calculations of the effect of such
displacement on the primary mirror show that the measured amount, 1.3 mm,
accounts in detail for the amount and character of the observed image blurring.
No verification of the reflective null corrector's dimensions was carried out by
Perkin-Elmer after the original assembly. There were, however, clear indications
of the problem from auxiliary optical tests made at the time, the results of which
have been studied by the Board. A special optical unit called an inverse null
corrector, designed to mimic the reflection from a perfect primary mirror, was built
and used to align the apparatus; when so used, it clearly showed the error in the
reflective null corrector. A second null corrector, made only with lenses, was used
to measure the vertex radius of the fished primary mirror. It, too, clearly showed
the error in the primary mirror. Both indicators of error were discounted at the
time as being themselves flawed.
The Perkin-Elmer plan for fabricating the primary mirror placed complete
reliance on the reflective null corrector as the only test to be used in both
manufacturing and verifying the mirror's surface with the required precision.
NASA understood and accepted this plan. This methodology should have alerted
NASA management to the fragility of the process and the possibility of gross error,
that is, a mistake in the process, and the need for continued care and
consideration of independent measurements.
-T,he design of the telescope and the measuring instruments was performed well
by skilled optical scientists. However, the fabrication was the responsibility of the
Optical Operations Division at the Perkin-Elmer Corporation (P-E), which was
', insulated from review or technical supervision. The P-E design scientists,management,
and Technical Advisory Group, as well as NASA management and 1 NASA review
activities, all failed to follow the fabrication process with reasonable
diligence and, according to testimony, were unaware that discrepant data existed,
although the data were of concern to some members of P-E's Optical Operations
Division. Reliance on a single test method was a process which was clearly
vulnerable to simple error. Such errors had been seen in other telescope
programs, yet no independent tests were planned, although some simple tests to
protect against major error were considered and rejected. During the critical time
period, there was great concern about cost and schedule, which further inhibited
consideration of independent tests.
The most unfortunate aspect of this HST optical system failure, however, is that
the data revealing these errors were available from time to time in the fabrication
process, but were not recognized and fully investigated at the time. Reviews were
inadequate, both internally and externally, and the engineers and scientists who
were qualified to analyze the test data did not do so in sufficient detail.
Competitive, organizational, cost, and schedule pressures were all factors in
limiting full exposure of all the test information to qualified reviewers.


Mars Climate Orbiter Failure due to Metric Unit Mix up

Summary from Wikipedia

On November 10, 1999, the Mars Climate Orbiter Mishap Investigation Board released a Phase I report, detailing the suspected issues encountered with the loss of the spacecraft. Previously, on September 8, 1999, Trajectory Correction Maneuver-4 was computed and then executed on September 15, 1999. It was intended to place the spacecraft at an optimal position for an orbital insertion maneuver that would bring the spacecraft around Mars at an altitude of 226 kilometers on September 23, 1999. However, during the week between TCM-4 and the orbital insertion maneuver, the navigation team indicated the altitude may be much lower than intended at 150 to 170 kilometers. Twenty-four hours prior to orbital insertion, calculations placed the orbiter at an altitude of 110 kilometers; 80 kilometers is the minimum altitude that Mars Climate Orbiter was thought to be capable of surviving during this maneuver. Final calculations placed the spacecraft in a trajectory that would have taken the orbiter within 57 kilometers of the surface where the spacecraft likely disintegrated because of atmospheric stresses. The primary cause of this discrepancy was engineering error. Specifically, the flight system software on the Mars Climate Orbiter was written to take thrust instructions using the metric unit newtons (N), while the software on the ground that generated those instructions used the Imperial measure pound-force (lbf). This error has since been known as the "metric mix up" and has been carefully avoided in all missions since by NASA.[16]
The discrepancy between calculated and measured position, resulting in the discrepancy between desired and actual orbit insertion altitude, had been noticed earlier by at least two navigators, whose concerns were dismissed. A meeting of trajectory software engineers, trajectory software operators (navigators), propulsion engineers, and managers, was convened to consider the possibility of executing Trajectory Correction Maneuver-5, which was in the schedule. Attendees of the meeting recall an agreement to conduct TCM-5, but it was ultimately not done.



Scientific Explorations by Paul Doherty

© 2013

29 July 2013