In the public mind, the image of a scientist is often that of a superman capable of achieving miracles. If a statement or a calculation has been vetted by a scientist, it is bound to be correct and, as such, reliable. At least that is the layman’s impression. Reality may, however, advice caution: for, despite an aura of correctness in general, there may be exceptional situations wherein the scientist may have goofed up. The following examples illustrate the need for caution. One of the triumphs of space astronomy was the launch of the Hubble Space Telescope (HST). The idea of placing a space telescope in a satellite orbit round the Earth had been tempting the astronomers and space technologists right from the early days of space technology. Indeed, the progress from Sputnik to a space telescope represents the remarkable strides taken by space technology in the second half of the last century.
The advantages of an optical telescope working at a height of some 500-600 km above sea level are many. For example, when we view stars through a ground-based telescope, they twinkle. The twinkling may be good for poets and nursery rhymes, but for the astronomer it represents unsteady images; unsteady because the movements of air in the Earth’s atmosphere cause the stellar images to shake. A space telescope catches those images formed by light rays before they go through the atmospheric air. That is why the steadiness of a space telescope image cannot be matched by images from a ground-based telescope.
A second benefit of such a telescope is, of course, that its image has very little absorption by en-route layers of the atmosphere. So the images are brighter and the space telescope can reach out to considerably fainter objects than its ground based counterpart. And, of course, a third, but not the least, advantage is that a dark sky is available to the space telescope round the clock. This is in contrast to ground-based telescopes which have to wait till well after sunset before starting observations and wind them up before sunrise. Because of these unique features, a space telescope is supposed to be worth it, despite the fact that cost of making and launching one is far greater.
So, when HST, so named after the famous astronomer Edwin Hubble (who is credited with the discovery that the universe is expanding), was finally launched in 1990, a lot was expected from it. Unfortunately, however, the images seen from the new telescope did not come up to the level expected. They were blurred, like the view seen by a man with imperfect vision. Why was the telescope behaving in such an imperfect manner? The question required an urgent answer since so much money had already been spent on the exercise. In response, the prime agency Nasa established a commission of enquiry to decide why and how the error had crept in and how could the situation be retrieved.
The commission found that the primary mirror of the HST had been ground to a shape that was incorrect. The level of precision to be achieved was such that the difference between the theoretically required curved shape and that found in the telescope should have been less than 2.2 nanometres, whereas it actually exceeded up to 2.2 micrometers near the perimeter. Just as a human being with imperfect vision uses glasses, it was necessary to correct this defect by adding a corrector lens. Called “COSTAR”, it provided a Collective Optics Space Telescope Axial Replacement. The astronauts who went on the first service mission of the telescope in 1993 applied the corrective treatment and brought the telescope to its expected capability. One could call COSTAR the glasses that HST needed. In retrospect, one sees that all this expense and fuss could have been avoided if the mirror had been thoroughly checked while it was on the ground waiting to be launched.
Another example of how an error cost Nasa the entire mission, worth $125 million, is seen in the history of the Mars Climate Orbiter back in October 1999. The error was overtly trivial but potentially serious. It came from the use of different units by different sections of the project. the jet propulsion laboratory used the metric system of units like centimetre, gram, etc., while the builder of the spacecraft, Lockheed Martin Astronautics, in Denver, used the English units of pound, foot, etc. Thus, there was a natural mistake in using data in one set of units to work out the answer in the other units. As a result, the Mars probe was lost. This shows how overconfidence in one’s work may lead to errors of a serious nature.
Lest these examples give the impression that errors occur only in experimental projects, here is an example to show that theoreticians as great as Albert Einstein are not immune from them. Einstein’s coworker in Princeton, Leopold Infeld describes in his autobiography, how Einstein came to the conclusion that gravitational waves cannot exist.
His conclusion was based on a long calculation and was important enough for announcement in a technical seminar. Accordingly, a seminar was announced and drew good attendance as all of Einstein’s seminars did. In the meantime, however, Einstein discovered an algebraic error in his calculation, which on correction vitiated his important conclusion. So at the seminar, Einstein gave his full calculation and then described where his error had crept in. Thus, we have many different situations where error bugs creep in unannounced and one needs to be careful about their intrusion! Are scientists error-prone? They are after all human. But they are aware of the possibility and are expected to take care that they have debugged their work. Only, sometimes they don’t!
The writer, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy and Astrophysics, Pune University Campus.
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