A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Circular arguments

When something exciting was happening and astronomers wanted other astronomers around the world to know, they used to send a telegram to one central location from which subsequent telegrams were then exploded to observatories around the world. The CBAT --Central Bureau for Astronomical Telegrams -- still exists today to provide the same service, except that it is all done by email today. The CBAT issues International Astronomical Union Circulars, which used to be, in fact, rectangular, approximately the size of an index card, on which new exciting information was mailed. Again, today, it is all email. The switch from telegrams arriving at observatories to emails arriving in everyones already overstuffed in box is another sign of the quaintness being snuffed out of astronomy in favor of efficiency. And I, for one, say thank goodness for efficiency.

Today, we are issuing our IAU Circular describing the moon shadows. It will read something like this:

Mutual Events of 2003 EL 61 and its Inner Satellite

Daniel Fabrycky (Harvard University), Darin Ragozzine & Michael Brown (California Institute of Technology) and Matthew Holman (Smithsonian Astrophysical Observatory)

Orbital fits to the relative astrometric positions of dwarf planet 2003 EL61 (IAUC 8577) and its inner satellite, S/2005 (2003 EL_61) 2 (IAUC 8636), have revealed a near edge-on orbit, implying likely mutual events. The orbital model was based on images from HST (WFPC2) and Keck (LGS-AO). Due to the changing orientation of the Earth-EL_61 line of sight, the orbit is moving closer to edge-on until August 2008, after which the orbit will open up again. The current distance of closest projected approach is ~500 km, nearly the same as the semi-minor axis of the triaxial primary (Rabinowitz et al. 2005, ApJ, 639, 1238), so events will likely be grazing. Shadows of the satellite and EL_61 will likely miss each other. The unocculted lightcurve has double-peaked rotational modulation of full amplitude 0.25 mag and period 3.9 hours; template lightcurves of this variation are available from Holman. The duration of the events will be between 0 and ~6 hours; ingress and egress will consist of ~0.03 magnitude changes on a timescale of ~10 minutes. Telescopes distributed in longitude are needed to follow events as the orbital period is 18.36 d. The main body is rather faint (V~17.4 mag), so high-precision photometry requires moderate (~1 m) collecting area.

Due to orbital eccentricity, events in which the main body occults the satellite are more likely to occur than events in which the satellite occults the main body. Our orbital model predicts mid-event times as follows (add or subtract up to 3 hours for ingress or egress times).
For the satellite occulting the main body:
HJD 2454617.58 +/- 0.07 = 5/31 01:50+/-1:40 UT
HJD 2454635.84 +/- 0.07
HJD 2454654.11 +/- 0.07
HJD 2454672.48 +/- 0.08
For the main body occulting the satellite:
HJD 2454625.22 +/- 0.07 = 6/07 17:10+/-1:40 UT
HJD 2454643.45 +/- 0.07
HJD 2454661.79 +/- 0.1


What does all of this mean? First, a little sadness. We missed most of the shadows by a few years. There is only a chance to observe about 3 more this year, and then not again for 130 years.

The next event is visible over Asian/Europe, but we think it is likely to just be a graze, so nothing will be clear. After that we are on to Hawaii and Japan again for June 7th.

Honestly, I think we're too late. But what a great project it will be for our great-great-great-grandkids.


  1. I hope our great-great-great-grandkids will be in orbit around dwarf planet 2003 EL61 !

  2. No chance of hacking HST, eh? That must be scheduled and programmed well in advance, and yet....Or could you use space-based telescopes in other wavelengths? Spitzer...etc.

  3. While it may be too late for our Earthly telescopes, there's a fair bit of instrumentation scattered about the solar system. Could the HiRise telescope currently in orbit around Mars catch it? What about EPOXI, which is currently being used for exoplanet transit detection?