Hubble's Universe Unfiltered

  • September 19, 2013

    The Edge of Space

    by Frank Summers

    Last week, NASA announced that the Voyager 1 spacecraft had become the first human-made object to enter into interstellar space. As a writer, I liked how they said "human-made" in the press release, as it both directly avoids the chauvanistic "man-made" term and implicitly allows for non-human species to have beaten us to this new frontier. As an astronomer, I didn't like some of the resulting press that made the natural, yet wrong, conclusion that Voyager has left the solar system. The blame is partly on NASA, who did not make the distinction in the release, but rather in an accompanying article.

    First, let's deal with interstellar space. Interstellar means "between the stars," and hence refers to anyplace that is inside our galaxy and outside of the atmosphere of a star. The important question for today is defining where the atmosphere of a star ends. (I'll leave the question of "inside our galaxy" for a later disussion.)

    As seen during total solar eclipses and by several space missions, our Sun has a large extension to its visible atmosphere called the solar corona. The corona, in turn, blends into the solar wind that streams past Earth and the other planets. The boundary where the solar wind dies out is the place where interstellar space begins.

    As the solar wind extends to larger and larger volumes about the Sun, the pressure it exerts drops. At some point that pressure is counter-balanced by the pressure of the gas in interstellar space. For Voyager 1, the boundary crossing was marked by measurements that indicated changes in density starting on August 25, 2012. That date is now accepted as the beginning of humanity's interstellar adventure.

    Second, let's consider the extent of the solar system. Earth is 93 million miles from the Sun; a distance we define as one astronomical unit (AU). The farthest planet, Neptune, orbits at 30 AU. The thousands of small, icy objects (including Pluto) discovered in the Kuiper Belt are generally out to 50 AU. The most distant object yet discovered orbiting our Sun is Eris, at about 97 AU. With Voyager 1 at 125 AU and Voyager 2 at 102 AU, both are beyond these distances.

    However, the orbits of long-period comets extend out to around 500 times farther still. The comets are too small and faint to be seen beyond the outer planets, but the parameters of their orbits are well measured. At an outer distance of roughly 50,000 AU, the Oort Cloud of comets represents the boundary of objects orbiting the Sun. Considering a scale model where Earth is one inch from the Sun, Voyager 1 is about 10 feet away and the Oort Cloud extends for over three-quarters of a mile. It will take tens of thousands of years for the Voyager spacecraft to reach such distances and truly exit the solar system.

    The apparent contradiction is that Voyager 1 has crossed into interstellar space, yet remains inside our solar system. It is not really a conflict, as the two measures use different criteria. The edge of interstellar space is a pressure boundary, while the extent of solar system orbits is a gravitational boundary. There is no reason the two boundaries should coincide, and, in fact, they differ greatly.

    Voyager 1 is not the only long-distance space journey in the news these days. Comet ISON, when its orbit is extrapolated backward, has spent most of its time well outside the boundary of the solar wind. Its passage into the inner reaches of the solar system is vanishingly brief compared to its entire orbit. If and when it flares up this fall / winter, consider it a beautiful and dynamic greeting from a passing interstellar traveller.

  • September 12, 2013

    A Most Curious Solar Eclipse

    by Frank Summers

    It is not unusual for enthusiasts to make grand travel plans in order to observe solar eclipses. Friends of mine have journeyed to China, climbed one of the highest mountains in Mexico, and set sail to the middle of the ocean in order to witness these rare celestial events. Eclipse chasers say they are drawn to the incredible views of the solar corona as well as that indescribable feeling from the natural world as day slips into night and back again over a brief period.

    But no one travels farther to observe solar eclipses than NASA. One of the most famous solar eclipse shots was captured by Apollo 12 on the return flight back from the Moon. Passing through Earth's shadow, the crew captured a stunning image of a solar eclipse and were perhaps the first to use Earth as the occulting object. A similar and even more amazing image was taken by the Cassini Mission at Saturn. With the Sun in eclipse, the rings of Saturn were illuminated from behind and showcased as never before.

    NASA's latest solar eclipse tour de force comes from the Mars Science Laboratory, which everyone calls the Curiosity rover. The Curiosity team was able to position the rover to observe a solar eclipse by the larger of Mars' two moons, Phobos (the smaller moon is called Deimos). Given that Curiosity is a rover on the surface of Mars, and not a spacecraft that can fly into a shadow, that was some incredible feat of planning. Further, Phobos is a tiny moon (probably a captured asteroid) about 15 miles across -- much, much smaller, and with a correspondingly much, much smaller shadow, than the Moon, Earth, or Saturn.


    The above video shows the full timelapse sequence of 89 images captured by Curiosity, and is shown in realtime. It took just 32 seconds for Phobos to pass across the face of the Sun. The geometry of this eclipse makes it about as close to a total solar eclipse as is possible on Mars, with Phobos' silhouette as large as ever. The irregular shape of this tiny moon is brought out in sharp relief, allowing for more precise measurements of its size and orbit.

    While the visual image is not as spectacular as the Sun's corona or Saturn's backlit rings, the philosophical impact is just as deep. NASA is not just exploring the geography, geology, hydrology, etc., of Mars; we can also do astronomy observations from its surface. For those of us somewhat jaded by picture after picture of Mars' surface rocks, this image of a rock in Mars' sky is a great booster shot of awe and wonder.

    And, if you think this observation is something, just wait until they observe Comet ISON from Mars in the coming months. Doesn't that pique your curiosity?

  • September 5, 2013

    Episode 20: A Horse of a Different Color

    by Frank Summers

    Download this episode


    A Horse of a Different Color

    The Horsehead Nebula is a striking, dark gas cloud just below Orion's belt. It is a favorite of both professional and amateur astronomers. However, as a dark nebula, most of its true structure is hidden from visible light observations. To celebrate the 23rd anniversary of the Hubble Space Telescope, we revealed the considerable detail of that unseen nebular structure via an infrared portrait. The result is even more striking, and something one just doesn't see very often: a veritable astronomical horse of a different color.


    Hubble press release:



    • When I introduce the image of the Orion constellation, including the Hydrogen Alpha filter, I mistakenly imply that the image is only in Hydrogen Alpha. It is not. It is an image that includes the standard visible light plus the Hydrogen Alpha light to augment it. Astronomical images are often taken repeatedly through a series of filters which isolate different wavelength regions of light. Hubble's images are always done one filter at a time and then added together as needed with image processing software. In fact, everyday digital cameras that take "color" pictures are really processing the red, green, and blue light separately before combining them into a color image. Similarly, computer monitors display separate tiny red, green, and blue dots that your eye sees as a single color. Filters and color science are not just for geeks, they are part of everyday life.

    • Hydrogen Alpha is a friendlier term for the wavelength of light emitted when the electron in a hydrogen atom drops from the third to the second orbital level. Its wavelength is 656.28 nanometers and it provides a characteristic pinkish glow. Because hydrogen is by far the most abundant element in the universe, and hydrogen alpha is the dominant hydrogen emission in visible light, pink is the standard color of nebulae. By using filters, we can select out and showcase other colors, but the human eye would see most nebulae as pink.

    • The VISTA telescope is an acronym for the "Visible and Infrared Survey Telescope for Astronomy." It is run by the European Southern Obsevatory and located at Cerro Paranal in Chile. As a wide-field telescope, it can cover much larger swaths of the sky than Hubble and obtain complete astronomical portraits. By contrast, Hubble just studies the details, albeit with much higher resolution. A beautiful collection of VISTA images shows off its diverse capabilities and includes several nice visible vs. infrared comparisons.

    • The flight into the Horsehead Nebula video shown in this episode is the second half of a two-shot sequence. The first part is a visible light zoom into the Horsehead region on the sky and a cross-fade from visible to infrared. The full zoom and fly sequence can be downloaded from this HubbleSite page. Indeed, it was the joining of the two shots as a seamless sequence that motivated us to start the fly-in shot as a compressed, flat model and expand it to an extended, sculpted model during the first five seconds.

    Image notes

    Orion and Monoceros Region
    Credit and Copyright: Akira Fujii/David Malin Images

    Orion Constellation
    Credit: Tyler Nordgren, University of Redlands

    Orion Constellation with Hydrogen Alpha
    Credit and Copyright: Robert Gendler and Stéphane Guisard

    Orion Constellation in Visible and Infrared Light, IRAS
    Credit: NASA, Infrared Astronomical Satellite

    Flame and Horsehead Nebula, DSS
    Credit: Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO

    Horsehead Nebula, NOAO
    Credit: Nigel Sharp, NOAO

    Horsehead Nebula, Hubble
    Credit: NASA, NOAO, ESA and The Hubble Heritage Team (STScI/AURA)

    Flame and Horsehead Nebula in Infrared Light, VISTA
    Credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

    Horsehead Nebula in Infrared Light, Hubble
    Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

    (video) The Gaseous Landscape of the Horsehead Nebula in Infrared
    Credit: NASA, ESA, and G. Bacon, T. Davis, L. Frattare, Z. Levay, and F. Summers (Viz 3D Team, STScI)