Author(s): , , , , ,
Institution(s): 1. JPL, 2. SETI
The number of NEAs smaller than 140 m (with absolute magnitude H>22) is estimated to be in the millions, but less than a percent have been discovered. Since 2012, small NEAs amount to 1/3 of the ~30 NEAs observed annually at Goldstone, and to date more than 40 have been detected. 90% of NEAs with H>22 are targets-of-opportunity observed within days or weeks of discovery. Most have weak signal-to-noise ratios (SNRs) at Goldstone and are detectable within about ten lunar distances, but in some instances, the SNRs are strong enough for delay-Doppler images with range resolutions as fine as 3.75 m/pixel. This high-resolution capability is relatively new and it has allowed us to image about a dozen objects with diameters between 30-140 m since 2010. Characterizing this population is important both for scientific assessment and because some of the objects could be destinations for future manned and/or robotic missions. Goldstone has observed 20 small objects on the Near-Earth object Human spaceflight Accessible Targets Study (NHATS) list to date. With the increasing number of objects detected by radar, we can start to investigate population properties. We see considerable variation in the shapes from very elongated and angular objects such as 2013 BV15 and 2014 SC324 to rounded ones such as 2013 FB8 and 2014 BR57. In contrast with larger NEAs, we have not detected any binary or contact binary systems among NEAs with H>22. Rotation periods range from only 16 s for 2014 RC (N. Moskovitz, pers. comm.) to more than half a day for 2003 HM; more than 75% have rotation periods of less than one hour. The recent increase in observing time and new radar capability at DSS-13 on the Goldstone complex will yield images of more objects in this size class. DSS-13 is a 34 m antenna that has a new 80 kW transmitter. DSS-13 is more than an order of magnitude less sensitive than the 70 m DSS-14 facility often used for radar observations, but for targets with strong SNRs, DSS-13 can obtain images with a resolution of 1.875 m that is twice as fine as the highest resolution at DSS-14 and four times as fine as the highest resolution at Arecibo.