Hubble's Universe Unfiltered

  • October 3, 2013

    Update on Comet ISON from Mars

    by Frank Summers

    As an update to last week's blog post (Martian Encounter), the first images of Comet ISON taken from Mars have been released. Click on the image accompanying this blog post to see them in high resolution. But, even then, these are the kind of images that only an astronomer can love. Perhaps some explanation will help others to appreciate them as well.

    As expected, these images came from the High Resolution Imaging Science Experiment (HiRISE) camera of the Mars Reconnaissance Orbiter (MRO). The rovers Curiosity and Opportunity also took images, but no detections have been announced. The rover images are routinely posted in raw form, and a discussion thread among astronomy image enthusiasts has been examining those in detail to try to spot Comet ISON.

    The MRO images show not much more than a dot, but dots in astronomy can be really important. Remember that images of planets around other stars and of distant galaxies in the farthest reaches of the universe are similarly also just dots. Not to imply that Comet ISON rises to that level of importance, but rather to exemplify that the cutting edge of science is filled with such dot-like observations.

    Here, the dot of Comet ISON shows the image did not capture much of the coma, the gaseous cloud surrounding the icy and rocky nucleus. Part of the reason is the exposure time of the image, and part is that Comet ISON is not as bright as expected. When you recognize that Comet ISON, at this time, is almost 200 million miles from Earth and only 7 million miles from Mars, these close up observations provide a great constraint on the overall brightness.

    The size of the dot also helps constrain the size of the comet's nucleus. The resolution of the MRO images is approximately 8 miles per pixel. The nucleus has previously been estimated at sizes smaller than that, so the true size is hidden in the pixelation of the image. However, better estimates of the size can be obtained through deeper analysis of these shots.

    More observations from Mars are ongoing, though the view from Mars will see less and less reflected light as Comet ISON passes farther inside Mars' orbit. These observations confirm, augment, and refine the size and brightness estimates. Generally, there is a somewhat pessimistic feeling that Comet ISON is not going to develop into a spectacular visual comet. But unpredictability is a hallmark of comets, and there is the important unanswered question of whether it will break up or survive its passage by the Sun. We do not yet know whether ISON will end in a bang or a whimper.

  • 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.