Hubble's Universe Unfiltered

  • September 26, 2013

    Martian Encounter

    by Frank Summers

    Comet ISON was introduced to the public earlier this year with much fanfare about a potential “comet of the century." As observations have continued, and especially as recent measures show it to be fainter than predicted, there has been much public speculation as to whether it will flourish or fizzle. While no one can definitively answer that question, it is pretty safe to say that Comet ISON will at least be the "comet of the year."

    On Nov 28th, 2013, Comet ISON will reach its closest point to the Sun, called perihelion. It will swing about 680,000 miles above the surface of the Sun, which is extremely close when you consider that the Sun’s diameter is only a bit larger at about 860,000 miles. Scientifically interesting events will certainly unfold before, during, and/or after that perihelion passage. Whether or not the passage is visually interesting is the big unknown, and one that is hampered by the fact that Comet ISON’s closest approach to Earth is about 40 million miles on Dec 26, 2013.

    However, an encounter with Mars occurs well before either of those milestones. On Oct 1, 2013, Comet ISON will fly by Mars at a distance of about 6.7 million miles. Though it is about 10 times farther than the closest approach to the Sun, it’s also about one-sixth the closest approach to Earth. This Martian encounter brings some intriguing opportunities.

    There are five active Mars missions: the rovers Opportunity and Curiosity as well as the orbiters Odyssey, Express, and MRO (Mars Reconnaissance Orbiter). In a recent blog post, I noted how Curiosity had observed a solar eclipse, but its instruments are not as well suited for observing a much fainter comet. If it does succeed, that would mark the first comet observation from the surface of another planet.

    The Mars mission best equipped to see Comet ISON is MRO. Its instruments include a 20-inch telescope, the largest ever carried into deep space. MRO should easily detect the comet, although there are some complications -- it was designed for rapid observations of the surface, not long exposures of the sky. To create an analogy with Earth satellites, MRO is intended to be used like Landsat, but is being re-purposed to be used like Hubble. Comet ISON encounter observations are slated to begin this weekend.

    These views from Mars will be the closest snapshots yet, and should provide clues to the size of the comet. A larger comet is more likely to survive its close passage by the Sun, while a smaller one may break into pieces. In addition, as Comet ISON passes inside the orbit of Mars and continues to warm, the chance of outbursts from vaporizing ices has grown considerably. Surprises would not be unexpected.

    The Martian encounter data will join the accelerating campaign of Comet ISON observations. Also scheduled for October are more Hubble observations. In total, 16 NASA spacecraft will study the icy visitor as it sweeps through the inner solar system. Add to that thousands of ground-based telescopes and this will be one heavily scrutinized comet. What would be nice is if billions of human eyes could join the party in a couple months. Stay tuned.

  • 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?