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					                               The Final Flight of a Sun-Diving Comet
                               Carey M. Lisse
                               Science 335, 296 (2012);
                               DOI: 10.1126/science.1217168




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             http://www.sciencemag.org/content/335/6066/296.full.html#ref-list-1
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Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the
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2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is a
registered trademark of AAAS.
PERSPECTIVES

      ASTRONOMY


      The Final Flight of a                                                                                                 A suite of space-based observatories has
                                                                                                                            captured the details of a comet as it
                                                                                                                            disintegrates in the solar corona.
      Sun-Diving Comet
      Carey M. Lisse



      O
               ver the past decade, solar monitor- those fragments, may reveal details about the mass loss rates. The exact physical details of
               ing observatories have detected and sizes of the constituents from which the com- the process are still to be worked out—i.e.,
               discovered more than 1600 discrete ets were formed.                                                exactly what happens to a chunk of cometary
      members of the Kreutz family of comets.                Schrijver et al. tracked comet C/2011 matter once it breaks off or is ejected from
      These comets are associated by common N3 (SOHO) and monitored its optical and the nucleus—but it is highly likely, as the
      orbits and their propensity to come within extreme-ultraviolet (EUV) emissions and authors argue, that the dominant mechanisms
      a few solar radii (RSun) of the Sun. They can absorptions as it passed at tremendous speed are related to coronal plasma collisions. By
      be detected as they evaporate and disinte- (~650 km/s) through the solar corona on 6 studying more passages of comets through




                                                                                                                                                                                                                      Downloaded from www.sciencemag.org on January 19, 2012
      grate, throwing out huge amounts of fine dust July 2011 UT (Universal Time) only 100,000 the corona, both in new events [e.g., the
      and gas, which can be seen even against the km (~0.15 Rsun) above the Sun’s photosphere recent sun-diving passage of comet C/Love-
      Sun’s glare. Thought to be the fragmented (see the figure). The first observations of joy on 16 December 2011 (8)] and by search-
      remnants of the passage of a giant (~10 to their kind, the results pave the way for future ing through the archival SOHO, STEREO,
      50 km radius) parent comet several thou- measurements of sungrazer cometary dis- and SDO records, we will increase our under-
      sand years ago, the Kreutz family has been integration times, length scales, masses and standing of how comets fall apart and are put
      the subject of intense study by both
      amateur and professional astrono-           1000
      mers using a plethora of optical and
      ultraviolet instrumentation onboard
      a number of spacecraft designed to
                                                                             00:01:12
      study the Sun. On page 324 of this                                                  23:59:36
                                                                                                        23:57:00
      issue, Schrijver et al. (1) report com-                                                                         23:56:00
                                                   800
      bined observations from the Solar                                                                                            23:51:36
                                                                                                                                                 23:49:48
      Dynamics Observatory (SDO),
      the Solar Heliospheric Observa-
      tory (SOHO), and Solar-Terrestrial
                                                       North to South (arc sec)




      Relations Observatory (STEREO)
                                                   600
      detailing the path of comet C/2011
      N3 (SOHO) as it passes through
      and disintegrates in the Sun’s lower
      corona. Such a method of cometary
      study may provide insight into the
      makeup of the parent body as well as         400
      the constituent material of the early
      solar system.
          Some of the brightest comets ever
      seen have been large Kreutz sun-
      grazers—the Great Comets of 1106,            200
      1843, 1882, and C/1965 Ikeya-Seki
      (2, 3); and there is a steady stream
      of smaller Kreutz fragments arriv-               0                 200              400                600               800             1000             1200
      ing more or less continuously.                                                               East to West (arc sec)
      The various pieces of the original
      Kreutz comet are estimated (4) to be A flight path to destruction. Extreme-ultraviolet image of the solar corona (in SDO’s 171 Å channel, most sensitive to
      roughly the same size (~10 to 1000 emission from coronal plasma near 10 K), on 6 July 2011 00:00:01 UTC (coordinated UT). Overlaid (dashed-white box)
                                                                                      6


      m in radius) as the fragments pro-      is the projected orbit of the comet C/2011 N3 between 5 July 2011 23:46 UTC and 6 July 2011 00:06 UT. Insets (with
      duced by disrupting comets D/1999 UTC times) show a subframe region containing the comet nucleus, coma, and debris trail, after subtraction of the same
                                                                                                                                                                                     CREDIT: C. J. SCHRIJVER ET AL.




                                              region imaged 36 s earlier to remove the background corona. The nucleus and coma region were found to be consistently
      S4 (LINEAR) (5) and 73P/Schwass-
                                              dark and EUV absorptive in all images; the trail was found to be EUV bright, likely due to energy deposition and heat-
      mann-Wachmann 3 (6, 7) and, like ing from collisions with the solar coronal plasma. Detailed modeling shows that the comet passed at perihelion through
                                                        the closed-field solar corona rather than through the solar wind, although the EUV-bright tail does not align with the
      John Hopkins University, Applied Physics Labo-    observed coronal magnetic field lines. The comet is moving from right to left across the face of the Sun in this image,
      ratory, Laurel, MD 20723, USA. E-mail: carey.     and the nucleus is on the extreme left of each subframe; the debris trail is becoming fainter with time, as the comet dis-
      lisse@jhuapl.edu                                  sipates and disintegrates in the corona. One arc sec equals ~740 km.


296                                                    20 JANUARY 2012 VOL 335 SCIENCE www.sciencemag.org
                                                                                   Published by AAAS
                                                                                                                                                     PERSPECTIVES

together. The results from comet C/2011 N3                  frequency distribution of sun-grazing comet         quasi-static magnetohydrodynamic models
(SOHO) are thus pioneering a new method of                  fragments may be able to provide important          used in the past. Understanding how the roil-
cometary study.                                             information about the formation mechanisms          ing 5780 K convective surface of the present-
    Understanding the physical construc-                    of comets.                                          day Sun, perfused with magnetic field lines
tion of comets sheds important light on how                     Sun-grazing comets also probe a local           extending out into interplanetary space, cre-
matter accreted from tiny, micrometer-sized                 temperature regime, from 1000 to 4000 K,            ates the ~106 K tenuous corona exosphere is
specks of dust and molecules of gas to build                that is not otherwise encountered in the solar      not only the prime goal of NASA’s next big
kilometer-sized ice and rock-rich bodies, the               system, emitting material via sublimation and       solar mission, Solar-Probe Plus, but is also
comets, in the first million years of the solar              thermal desorption as they do so. Thus, from        vital to our existence as human beings living
system’s existence (9). This is still a great               remote spectroscopic studies of sungrazers,         93 million miles away, as this is the region of
mystery—most studies of the aggregation                     we can learn about the least-volatile compo-        space where the giant solar flares and coro-
of protoplanetary gas and dust, using their                 nents that make up comets (and presumably           nal mass ejections are created and launched
known physical parameters (bulk modulus,                    the rest of the bodies in the solar system), like   toward the planets.
porosity, surface cohesion, dielectric con-                 the rocky silicates and metal sulfides that are
stant, etc.) at the expected interaction speeds             some of the first materials to condense out of            References
                                                                                                                 1. C. J. Schrijver et al., Science 335, 324 (2012).
of a few kilometers per second or more, show                the protosolar nebula and protoplanetary disk        2. B. G. Marsden, Astron. J. 98, 2306 (1989).
that particles should build up to millimeter-               and make up the bulk of Earth and the other




                                                                                                                                                                                         Downloaded from www.sciencemag.org on January 19, 2012
                                                                                                                 3. Z. Sekanina, P. W. Chodas, Astrophys. J. 607, 620
to centimeter-sized objects quite easily along              terrestrial planets.                                    (2004).
the plane of the early solar system, but larger-                The work of Schrijver et al. also holds          4. M. M. Knight et al., Astron. J. 139, 926 (2010).
                                                                                                                 5. H. A. Weaver et al., Science 292, 1329 (2001).
sized particles disintegrate upon impact, cre-              great promise for improving our understand-          6. H. A. Weaver et al., Bull. Am. Astron. Soc. 38, 490 (2006).
ating an “aggregational barrier” to planetesi-              ing of the solar corona. By using comets as          7. W. T. Reach, J. Vaubaillon, M. S. Kelley, C. M. Lisse, M. V.
mal formation (10, 11). (On the other hand,                 standard test particles and “running” them              Sykes, Icarus 203, 571 (2009).
                                                                                                                 8. http://science.nasa.gov/science-news/science-at-
once billions of kilometer-sized comet bodies               through the corona, observations of the pas-            nasa/2011/16dec_cometlovejoy
were formed, accretion into the known plan-                 sage of many comets at different heights             9. C. M. Lisse, Bull. Am. Astron. Soc. 42, 965 (2010).
etary-sized objects was relatively straightfor-             above the photosphere, at different times,          10. K. Wada, H. Tanaka, T. Suyama, H. Kimura, T. Yamamoto,
                                                                                                                    Astrophys. J. 702, 1490 (2009).
ward.) Since comets are weak bodies formed                  and in different solar latitudes and longi-
                                                                                                                11. A. Zsom, C. W. Ormel, C. Güttler, J. Blum, C. P. Dullemond,
relatively gently, it is likely they fragment               tudes, will also help us to map out the three-          Astron. Astrophys. 513, A57 (2010).
and disrupt into pieces similar to those from               dimensional density structure of the corona
which they were assembled. Thus, the size-                  in a completely new way, independent of the                                              10.1126/science.1217168




GEOPHYSICS


Transforming Earthquake                                                                                         Citizen science projects have the potential
                                                                                                                to transform earthquake science if data quality

Detection?                                                                                                      standards are maintained.


Richard M. Allen



E
        arthquakes are a collective experience.             discovery, and what is the role of education?       mation of each report is converted to lati-
        Citizens have long participated in                      Modern geophysical instruments can              tude-longitude coordinates and the data are
        earthquake science through the report-              record a magnitude 5 (M5) earthquake from           mapped. Online tools allow users to explore
ing, collection, and analysis of individual                 the other side of the world. However, to            the data set that includes their contribution.
experiences. The value of citizen-generated                 map, track, and analyze the details of large        The project also has an educational compo-
status reports was clear after the 1995 Kobe,               destructive earthquake ruptures, and to elu-        nent explaining earthquake phenomena.
Japan, earthquake (1). Today’s communica-                   cidate how the rupture process links to earth-          The DYFI database now contains nearly
tions infrastructure has taken citizen engage-              quake impacts, requires detailed data from          2 million entries available for download (8–
ment to a new level: Earthquake-related Twit-               close to the event. Currently, the best tradi-      10). The DYFI data are used to complement
ter messages can outrun the shaking (2),                    tional geophysical networks only have sta-          the traditional network data. Combined with
Internet traffic detects earthquakes (3–7) and               tions every ~10 km and cover limited areas.         reports of building damage, they can also
maps the distribution of shaking in minutes                 Contributions of citizens have the potential        help to determine how well building infra-
(8–10), and accelerometers in consumer elec-                to provide much higher resolution, especially       structure can withstand earthquake shaking
tronic devices record seismic waveforms (11–                in residential areas.                               in different locations.
16). What are we learning from this flood of                     The best-developed citizen-based earth-             An individual’s reaction to an earthquake
data, and what are the limitations? How do we               quake science project today is the U.S. Geo-        can also be tracked for scientific purposes
harness these new capabilities for scientific                logical Survey’s (USGS) “Did You Feel It?”          without that individual’s active participation.
                                                            (DYFI) (8–10). After an earthquake, indi-           The European-Mediterranean Seismological
Seismological Laboratory and Department of Earth and
                                                            viduals can go online and answer questions          Center (EMSC) tracks the hits on its Web site
Planetary Science, University of California, Berkeley, CA   designed to capture the data necessary to esti-     and uses the hit rate and Internet protocol (IP)
94720, USA. E-mail: rallen@berkeley.edu                     mate shaking intensity. The location infor-         addresses to extract information about earth-

                                            www.sciencemag.org SCIENCE VOL 335 20 JANUARY 2012                                                                                     297
                                                                             Published by AAAS

				
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Description: he Final Flight of a Sun-Diving Comet Carey M. Lisse Science 335, 296 (2012); DOI: 10.1126/science.1217168 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.