PHOTO: NASA - Asteroid (253) Mathilde taken up by the space probe NEAR on 27 June 1997 from a distance of 2400 km. It is lit up from above right by the sun. On the surface many large craters are visible, like that probably more than 10 km deep crater provided with long shade in the picture center.

INTRODUCTION

The scientific interest in asteroids is due largely to their status as the remnant debris from the inner solar system formation process. Because some of these objects can collide with the Earth, asteroids are also important for having significantly modified the Earth’s biosphere in the past. They will continue to do so in the future. In addition, asteroids offer a source of volatiles and an extraordinarily rich supply of minerals that can be exploited for the exploration and colonization of our solar system in the twenty-first century.

WHAT IS AN OCCULTATION?

Occultations occur when one celestial object passes in front of another celestial object. For example, when the Moon passes in front of a background star, light from the background star is prevented from reaching the Earth. A shadow of the Moon is cast by the star onto the Earth, and this shadow sweeps across the Earth at roughly the same speed as the Moon is moving. A special case of an occultation is a?Total Solar Eclipse, in which the Moon passes in front of the Sun, obscuring it from view. My recent work involves Planetary Occultations because they occur when planets or minor planets (asteroids) pass in front of background stars. These events occur less?frequently?that lunar occultations because the apparent size of the planet or asteroid is much smaller than that of the moon.

While occultations of bright stars by major planets are very rare, occultations by asteroids are a little less so. This is not because any one asteroid has a greater chance of passing in front of a star. Rather, it is because there are so many more asteroids to choose from!?Typically, the occultation shadows of more than 100 asteroids will sweep across parts of the US annually. Each shadow is usually about the same width as the minor planet (typically 60-120 miles), although the shadow itself does sweep out a strip many thousands of miles long across the Earth. Thus the same asteroid occultation can potentially be observed on both the east and west coasts of the US.

This diagram shows a typical asteroid path across part of the globe. The dark horizontal lines stretching from Nevada through Michigan define a probable shadow path of this particular asteroid. Lighter parallel lines indicate a margin of error or 1 sigma or 31%, where the asteroid may cast a shadow or partial shadow.

WHY OBSERVE ASTEROID OCCULTATIONS?

Asteroids represent the bits and pieces left over from the process that formed the inner planets, including Earth. Asteroids are also the sources of most meteorites that have struck the Earth’s surface and many of these meteorites have already been subjected to detailed chemical and physical analyses. If certain asteroids can be identified as the sources for some of the well-studied meteorites, the detailed knowledge of the meteorite’s composition and structure will provide important information on the chemical mixture, and conditions from which the Earth formed 4.6 billion years ago. During the early solar system, the carbon-based molecules and volatile materials that served as the building blocks of life may have been brought to the Earth via asteroid and comet impacts. Thus the study of asteroids is not only important for studying the primordial chemical mixture from which the Earth formed, these objects may hold the key as to how the building blocks of life were delivered to the early Earth.

One such method for studying asteroids can be undertaken by the amateur?astronomer?with modest equipment.

If the occultation shadow band is a few tens to a few hundred miles wide, then observers situated within the band and perpendicular to the direction of motion of the shadow will each see the star occulted (or dimmed) by a different part of the asteroid. If enough observations are obtained, one can essentially “join the dots” to build up a picture of the shape of the asteroid. Even if only two observers see an event, so long as they are separated by a reasonable distance an average diameter for the asteroid can be deduced – the only direct way in which this information can be obtained. Determining the diameter of a minor planet is important because it can provide clues to the asteroid’s density, which in turn tells us something about its bulk composition and thus its origin.

Because the chances of a specific asteroidal occultation track passing across an established observatory are not high, the contribution that amateur astronomers with portable telescopes can make is considerably enhanced. It is the ability of amateurs to relocate themselves and their telescopes at short notice which is frequently crucial to the success of an observation.

PREDICTIONS

Predicting exactly where an asteroid’s shadow will pass as it moves across the Earth is quite difficult. To do so the relative positions of the star and moving asteroid need to be measured very accurately, which is a difficult task given that both appear in a telescope as a point source of light. Until recently the best astronomers have been able to do is to predict an occultation track with an error of perhaps one hundred to a few hundred miles. This has meant that the chances of seeing an actual occultation have not been high, and persistence in observing many events has been the name of the game. Fortunately though, through the use of “last-minute astrometry” (in which the positions of both the asteroid and star are measured on the night before the occultation, when they are very close together), together with recently-released high-precision star catalogues, the accuracy of the predictions is being enormously improved.

PHOTO: Kelly Beatty - Gary Jacobson (ATMoB) recording data for a daytime Lunar Occultation of the bright star Antares.

RECORDING OCCULTATIONS

The most complex, and not?coincidentally?the most expensive method to study asteroids is to send an intercepting spacecraft to take photographs and collect other data. The asteroid (253) Mathilde pictured at the top of this article is one such example. But asteroid occultations are the most effective way?that?amateur astronomers can?accurately?measure these objects. IOTA has?over?a thousand such observations, compared to the ten asteroids that have been imaged by spacecraft.

Several methods are used to record the occultations. The most simple, yet least accurate way to record an occultation is visually, through a telescope, using a time source such as a stopwatch synchronized to a GPS or radio time standard such as WWV, and a tape recorder. The observer watches the target star and speaks the details of the event while also recording the radio time signal or calling out the time from the stopwatch.

Other methods greatly increase the accuracy of the?observation. Recording video through the telescope while also recording the WWV radio announcement is one method, while employing a GPS-fed video injection device to?display?the exact?time?in milliseconds on the captured frames is still?more?accurate. When the target star is below a certain brightness threshold?video?may not be sensitive enough to capture the event. This is why I used another method while recording Pulcova and 2002 TX300. A sensitive CCD astro-camera was attached to the telescope and to a PC, and a research-grade GPS unit was used to trigger the camera at precise intervals. This allowed for longer integrations (exposures) in order to capture the magnitude ranges of 12.4 to ~19 magnitude.

Since the exact location and altitude of the observer greatly affects the accuracy of the data, these must be recorded along with the event.

SUBMITTING DATA

Data is submitted on a form to the International Occultation Timing Association (IOTA).?Relevant?information includes observer ID; latitude, longitude and elevation; equipment used; time recording started, (D) Disappearance and (R) Re-appearance times; and time recording ended. This data is then analyzed and?combined?with other successful observations to further define the asteroid.

RESULTS

It’s pretty amazing what these observations yield! By ‘doing the math’ these scientists and amateurs can profile an asteroid, define its width, determine whether there are any companion objects or moonlets, and with lots of observations, make a better model.

It is difficult to define more than the diameter with only 2 readings

More readings, each here represented by a different color line, help do define the asteroid's shape more accurately.

Still more amazing is that without directly observing the asteroid, a greater number of accurate predictions can even enable us to put together a 3D model as seen below. Compare these models to the actual photo of Mathilde at the top of this article.

With sophisticated software and very accurate readings, the actual shape of the asteroid begins to take form in this 3D rendering.

Bruce

Sources by permission:
Royal Astronomical Society of New Zealand -?http://occsec.wellington.net.nz

Astronet -?http://www.xs4all.nl/~carlkop/comastr.html

International Occultation Timing Association -?http://www.asteroidoccultation.com