Asteroids on Galaxy Zoo

August 20, 2009 15:51 by scibuff

Galaxy Zoo is a highly successful citizen science project. It harnesses the power of the Internet and the great ability of human brain to recognize patterns and shapes (something computers are notoriously hard to “teach”) to classify a million of galaxies. The data is provided by the Sloan Digital Sky Survey (SDSS) using a dedicated 2.5 m f/5 modified Ritchey-Chretien altitude-azimuth telescope located at Apache Point Observatory, in south east New Mexico.

It is not uncommon in science to make accidental discoveries. Alexander Fleming certainly did not intend to contaminate one of his research cultures with a Penicillium genus. Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background radiation because of their “inability” to account for the 3.5 K excess temperature of their radiometer. Thomas Bopp (co)discovered the most observed comets of the 20th century when looking at the globular cluster M70 (and actually did not even own a telescope at the time).

Other times, data from a research project with a specific goal can be used to make discoveries never anticipated by the project team. Several Galaxy Zoo participants noticed interesting artifacts present in images of galaxies from the SDSS. The rainbow-like artifact in the image below is actually an asteroid.

Galaxy designated as SDSS J130941.81+063637.8 with an asteroid trail - Image Source: SDSS

Galaxy designated as SDSS J130941.81+063637.8 with an asteroid trail - Image Source: SDSS

The SDSS instruments use couple-charged device (CCD) cameras to collect light. A CCD chip can be thought of as a 2 dimensional array of light collectors that accumulate electric charge proportional to the intensity of incident light. The electric charge is then converted to voltage in a charge amplifier and digitized by a computer to create an image. CCD data in astronomy is used to create (only) gray-scale images. To make a color image, the camera takes exposures with different filters. The individual filter images can be later combined into a single color image (similar to combining theĀ  R, G and B channels of a color picture).

The SDSS camera uses five different filters: u (ultraviolet), g (green), r (red), i and z (both infrared). Thus, five exposures are required to produce an image of any single region of the sky. The actual order in which the filters are used is r, i, u, z and g. Although SDSS takes images through five filters, only three are combined to produce colored pictures. The i filter makes the red picture, r filter makes the green picture, and the g filters makes the blue picture.

Because of the technique called “drift-scanning” employed by the SDSS, exposure through a single filter takes roughly 54 seconds. Since asteroids are relatively close to us, they move rather quickly across the sky. This motion is easily noticeable at the resolution of 0.396 arc second per pixel achieved by SDSS. Furthermore, because of the order in which different filters are used, the red and green streaks are closer together and for slower moving objects may even be combined into single yellow/orange/brown streak. The dark blue/violet streak, however, is always easily recognizable at a sufficient distance from the other two (because the g filter is used last) .

Galaxy designated as SDSS J105646.89+050055.3 with an asteroid trail beneath it - Image Source: SDSS

Galaxy designated as SDSS J105646.89+050055.3 with an asteroid trail beneath it - Image Source: SDSS

The animation below shows what happens when the SDSS’s filters scan a part of sky with a slow-moving asteroid. The asteroid is the brown dot moving across the animation. The animation shows the camera’s r, i, and g filters sweeping across the sky. (In reality, it works the opposite – the cameras stay still and the sky moves during the night.) The camera takes a picture of the asteroid through the r and i filters (which are next to one another), leaving a yellow dot (or red and green dots). When the g filter scans the asteroid, the asteroid has moved; it shows up as a blue dot in its new place. In the last frame of the animation, the asteroid is removed, leaving only the image that would be seen by the SDSS.

Click on animation to play

Although SDSS is focused on looking for distant galaxies and quasars, it also sees objects in our cosmic neighborhood. The first data release (SDSS DR1) alone contains more than 100,000 asteroids. Occasionally, it manages to photograph a wobbling satellite in low-earth orbit (below) and even a bright meteor (below).

A satellite in Low Earth Orbit captured by SDSS camera using the r filter. The discontinuity in the satellite's path is a result of the data for that particular area being taken at different time - Image Source: SDSS

A satellite in Low Earth Orbit captured by SDSS camera using the r filter. The discontinuity in the satellite's path is a result of the data for that particular area being taken at different time - Image Source: SDSS

Bright meteor captured in the act of burning up in the earth's atmosphere. The trail is colored green because the image of the meteor was captured in only one of the 5 SDSS filters - Image Source: SDSS

Bright meteor captured in the act of burning up in the earth's atmosphere. The trail is colored green because the image of the meteor was captured in only one of the 5 SDSS filters - Image Source: SDSS