Lucky Starr and the Big Sun of Mercury by Isaac Asimov

the circumstances, he felt he could have maintained such a stride even under Earth's gravity.
    And then, with no warning, no premonitory glow in the sky, no hint from any atmosphere,
there was the
Sun!
    Rather, there was a hairline that was the Sun. It was an unbearable line of light edging a notch of broken rock on the horizon, as though some celestial painter had outlined the gray stone in brilliant white.
    Lucky looked backward. Across the uneven ground that lay behind him there were the splotches of prominence-red. But now, just at his feet, there was a wash of white, catching crystal formations in glinting highlights.
    He moved onward again, and the line of light became first a small splotch and then a larger one.
    The boundary of the Sun was clearly visible, lifting a bit above the horizon in its center, curving gently down on each side. The curve was awesomely flat to one whose eyes were accustomed to the curvature of Earth's Sun.
    Nor did the Sun's blaze drown out the prominences which crawled along its edge like flaming red snake-hair. The prominences were all over the Sun, of course, but only at the edge could they be seen. On the Sun's face, they were lost amid the glare below.
    And over all was the corona.
    Lucky marveled, even as he watched, at the manner in which the inso-suit had been adapted to its purpose.
    A glance at the edge of Mercury's Sun would have been blinding to unprotected eyes, blinding forever. The visible light was bad enough in its intensity, but it was the hard ultra-violet, unfiltered by atmosphere, that would have meant death to vision… and to life itself, eventually.
    Yet the glass of the inso-suit's face-plate was so arranged molecularly as to grow less transparent in direct proportion to the brightness of the light that fell upon it. Only a small fraction of a percent of the Solar blaze penetrated the plate, and he could stare at the Sun without danger, almost without discomfort. Yet at the same time, the light of the corona and the stars come through undiminished.
    The inso-suit protected him in other ways. It was impregnated with lead and bismuth, not enough so as to raise its weight unduly, but enough to block out ultraviolet and x-radiation from the Sun. The suit carried a positive charge to deflect most of the cosmic rays to one side. Mercury's magnetic field was weak, but Mercury was close to the Sun and the cosmic ray density was large. Still, cosmic rays are composed of positively-charged protons, and like charges repel like.
    And, of course, the suit protected him against the heat, not only by its insulating composition but by its mirrorlike reflecting surface, a pseudo-liquid molecular layer that could be activated by a touch on the controls.
    In fact, Lucky reflected, when the advantages of the inso-suit were considered, it seemed a pity that it was not standard protection under all conditions. Unfortunately, he realized, its structural weakness, as a result of lacking metal in real quantity, made it impractical for use except where protection against heat and radiation were paramount considerations.
    Lucky was a mile into the Sun-side now and not conscious of undue heat.
    This did not surprise him. To stay-at-homes who confined their knowledge of space to the sub-etheric thriller shows, the Sun-side of any airless planet was simply a solid mass of undeviating heat.
    This was an oversimplification. It depended on how high the Sun was in the sky. From this point on Mercury, for instance, with only a portion of the Sun above the horizon, comparatively little heat reached the surface, and that little was spread over a lot of ground as the radiation struck almost horizontally.
    The "weather" changed as one went deeper into the Sun-side and finally, when one reached that portion where the Sun was high in the sky, it was everything the sub-etherics said it was.
    And besides, there were always the shadows. In the absence of air, light and heat traveled in a straight line. Neither