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CBA Belgium Observatory
Observations of Near Earth Object 1999 KW₄

Minor planet 1999 KW4 was discovered in 1999 using the LINEAR 1-meter telescope in New Mexico. It had an extreme close flyby in May 2001, when it came within 4.8 million kilometers of Earth on May 25th. It won't come this close again until 2019. The minor planet belongs to the special class of Aten asteroids, which circle the Sun in less than a year and range only briefly outside the Earth's orbit. Its orbit is shown in the diagram below in brown color (the red circle represents the Sun. Visible also are the orbits of Mercurius, Venus, Earth and Mars).

We made our first observations of 1999 KW4 on 2001, May 24.08 UT, when this Near-Earth Object (NEO) was visible very low in the morning sky (about 10 degrees altitude), just before dawn. We collected 31 CCD images of 30 seconds each, showing the extreme speed of the minor planet. The session clearly was too short to perform any decent photometry. The next night was clear again, and allowed a somewhat longer session, that revealed a first fraction of the light curve of 1999 KW4. The nights of May 25/26, May 27/28 and May 29/30 all were clear again. We collected several hours of high quality images and were capable of building very spectacular light curves, showing the complex modulations in 1999 KW4's light curve. The table below presents an overview of our photometric observations at CBA Belgium Observatory.

Log of observations :

Date (UT)	Duration (h)	Nbr frames
2001 May 24.081 - 24.096	0.36	31
2001 May 25.040 - 25.097	1.37	88
2001 May 25.909 - 26.096	4.49	284
2001 May 27.942 - 28.097	3.72	235
2001 May 29.895 - 30.099	4.90	308
Total	14.84	946

The image above is a composition of 17 CCD exposures of 30 sec each, taken with the 0.35-m f/6.3 telescope and ST-7 CCD at CBA Belgium, on the night of May 25/26, 2001. At that moment, 1999 KW4 was near to its closest point to Earth. This is just a small selection of images out of a total of over 280 exposures made that night.

In June 2000, Petr Pravec and Lenka Sarounova from Ondrejov Observatory (Czech Republic) detected non periodic brightness fluctuations of 0.1 to 0.2 mag in the light curve of 1999 KW4. They suspected that maybe a secondary component was involved, which would classify 1999 KW4 as one of the few binary Near-Earth Objects. Confirmation for their assumption came on May 23rd, 2001, when astronomers working with radar images at Goldstone clearly revealed the binary nature of the object (IAUC 7632). However, none of the observations allowed to determine precise parameters of 1999 KW4's binary system.

Evidently, the unusual photometric behavior of 1999 KW4 and its favourable flyby were an excellent trigger for us to devote a few nights of our observing schedule to this object, in an attempt to gain some insights in its light modulations. We were thrilled by the resulting light curves of 1999 KW4 : they were surpassing all of our dreams !! The object changed light in a couple of hours by as much as 0.3 - 0.4 magnitudes, and showed small scale modulations of 0.1 to 0.2 magnitudes in time ranges of 20 to 40 minutes. We would hardly ever have believed that a minor planet could be this interesting from a photometric point of view.

Below is a selection of the light curves we obtained. Needless to say that the light curve behavior is much too complex to be attributed to a single component system. Its complexity, however, makes it extremely difficult to derive the binary parameters of 1999 KW4. We hope that a combination of our set of observations with others, obtained at different longitudes, will allow to solve that puzzle !

Want to learn more about NEO's ?
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