Observations using the Arecibo S-band (2380 MHz, 12.6 cm) planetary radar system began in 1976 and its power and sensitivity were greatly improved with the telescope upgrade and Gregorian-dome installation in the mid-1990s. Below, we show the number of near-Earth asteroids attempted and detected since the radar program resumed in 1998. At first, observations of a couple dozen objects were attempted per year, mostly the largest and/or closest targets, especially those classified as asteroids potentially hazardous to Earth (PHAs). In 2011, the radar program became fully funded funded through the NASA Near-Earth Object Observations program and there was a push to observe more objects, especially smaller asteroids that are of interest for missions by robotic or manned spacecraft or those that could cause significant regional, rather than global, damage in the event of an impact.
In 2017, we detected 67 objects (out of 81 attempts) despite nearly five months of downtime due to the scheduled replacement of the heat exchangers that cool the radar system and for recovery from Hurricane Maria. Some objects were not detected because they are simply too small and/or too far away even for our giant telescope to detect, but """you miss 100% of the shots you don't take. --Wayne Gretzky" --Michael Scott" --#TeamRadar"
Small asteroids specifically of interest to NASA for possible future robotic or crewed missions are termed Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) compliant. The number of NHATS objects detected with Arecibo similarly jumped in 2012.
As we push to observe smaller and fainter targets with Arecibo, the mean and median absolute magnitudes of the detected objects has increased to H ~ 22, equivalent to a diameter of roughly 100 meters.
As we increase the cadence of observations, many of our detections are newly discovered objects observed as targets of opportunity during previously scheduled radar tracks or during pre-planned "survey" nights scheduled near new moon when optical survey programs are most efficient. Roughly 50% of asteroids detected with Arecibo, dozens per year, are those on their discovery apparition.