The Sun is at times serene-and sometimes violent-but always indispensable for life on Earth. How well do we know our Sun? What have we learned of it from Skylab that we did not know before? Are any major questions left unanswered? Are there any more surprises on the Sun?
By any yardstick Skylab's solar observations were an unqualified success, and probably the biggest step forward in all the centuries of man's efforts to understand our Sun and nearest star. Nine men brought back to Earth, in three Pacific splashdowns, a treasure trove of new knowledge of the Sun that has resolved fundamental questions about the most important object in the sky. Their efforts have changed the path of research in a field that was mature and presumably well understood when Skylab's fiery rocket left the sands of Florida for the sky.
Changes in solar astronomy followed almost instantly because the tightly organized Skylab program disseminated new results immediately to scientists around the world. When Skylab's last crew landed, on February 8, 1974, solar data from their program had already become a keystone in modern understanding of the Sun, much as the Apollo lunar findings almost instantly rewrote astronomical textbooks on the Moon. As with Apollo, the splashdown was but the beginning; since 1974 an ever increasing number of important new results have come from the Skylab data, and we have only begun to use them.
We now know of a host of previously hidden and important relationships between the separate layers of the solar atmosphere: how the various masks of the Sun-photosphere, chromosphere, transition region, and corona-are connected and related, for Skylab showed us these faces simultaneously, and in their many moods. We had never before had that observational advantage. For the first time Skylab allowed us an extended view, in sharp detail, of the mysterious outer atmosphere of the Sun. In X-ray wavelengths we saw the corona in full face, not just at the edges as was all that had been possible for previous generations of astronomers, who had been limited to observations during natural eclipses or observations with ground-based coronagraphs. It was the answer to a long-held dream of astronomers, who since Benjamin Franklin's day had followed the paths of solar eclipses to the ends of the world for a few minutes' look at this important layer of the Sun.
Skylab's X-ray photographs revealed the secrets of coronal holes, which were of necessity masked in eclipse views, and which may prove to be the most terrestrially important of all solar phenomena. The quick responses of Skylab's astronaut observers, coupled with its unequaled instrumentation, caught at last the secrets of the start of solar flares, another feature of profound terrestrial importance. X-ray and ultraviolet pictures of early stages of flares from Skylab force a reappraisal of old concepts of these explosive events and demonstrate  that the visible light appearance of flares, which has been studied for generations, is but a late and perhaps unimportant phase of the real flare event, which comes earlier and higher in the solar atmosphere.
Skylab found a whole new realm of solar activity on a cosmic scale in coronal transients-the Sun's biggest show-great blobs of coronal matter, larger than the Sun itself, that stream outward past the planets dwarfing everything in their path. We may presume that the Sun has been blowing these gargantuan bubbles toward the planets for thousands of years; and yet their presence went almost unnoticed and largely unseen until the time of Skylab.
Thanks to Skylab's ultraviolet and X-ray cameras we now know about another new sign of action on the Sun-bright points in the corona and upper chromosphere that may well prove to be more fundamental than the more obvious signs of magnetic activity, like sunspots, which have for so long dominated our attention. It seems safe to predict that from continued studies of all these newfound features will come the sorts of advances which followed Galileo's first detailed looks at sunspots or Hale's monochromatic pictures of the chromosphere.
Yet any look at history warns us how wrong we can be in attempting to assess the progress of our own generation in learning about nature. That assessment seems better reserved for retrospect, and the longer the view the better. Then, almost invariably, the path of progress proves different from what it seemed along the way: not a beeline from one discovery to another, but a twisting and tortuous path with as many false starts as good ones. Truth is elusive, and meaningful science was never easy.
Some contend that science and learning advance largely through revolutions in which previously held truths are overturned by new evidence or new inquiry. If that is so, we can still be certain that in solar physics Skylab was such a turning point, and a source of a wholly new and clearer picture of the Sun.
We can be even more certain that all the questions are not yet answered, or even asked, nor all the surprises found. Skylab gave us a detailed look at the Sun during only one phase of its varied behavior, near the minimum of the 11-yr solar cycle. What are the natures of coronal holes, coronal transients, and bright points when the Sun is at the maximum of its well-known activity cycle? What is the cause of the I l-yr cycle of sunspots and of more anomalous times when solar activity seems to disappear altogether from the Sun? How constant is the Sun? Solar variation clearly affects our upper atmosphere, producing the aurora borealis, distortions of our magnetic field, and changes in our ionosphere. Do these or other changes on the Sun affect short-term weather or long-term climate on Earth? And if so, which solar changes, and how? What powers the Sun? How long will that fuel source last, and how constantly will it burn? Are our theories of the solar interior (which no one has ever seen) correct, or, as so often is the case, only a present approximation in the winding path toward truth?
Skylab seems certain to confirm another lesson from history: that from every new instrument and endeavor in science we can expect a few answers and many more new questions. So it was with Galileo's small refractor and with Tycho Brahe's great astronomical quadrant instrument before that. Our pictures of a new Sun from Skylab have answered urgent questions but they, too, will raise a host of new ones. It is the way we learn, and it is not so much the answers as the questions that lead us toward a better understanding of the all-important Sun.