Spacewatch Discovery of 2003 SW130
Subsequent observations of this tiny asteroid were a bit tricky for the Spacewatch observers. On the second night Ms. Gleason acquired follow-up measurements, the length of the trail had decreased to 1/4 its original size! What could account for this change? The object's apparent angular rate was slowing down. In other words, as an object travels farther away from the Earth its apparent motion on the sky decreases, manifesting in Spacewatch images as a decrease in trail length. Prof. Gehrels's follow-up observations also showed the trail length shrinking as it continued to recede from the Earth. The change in trail length from 73 arcsec to 3 arcsec is a function of decreasing apparent angular rate. The actual velocity (9.2km/s) of this object has not changed significantly. In addition to this drastic change in trail length overnight, the object's location was always quite far off the ephemeris prediction. A bit of hunting around the sky was required by Ms. Gleason and Prof. Gehrels to make follow-up measurements, now complicated by both the location and the expected trail length being in question.
Fortunately, enough data were acquired to establish a reliable orbit and on Sept 22, this Spacewatch object was given the official designation of 2003 SW130 which is an Aten-type asteroid. By definition, an Aten is an asteroid with a semi-major axis less than 1 AU, whose orbit crosses that of Earth near its aphelion, or farthest point from the Sun.
As the smallest asteroid detected by Spacewatch and designated by the MPC, its estimated size is between 5 - 9 meters in diameter, calculated as a function its absolute magnitude (H = 29.2), An object of this size poses no threat to the Earth, despite its close proximity on Sept 19. If the orbit were ever perturbed, predicting an impact with the Earth, it would probably disintegrate in the Earth's upper-atmosphere, appearing as 'shooting-stars'. Small pieces of the asteroid might also fall to the ground as meteorites. It has been roughly calculated that objects of this size (about 5 meters) impact somewhere on the Earth's surface once each month! These are the largest objects a person could expect to see as a brilliant shooting-star in a lifetime. Although the smaller objects such as 2003 SW130 are not hazardous, the statistics of their population provide information about the processes of fragmentation and cohesion of asteroids due to their mutual collisions and gravitation, respectively.