Planetary Radar

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The Arecibo Observatory has the world's most powerful planetary radar system, which provides ground-based observations whose quality could only be exceeded with a spacecraft flyby. The 305 meter Arecibo telescope equipped with a 1 MW transmitter at S-band (12.6 cm, 2380 MHz) is used for studies of small bodies in the solar system, terrestrial planets, and planetary satellites including the Moon.

Radar Astrometry

Since NASA’s near-Earth object observations program started funding the planetary radar program in November 2011, the annual number of radar-observed asteroids increased from less than 20 per year to more than 100. Roughly one half of the targets observed each year are recently discovered near-Earth asteroids (NEAs), which usually have large orbit uncertainties. Radar astrometry is a valuable tool for orbit refinement, providing precise measurements that can significantly improve the accuracy of orbit determination, preventing the object from becoming lost and requiring re-discovery in the future. In addition, Doppler and range measurements can increase the orbit predictability window from years to centuries, and quickly eliminate impact false alarms with the improvement of estimates of an asteroid’s orbital elements.

Physical Characterization

The Arecibo planetary radar is a powerful tool for post-discovery characterization of near-Earth objects, planets, and moons. In addition to precise line-of-sight velocity and range information, depending on the target’s size and distance, planetary radar is useful for quickly estimating the instantaneous rotation rate of near-Earth asteroids, resolving the target’s size, detecting potential satellites, and ultimately resolving the shape through inverse modeling efforts. Although comets rarely come close enough to Earth to allow strong enough echo, when an approaching comet becomes detectable by the planetary radar systems, it is possible to get clues to the size and spin period of the nucleus. Furthermore, radar signal can penetrate through clouds (such as the thick atmospheres on Venus or Titan), or several wavelengths below the regolith surface, providing insight to geologic features hidden from optical wavelengths, and provide clues to the near-surface bulk density or metal richness of the target based on its reflectivity at radar wavelengths. Radar polarimetry can give clues to the decimeter-scale surface structure, which is crucial for landing spacecrafts. The physical properties obtained with radar are fundamental information to support space mission’s planning, hence the majority of small bodies missions select targets that can be characterized with radar prior to the mission.

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The Arecibo Observatory Planetary Radar program is fully funded by NASA's Near Earth Object Observations Program and proudly supports NASA's efforts to track and characterize near-Earth objects for planetary defense. For information about asteroid and comet orbits, including close approaches to Earth, please see the websites of the NASA Center for Near-Earth Object Studies and the NASA Planetary Defense Coordination Office.

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S-Band Spotlight

Arecibo Radar Observations Remove Asteroid 2020 NK1 from CNEOS Sentry List

Arecibo, Puerto Rico – August 7, 2020

Arecibo Observatory radar observations have removed the near-Earth asteroid 2020 NK1, originally estimated at more than 500 meters (1600 feet) across, from the CNEOS Sentry List. Before the Arecibo observations, 2020 NK1 was calculated to be one of the biggest threats out of all known asteroids on NASA’s list of potential impactors, with about one chance in 70,000 of impacting the Earth between 2086 and 2101.

Arecibo’s Planetary Radar Group made it a priority to observe 2020 NK1 when it came within 5 million miles of the Earth and therefore in range for the AO radar system. However, the approaching Tropical Storm Isaias, due to slam into the island at the same time, threatened the observation plans since any storm damage to the telescope or sustained winds would prevent the observations from taking place.

Radar range-Doppler image of 2020 NK1 reveals an elongated asteroid approximately 1 km (0.6 mi) along the long axis. The image resolution in the vertical dimension is 30 m/pixel. Image credit: Arecibo Observatory/NASA/NSF.

“Fortunately, the storm passed quickly without damage to the telescope or the radar system, and the Observatory staff made all the possible efforts to reactivate the telescope from hurricane lockdown in time for the observations scheduled for July 31”, described Dr. Sean Marshall, an observatory scientist who led the radar observations of the Potentially Hazardous Object (PHO) at Arecibo.

“Fortunately, the storm passed quickly without damage to the telescope or the radar system, and the Observatory staff made all the possible efforts to reactivate the telescope from hurricane lockdown in time for the observations scheduled for July 31” - Dr. Sean Marshall, Observatory Scientist at Arecibo Observatory

The team of scientists and telescope operators were able to observe the asteroid on July 31st for two and half hours, collecting precise measurements of the asteroid’s velocity and distance from Earth, and high-resolution images of the asteroid itself. “These measurements greatly improve our knowledge of 2020 NK1’s orbit and allow for predictions of its future whereabouts for decades to come,” says Dr. Patrick Taylor, a scientist at the Lunar and Planetary Institute, part of Universities Space Research Association, who participated in the observation remotely.

The radar images obtained reveal an elongated shape with a diameter along its longest axis of approximately 1 km. The observations showed the asteroid is not expected to get close enough to Earth to pose a danger in the future, with its closest approach coming in 2043 when it will pass about 2.25 million miles from Earth – or more than 9 times farther away than the Moon, the team concluded.

“This event was a great example of the important role that the Arecibo planetary radar system plays in planetary science and planetary defense, with very quick response times and high-precision radar measurements and imaging capabilities, in spite of storms, the COVID-19 pandemic, and earthquakes with which Puerto Rico has dealt with this year”, says Dr. Anne Virkki, the head of the Planetary Radar group at the Arecibo Observatory.

2020 NK1 is one of many PHOs that NASA tracks. Asteroids are considered PHOs if they are bigger than 500 feet in diameter and come within 5 million miles of the Earth’s orbit. No known PHOs pose an immediate danger to the Earth, but observations like those conducted at the Arecibo Observatory are used to determine their future trajectories and risk.

Other team members participating in the radar observation of 2020 NK1 were Dr. Flaviane Venditti and Telescope Operators Israel Cabrera and Juan Marrero at Arecibo. Virkki and Dr. Dylan Hickson, also from Arecibo, participated in the data analysis.

UCF manages the NSF facility under a cooperative agreement with Universidad Ana G. Méndez and Yang Enterprises Inc. The Arecibo Planetary Radar Project is fully supported by NASA’s Near Earth Object Observations Program in NASA’s Planetary Defense Coordination Office through a grant awarded to UCF. Arecibo has played a role in analyzing NEOs for decades, observing up to 130 objects per year.

Click here for the UCF press release about these observations!

The Arecibo Planetary Radar program is a project of the Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office.


Publications making use of Arecibo Observatory Data

NOTE: Any opinions, findings, or recommendations expressed here do not necessarily reflect the views of the Arecibo Observatory, the National Science Foundation (NSF), University of Central Florida (UCF), Yang Enterprises (YEI), and Universidad Metropolitana (UMET), or the National Aeronautics and Space Administration (NASA). This website section is maintained by Dr. Sean Marshall