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Chance Pass Through a Cometary Tail

Chance favors the prepared mind. So goes the old saying, never vindicated as clearly as in the encounter between Comet McNaught and the spacecraft called Ulysses. Thomas Zurbuchen (University of Michigan) notes that having a spacecraft on a mission to study the polar regions of the Sun pass through a cometary tail is chancy enough — he likens it to putting your hand in Lake Michigan and pulling out a fish — but having Ulysses already equipped with the needed instruments to study the solar wind means we had an unexpected chance to study the interactions between cold cometary materials from the Solar System’s infancy and hot solar plasmas. Talk about being in the right place at the right time…

A cometary encounter

Complex chemistry emerged from the serendipitous encounter, with O3+ oxygen ions showing the effects of cometary materials on the outgoing stream of solar wind ions. O3+ ions are oxygen atoms with a positive charge, resulting from the presence of five electrons instead of eight. Solar wind ions that were originally missing most of their electrons evidently picked up replacements from McNaught’s gaseous envelope. Moreover, even though Ulysses made its pass some 160 millon miles from the cometary nucleus, it found the solar wind had dropped to half speed.

Image: In May of 1996, Ulysses passed through the tail of Comet Hyakutake, with results significantly different than its recent pass through Comet McNaught’s tail. Making sense of the discrepancies may help us to a better understanding of how the solar wind operates. Credit: NASA/David A. Hardy.

Michael Combi, another author on the paper, takes note of the unexpected find:

“This was very surprising to me,” Combi said. “Way past the orbit of Mars, the solar wind felt the disturbance of this little comet. It will be a serious challenge for us theoreticians and computer modelers to figure out the physics.”

Combi’s surprise is understandable given that in 1996 Ulysses passed through the tail of comet Hyakutake and found no real change in solar wind velocity. The recent findings showed the solar wind moving at less than 249 miles per second while somewhere in the neighborhood of 435 miles per second would be expected at that distance from the Sun. The solar wind itself was discovered partially through the observation that cometary tails always point away from the Sun, a reflection of the magnetism and velocity of the high-speed plasma streams pushing outward from the upper reaches of the solar atmosphere.

Using the solar wind to give a magnetic sail a push to the outer Solar System, as in the Mini-Magnetosphere Plasma Propulsion concepts of Robert Winglee, offers another possible solution for future planetary transport (and, long-term, for interstellar propulsion via particle beam), but the Comet McNaught findings point out how far we are from a robust understanding of how the solar wind functions. Setting sail magnetically upon this particular wind may tax the skills of the ablest navigator, but future studies should clarify how variable is its flow.

The paper is Neugebauer, Gloeckler et al., “Encounter of the Ulysses Spacecraft with the Ion Tail of Comet McNaught,” Astrophysical Journal 667 2007 October 1), pp. 1262-1266 (abstract).

Comments on this entry are closed.

  • ljk February 22, 2008, 9:36

    Ulysses, the mission to study the Sun’s poles and the influence of our star on
    surrounding space is coming to an end. After more than 17 years in space –
    almost four times its expected lifetime – the mission is finally succumbing to
    its harsh environment and is likely to finish sometime in the next month or two.

    More at:


  • ljk July 7, 2008, 22:55

    Atomic Oxygen in the Comae of Comets

    Authors: Anita L. Cochran

    (Submitted on 3 Jul 2008)

    Abstract: We report on the detection of atomic oxygen lines in the spectra of 8 comets. These forbidden lines are a result of the photodissociation of the parent oxygen-bearing species directly into an excited state. We used high resolution spectra obtained at the McDonald Observatory 2.7m telescope to resolve the cometary oxygen lines from the telluric oxygen lines and from other cometary emissions.

    We find that the relative intensities of the two red lines (6300.304 and 6363.776A) are consistent with theory. The green line (5577.339A) has an intensity which is about 10% of the sum of the intensities of the two red lines. We show that collisional quenching may be important in the inner coma. If we assume the relative excitation rates of potential parents which have appeared in the literature, then H2O would be the parent of the cometary green oxygen line. However, those rates have been questioned.

    We measured the width of the three oxygen lines and find that the green line is wider than either of the two red lines. The finding of a wider line could imply a different parent for the green and red lines. However, the constancy of the green to red line flux ratio suggests the parent is the same for these lines but that the exciting photons have different energies.

    Comments: 20 pages; 9 figures; 5 tables; Accepted for publication in Icarus

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.0652v1 [astro-ph]

    Submission history

    From: Anita L. Cochran [view email]

    [v1] Thu, 3 Jul 2008 22:07:59 GMT (53kb)


  • ljk June 29, 2009, 0:53


    Joint ESA/NASA Ulysses mission to end

    European Space Agency

    26 June 2009

    Upon receipt of the last command from Earth, the transmitter on Ulysses will switch off on 30 June, bringing one of the most successful and longest missions in spaceflight history to an end.

    After 18.6 years in space and defying several earlier expectations of its demise, the joint ESA/NASA solar orbiter Ulysses will achieve ‘end of mission’ on 30 June 2009. The final communication pass with a ground station will start at 17:35 CEST and run until 22:20 CEST (15:35-20:20 UTC) or until the final command is issued to switch the satellite’s radio communications into ‘monitor only’ mode.

    No further contact with Ulysses is planned. Ulysses is the first spacecraft to survey the environment in space above and below the poles of the Sun in the four dimensions of space and time. Among many other ground-breaking results, the hugely successful mission showed that the Sun’s magnetic field is carried into the Solar System in a more complicated manner than previously believed. Particles expelled by the Sun from low latitudes can climb up to high latitudes and vice versa, even unexpectedly finding their way down to planets.