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Vulcanoids by Landon Curt Noll

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Searching for Vulcanoids by LANDON CURT NOLL the title of this article may sound like an episode from Star Trek, but to astron- omers, this is an inner-solar-system quest that has been in the making since 1859. It’s a quest that you can join today! Hunting for minor planets The innermost region of our solar system is known as the Vulcanoid zone, and it has remained largely unexplored. (Even Mercury has been only partially during a total solar eclipse photographed by just one spacecraft, Mariner 10, and that was in the mid-1970s.) Discovering asteroids lurking in the Vulcanoid zone would greatly add to our is not as far-fetched as it understanding of the birth and evolution of our solar system — not to mention those of other Sun-like stars in the Milky Way. may seem. The biggest challenge in searching for Vulcanoids is the intense glare from the Sun. A total solar eclipse, such as the one occurring on March 29th (see page 115), offers an excellent, albeit brief, opportunity to probe this normally hidden zone. With the advent of inexpensive CCDs, powerful image-processing software, and the Internet, amateur astronomers are now in a position to exploit the short window into the Sun’s inner sanctum that a total eclipse provides. What is this Vulcan stuff all about? In the mid-19th century, French astronomer Urbain J. J. Le Verrier demon- strated that Mercury’s motion around the Sun was not behaving as prescribed by Newtonian mechanics. In 1859 Le Verrier proposed that the gravitational effect of a small inner planet, which he later named “Vulcan,” could account for Mercury’s unusual motion, and the search was on (S&T: October 1998, page 112). Not until 1916 did Einstein’s general theory of relativity finally explain Mercury’s motion, and searches for the putative intra-Mercurial planet (perhaps several such bodies) were largely abandoned. Today we know of no sizable object that per- manently occupies the space inside Mercury’s orbit. Nevertheless, This artwork by planetary scientist Daniel D. Durda (Southwest many astronomers still Research Institute) depicts how a Vulcanoid might look as it refer to this region as orbits the Sun. Vulcanoids are thought to be a possible popula- the Vulcanoid zone, be- tion of small asteroids residing well inside the orbit of Mercury (shown here as the bright “star” in the background). cause of its association with the mythical planet Vulcan. A minor planet whose mean distance to the Sun is less than that of Mercury (0.387 astro- nomical unit, or 57.9 mil- lion kilometers) would be considered residing within the Vulcanoid zone. Due to the intense 26 ECLIPSE 2009 heat of the Sun in this region, the Durda and S. Alan Stern (Southwest rizon (ignoring small obstructions only objects astronomers expect to Research Institute) suggest that along the horizon, such as trees, find here are asteroids. Vulcanoids are most likely to reside houses, and hills). Your test images Why is it important to participate toward the 0.18-a.u. edge of the zone, will help you determine the optimum in this search? You have a chance to and that the gravitational pull of exposure time to use during the become part of astronomical history. Mercury and the other planets could eclipse. (Taking an exposure while With a bit of effort and luck, you nudge the orbits of these bodies out part of the Sun’s disk is uncovered could be among those to find the very from the ecliptic plane by as much as could saturate the CCD, so plan to first Vulcanoids! 10° or more. have a 4-second margin of safety on In 2000, a team led by Durda used either side of the length of totality.) Modern Vulcanoid theory images from the Large Recent models by Neil Wyn W. Angle and Spectrometric Evans (University of Oxford) and Coronagraph aboard the Serge A. Tabachnik (Princeton Solar and Heliospheric University Observatory) suggest that Observatory (SOHO) space- Vulcanoid asteroids, if they exist, craft to conduct the most must reside in a narrow, dynamically extensive search to date for stable band in the Vulcanoid zone, Vulcanoids as faint as mag- close to the ecliptic. Recent updates nitude +8.0 (S&T: August to their model suggest that objects 2000, page 26). Although closer than 0.08 a.u. to the Sun would the team failed to find a be perturbed by extreme solar heat- single one, the existence ing and dynamical transport mecha- of fainter Vulcanoids is nisms and would either be pushed clearly a possibility. If the away or pulled in toward the Sun. largest Vulcanoids are just Mercury and the other planets under the search limits place an additional constraint on the (that is, between 20 and 60 stability of the Vulcanoid zone; Evans km in diameter), then the and Tabachnik’s latest model sug- team estimates that there gests that a Vulcanoid with a mean could be as many as 1,800 distance greater than 0.18 a.u. would to 42,000 Vulcanoids larger This view of the Sun’s corona and its surroundings was be ejected from the zone. Thus, any than 1 km! captured by the Large Angle and Spectrometric Coronagraph aboard the Solar and Heliospheric Observa- long-term Vulcanoid asteroid should finding Vulcanoids tory (SOHO) spacecraft on New Year’s Day 2005. The small orbit the Sun between 0.08 and 0.18 white circle in the central occulting disk represents the size a.u. As seen from Earth, this zone There may be plenty of of the Sun. SoHo / LASCo ConSortium (Nasa / ESA) extends as far as 10.5° from the center Vulcanoids out there, but of the solar disk. Studies by Daniel D. how can they be discov- ered? A CCD camera coupled to a Determine the faintest star you can telescope is clearly essential. As many record at or near the zenith. If you CCD users know, it’s possible to re- can capture stars fainter than 8th cord fairly faint stars even near a full magnitude, then your setup has what Moon at night, and the sky during a it takes to hunt for Vulcanoids. total solar eclipse has a similar bright- You will greatly improve your Stable ness. Here are a few things to con- ability to image faint Vulcanoids Vulcanoid zone sider that might help increase your by preventing wavelengths that are chance of success. shorter than orange from reaching If you plan to do the search during your camera. Vulcanoids are likely a total solar eclipse, test your tele- to be Mercury-like in color, greatly Merc ry u scope setup on the zenith weeks or favoring the red and near infrared (IR) months before you head off to your colors. By filtering out the blue of Venus observing site. You can simulate the the sky background (from O2 Rayleigh typical range of sky conditions at scattering), the green of the corona Earth totality by taking CCD exposures of (e.g., Iron emission lines), and the yel- a cloudless, moonless sky at dusk or low from Sun (scattered into the sky The dynamically stable Vulcanoid zone extends dawn. Capture images of the zenith from outside the lunar shadow), you 0.08 to 0.18 astronomical unit (12 to 27 million when the Sun’s center is between will significantly reduce background kilometers) from the Sun. 4.25° and 5.25° below the ideal ho- noise without sacrificing the bright- ECLIPSE 2009 27 Searching for Vulcanoids Search area Sun Search area 4ϒ ness of potential Vulcanoids. Recent ute prior to totality. E c l i p t i c Vulcanoid hunters have used OG Use these coordinates 590 (090) and RG 630 (091) Wratten to help align your filters to image stars as faint as +13.5 camera mount and to 8ϒ magnitude during the 2008 August 1 locate your Vulcanoid 10.5ϒ eclipse. observation point. Another technique that holds great Even experienced promise is to use cameras that are eclipse chasers often sensitive to both visual and near IR get so caught up in the frenzy and This diagram (drawn to scale) shows the prime wavelengths. The surface tempera- emotion at the onset of totality that Vulcanoid search area during totality. To reduce ture of objects residing within the carefully laid plans become chaotic. glare from the corona, aim your CCD-equipped stable Vulcanoid zone could be as So prior to the eclipse, practice set- telescope at least 2.5° from the Sun’s center. high as 825°C (1517°F) due to their ting up, alignment, and acquiring close proximity to the Sun. Such your Vulcanoid search coordinate circular orbit around the Sun would objects are expected to be strong ra- during the daytime. Practice it until complete one revolution in 8.26 to diators in the IR portion of the spec- you become very familiar being able 27.9 days. Thus, in a pair of short trum. SLR cameras made sensitive to to setup in a reasonable amount of exposures, say, 4 minutes apart, the the near IR, such as those modified time. Bring enough spare parts and asteroid would move as much as 0.5 by MaxMax (www.maxmax.com), of- batteries just in case. Create and use arcminute on the sky. fer a relatively inexpensive near IR a checklist that even includes the A far better strategy is for a pair detector solution while avoiding the obvious. More than one person has of observers working in tandem to cooling and thermal shield problems forgotten to remove a lens cap at the position themselves in different coun- associated with chilled mid and far IR right time! tries along the eclipse path.
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