Monday, April 4, 2011

Using Unsuspecting Amateur Comet Chasers and Interplanetary Probes as a Sensor Network

With this arxiv blog entry entitled "Astronomers Calculate Comet's Orbit Using Amateur Images" one realizes that astrophotographs posted online by amateurs all around the world represent a massive sensor network.

The sensor network described here is embodied by you, me, and whoever can take nightsky images and upload their photographs on the web.

How does this work? In a simplified  instance,  for each photo where the comet is displayed, there are also stars surrounding it. These stars can be estimated using a similar algorithm as the one used on satellite cameras called star-trackers. In effect, given a good timeline, the algorithm estimates the attitude of the image i.e. the pointing direction of your camera. Using this information and performing the same computation for different images, one can infer the trajectory of the comet. I mentioned a similar idea a year ago. However, the algorithm of the paper (see below) goes further than just perform star identification in that it picks the right star image out of a series of photos downloaded from a search on a search engine (i.e. they are not necessarily images of stars). 

The crowdsourcing project used to calibrate the images in this project is the system, From the presentation of the project:

So far,the sensor network really is the size of a planet but we can push this capability further. Instead of just using pictures taken from the ground, one can include pictures taken from space probes. For that, I asked the maintainer of to be an alpha tester while choosing this raw footage taken by the Cassini probe that is currently orbiting Saturn (about 1 billion kilometer from earth). N00171086.jpg was taken on March 31, 2011 and received on Earth March 31, 2011. According to the Cassini site "The camera was pointing toward SKATHI, and the image was taken using the CL1 and CL2 filters. This image has not been validated or calibrated."

The system was able to fit a pattern between their star catalog and the stars of the image:

The code puts your image in perspective as well as provide an evaluation the field of view (here it is evaluated at (RA, Dec) = (121.200, 30.085) degrees and spans 21.07 x 21.07 arcminutes .)

it also provides the photo  framed within a google maps GUI.

This is a very nice set-up and one can imagine how photos dating back from Mariner and Pioneer could be used to extract historical information of currently unknown comets.

We performed an image search on Yahoo for "Comet Holmes" on 2010 April 1. Thousands of images were returned. We astrometrically calibrated---and therefore vetted---the images using the system. The calibrated image pointings form a set of data points to which we can fit a test-particle orbit in the Solar System, marginalizing out image dates and catching outliers. The approach is Bayesian and the model is, in essence, a model of how comet astrophotographers point their instruments. We find very strong probabilistic constraints on the orbit, although slightly off the JPL ephemeris, probably because of limitations of the astronomer model. Hyper-parameters of the model constrain the reliability of date meta-data and where in the image astrophotographers place the comet; we find that ~70 percent of the meta-data are correct and that the comet typically appears in the central ~1/e of the image footprint. This project demonstrates that discoveries are possible with data of extreme heterogeneity and unknown provenance; or that the Web is possibly an enormous repository of astronomical information; or that if an object has been given a name and photographed thousands of times by observers who post their images on the Web, we can (re-)discover it and infer its dynamical properties!

Other projects mentioned in the paper include:


  1. Should be possible to reconstruct the 4D structure of a comet from such data. Cf.:

    Gordon, R. (2011). Cosmic Embryo #3: The ART of 3D Sun and Breast Cancer Imaging.

    Yours, -Dick Gordon