2007 XH16 -- A Bright Fast-Moving Earth-Approacher

2007 XH16 -- A Bright Fast-Moving Earth-Approacher


December of 2007 was a terrible month for asteroid hunting. After discovering 21 new minor planets in October and another 16 in November, I found only one during the last lunation of the year. The winter Milky Way was right in the way of the prime discovery area, and the weather was unusually bad at my remote observing site in New Mexico.

Instead of observing, I took the opportunity to catch up on some long-languishing projects for my website. I posted a few precovery images of some of my discoveries, and I finally completed a page of interesting statistics of my minor planets.

I also wrote a program to compute daily ephemerides of NEOs (near-earth objects) from the Minor Planet Center's daily NEO update file. Since I thought it would be most useful for amateurs, I restricted the list to those objects brighter than visual magnitude 19.5 and with a solar elongation greater than 45o. When I updated the web page on Dec. 23, I was surprised to see that 2007 XH16, a PHA (potentially hazardous asteroid) discovered on Dec. 08, had brightened to visual magnitude 12.9 and was moving at 54.88"/second! I was a bit shocked to realize that I was only alerted to this unusual object because of my own work; many observers were probably busy with Christmas just around the corner, and the main topic of discussion in related newsgroups was the possibility of 2007 WD5 impacting Mars on Jan. 30, 2008.

Skies were clear in New Mexico during the evening of Dec. 23, so I shot 24 consecutive 10-second exposures of 2007 XH16 shortly after it crossed the meridian. The exposures were kept short to reduce trailing and to minimize the sky brightness from the full moon. The object's motion was clearly visible in the preview JPGs as the exposure sequence continued for 7.5 minutes.

After reducing the images with Astrometrica, I submitted the following astrometry to the Minor Planet Center by stacking four frames with six consecutive exposures:

     K07X16H  C2007 12 24.17940003 15 48.730+10 05 24.30         12.9 R      H06
     K07X16H  C2007 12 24.18072203 15 44.289+10 06 53.81         12.9 R      H06
     K07X16H  C2007 12 24.18204203 15 39.870+10 08 23.57         12.9 R      H06
     K07X16H  C2007 12 24.18344803 15 35.139+10 09 58.94         13.0 R      H06
The animation below is from the 24 individual frames. At the time, the object was 0.022 A.U. from the earth and moving at 58.37"/minute. Although each exposure was 10 seconds, an additional 9 seconds was required to transfer the data to disc, so the time interval between adjacent frames is 19 seconds.

The following evening was also clear, and I reshot 2007 XH16 with the same sequence of exposures. The corresponding astrometry is:

     K07X16H  C2007 12 25.07138102 20 53.70 +27 27 50.3          13.1 R      H06
     K07X16H  C2007 12 25.07274102 20 47.74 +27 29 23.9          13.0 R      H06
     K07X16H  C2007 12 25.07416502 20 41.53 +27 31 01.3          13.0 R      H06
     K07X16H  C2007 12 25.07559002 20 35.33 +27 32 38.8          13.1 R      H06
The animation below is from the 24 individual frames. At the time, the object was 0.021 A.U. from the earth and moving at 62.43"/minute. The exposures were taken earlier in the evening compared to the previous night, and more stars are visible because the moon was lower in the sky.

The exposures were taken remotely at the RAS Observatory in Mayhill, New Mexico, with the GRAS-004 0.25-m f/3.4 Takahashi Epsilon E250 Astrograph.