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Comets and asteroids in STEREO

The STEREO Mission

In October 2006, the NASA Solar Terrestrial RElations Observatory -- STEREO -- was launched. The STEREO mission is a pair of near identical satellites designed to look at the Sun from two different locations in space and give us our first stereoscopic views of the Sun and solar eruptions. To achieve this, the two spacecraft are placed into an orbit that is the same as Earth's, except one spacecraft (the Ahead, or "A" spacecraft) moves faster than Earth and the other (the Behind, or "B" spacecraft) moves slower than Earth. The result of this is that over time, the spacecraft separate slowly from each other and from Earth. The STEREO Science Center has a plot that updates frequently showing the current location of the two spacecraft relative to Earth.

STEREO's eyes: the SECCHI package

The primary set of imaging instruments on the STEREO satellites is the Sun-Earth Connection Coronal and Heliospheric Investigation -- SECCHI. The SECCHI package is a set of five telescopes designed to monitor the solar disk, the outer atmosphere of the Sun (the "corona"), and the area of space between the Sun and the Earth. In terms of looking for comets and asteroids, the telescopes of interest are the COR2, HI1 and HI2 instruments.
  • COR2
    • COR2 is a coronagraph very similar to the LASCO coronagraphs. (In fact the entire SECCHI package was designed, built, tested and operated by a lot of the same NRL-based people that created LASCO.) It blocks the direct light of the Sun by using a solid 'occulter' disk -- essentially creating a false solar eclipse and allowing us to see the faint solar atmosphere (and comets!). The COR2 images have a maximum resolution of 2048x2048 pixels -- four-times larger than LASCO! Unfortunately, the filters that are on COR2 mean that the telescope is not as sensitive to the light emitted by comets as LASCO is. So we only see the brighter ones in COR2.
  • The Heliospheric Imagers - HI1 and HI2
    • The HI telescope is a completely new and revolutionary telescope concept that allows us to image the entirety of space between the Sun and the Earth. This is achieved with two imagers -- one with a 20-degree field of view looking at solar elongations from ~3.5-degrees to 23.5-degrees, and the second with a very large 70-degree field of view looking at solar elongations from 20-90 degrees.
    These telescopes are very sensitive, seeing stars down to about magnitude 13. This sensitivity is due mainly to the way we record data with them. Each exposure is actually a stack of exposures taken over a forty-minute period, the result of which is an image with a very low signal-to-noise ratio and hence high sensitivity.

    Comets and asteroids seen in SECCHI data

    Both the COR2 and HI instruments are sensitive to near-Sun comets and asteroids. The HI-1 imager in particular sees many Kreutz comets at different times of the year. In addition, minor planets (asteroids) are quite abundant in the HI-1 data. In fact in any randomly-chosen HI-1 image, it is usually possible to easily pick out one or two bright minor planets. The HI-2 imager also sees many asteroids, but they are often harder to spot due to the much larger HI-2 pixels. In COR2, the only comets we have seen so far are the brighter members of the Kreutz-group. We have seen one asteroid -- Vesta -- but the relatively small (6-degree) field of view means that very few minor planets cross that field of view.

    Discovering objects in SECCHI images

    Discovering comets in SECCHI data is tricky. The problem is that SECCHI is not a "realtime" instrument in the same sense that LASCO is. We only get a very low-resolution data product sent down from the spacecraft in realtime. Our full-resolution data product takes about 48hrs to reach us and another 24hrs for processing. Therefore, by the time you can get the images online, they are already quite old and the majority of comets will have already been seen and reported in LASCO data. There is, of course, always the chance of the discovery of an unknown object in SECCHI data (most likely HI-1), but these will be relatively rare.

    Locating "known" object in SECCHI images

    As previously mentioned, there are often several minor planets to be located in any given HI-1 image. Brighter ones are easy to spot if you "blink" two images (try out our online blink tool!) but to find the fainter ones you need to look carefully in the right spot. The difficulty here lies in finding the right spot to look in.
    Coordinates (locations) of comets and asteroids are given in terms of Right Ascention (RA) and Declination (Dec), but this coordinate system is Earth-based, and the STEREO spacecraft are already far from Earth and continually moving. Therefore, even though stars will always have the same RA-DEC no matter where you are in the solar system, objects such as comets and asteroids within the solar system will have very different RA-DEC values depending on where the spacecraft are currently located.
    Currently, there is no readily available tool that can be used to "convert" Earth-based coordinates of solar system objects to STEREO-based coordinates.