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1001 Near-Earth Objects for Planetary Radar

A century ago, when American magazine science fiction was developing, the Solar System seemed a relatively tidy place. At least, it did in comparison to today. The first issue of Hugo Gernsback’s Amazing Stories serialized a reprint of Jules Verne’s 1877 novel Off on a Comet and, indeed, in those days comets were the objects most likely to move around the system. The asteroids seemed distant in their belt and in stable orbits and there was little else between the planets. There was no Pluto.

Today, of course, we seem to have debris everywhere. The main belt asteroids are joined by trojan objects like the large population around Jupiter, and there is another belt of ancient material out beyond Neptune, the Kuiper Belt. In Earth’s neighborhood, interesting objects like 2021 PJ1, whose approach to our planet occurred on August 14 at 1.7 million kilometers, remind us that there is a large population of asteroids that move in orbits well inside the main belt, and could conceivably present a danger to us, at least enough of one to demand that we keep a close eye on their trajectories.

2021 PJ1 has a certain claim to fame, being the 1,000th near-Earth asteroid to be observed by planetary radar in the past 50 years. In this technique, we bounce a radar signal off an object and examine the photonic echo. The first asteroid to be viewed in this way was 1566 Icarus, all the way back in 1968, ‘painted’ by Goldstone radar, the same facility near Barstow, California that produced the PJ1 data this summer.

Image: The 70-meter Deep Space Station 14 (DSS-14) antenna at the Deep Space Network’s Goldstone Deep Space Complex near Barstow, California, was able to measure the Doppler frequency of the radio waves that reflected off asteroid 2021 PJ1’s surface. The figure shows radar echo signal strength on the vertical axis versus Doppler frequency (in units of hertz, or Hz) on the horizontal axis. The strong spike at a value of minus 70 Hz is the reflected signal (or “echo”) from 2021 PJ1; the other, smaller spikes are receiver noise. Credit: NASA/JPL-Caltech.

I always think primarily of Arecibo when planetary radar comes to mind, and in fact its radar capabilities were a prime reason for fighting to sustain its funding before its collapse in 2020. Well over half of existing NEO radar observations were made by its 305-meter dish. But the tally of the Goldstone Deep Space Complex is impressive via its DSS-14 70-meter and 34-meter DSS-13 antennae, with 374 near-Earth asteroids to date. Moreover, the Deep Space Network’s Canberra site, working with Australian observatories including Parkes, has notched up another fourteen.

NEA 1,001 came only a week after 2021 PJ1, an object labeled 2016 AJ193 that moved by at about 3.4 million kilometers. While 2021 PJ1 was small — between 20 and 30 meters wide — 2016 AJ193, although more distant, was a much easier catch because it’s some 40 times larger, with a diameter in the range of 1.3 kilometers. Originally observed by the NEOWISE mission, this asteroid gave us a lot more than plots on a graph: Ridges, hills, concavities and possible boulders appeared in the Goldstone observations, which also determined that it rotates with a period of 3.5 hours.

Image: This animation shows asteroid 2016 AJ193 rotating as it was observed by Goldstone’s 70-meter antenna on Aug. 22, 2021. 1.3-kilometers wide, the object was the 1,001st near-Earth asteroid to be measured by planetary radar since 1968. Credit: NASA/JPL-Caltech.

The 2016 AJ193 observations were led by Shantanu Naidu (JPL), who says:

“The 2016 AJ193 approach provided an important opportunity to study the object’s properties and improve our understanding of its future motion around the Sun. It has a cometary orbit, which suggests that it may be an inactive comet. But we knew little about it before this pass, other than its size and how much sunlight its surface reflects, so we planned this observing campaign years ago.”

The significance of planetary radar for simple security is obvious. Including telescopes on the ground and in space, we’re tracking close to 27,000 near-Earth objects, characterizing them through observations like the recent Goldstone work. The more we learn about the size, shape and composition of NEOs, the better we’ll be able to resolve questions about their trajectories and any future danger they might pose. This, in company with data from asteroid missions like Hayabusa2 and OSIRIS-REx, will help us tune our threat mitigation strategies if we ever do have to nudge an NEO.

Image: This series of images captured on Aug. 22, 2021, shows asteroid 2016 AJ193 rotate as it was observed by Goldstone’s 70-meter antenna. Credit: NASA/JPL-Caltech.

{ 9 comments… add one }
  • wdk September 9, 2021, 13:46

    “Off on a Comet” is often spoken of but seldom read these days, I suspect.
    Decades ago, while in service, two of us set off for Paris in August of ’68 and explored Versailles, stopping to pick up a souvenir or two in a shop.
    A paperback (livre de poche) with two gendarmes making giant leaps across a rugged but forested terrain provided cover illustration to a thick Jules Verne novel. “Hector Servadac”. Characteristically, Hector’s adventures began in preparation for a duel on the Algerian coast which would be interrupted by a comet’s collision with the Earth. Perhaps this spoils the end: since the comet was periodic Hector returns to where he began with the comet’s return passage.

    My French is/was that of a student; so it took me many years to finish this story on my own, but it encouraged me to do others, the durations of which exercises did not necessarily rival short period comets each time. Anyway Hector was not alone on the comet and there was plenty of mini-world exploration, observation of outer planets and interactions with fellow travelers arguing about what little provisions were available.
    And extreme seasons – because this was a comet, of course (?).

    The first I had heard of Off on a Comet was grade school in a science fiction anthology with some criticism included by the editor perhaps Damon Knight or a contemporary. The editor had some scathing things to say which I had long forgotten about; much akin to what could just as well have been said about the Merchant of Venice. I don’t know if you could call this quick review a caveat or not. But finally, after reading the story and having some other Restoration and 3rd Repulbic history to judge from I can see how books represent their times in many ways that contemporaries would not have noticed.

  • Christian G September 9, 2021, 18:33

    One of the things that strikes me about how our understanding of the Universe has evolved over my lifetime* is how much more dynamic we realize it is now, and the Solar System in particular. I used to be we thought of planets and asteroids and even comets as pieces of a clockwork that never moved from their tracks except in extraordinary cases, but now…

    *I’m just old enough to remember the Apollo-Soyuz rendezvous, for reference.

  • randomengineer September 10, 2021, 1:24

    Recently there were articles herein re SETI and of course discussion on METI. Wouldn’t planetary radar emissions constitute an inadvertent form of METI? If we were looking for ET signals ourselves, wouldn’t planetary radar for similar asteroid/comet detection be seen as a viable non-natural signal?

    • Paul Gilster September 10, 2021, 6:27

      Yes, I think so. And these signals would present themselves as one-off phenomena as they swept at random across our sky. As I understand it, these would be the only kind of signals that would be accessible from an installation like our Arecibo from the Alpha Centauri system, if such were listening from there. No “I Love Lucy” for them, despite that recurring trope… James Benford has looked into this in some detail.

      • Alex Tolley September 10, 2021, 11:21

        Isn’t METI assuming interstellar signaling/communication, rather than local. Even though there is talk of looking for active probes in our system, SETI folks assume that ETI is located at interstellar distances, and all the anti-METI talk is about directed messages to other stars, even galaxies.

        Would radar pulses show strong, narrow bandwidth signals at interstellar distances? How different are the signal strengths compared to radar used for terrestrial purposes?

        • Paul Gilster September 10, 2021, 16:05

          Good question. I think I can dig out some answers in papers around here, so will look.

  • Jeff Wright September 10, 2021, 4:56

    I have awful news I heard at http://www.up-ship.com : Nyrath-Winchell Chung of Atomic Rockets-project rho…has cancer. He needs a billionaire space angel right about now….publiusr

  • charlie September 13, 2021, 14:16

    “The strong spike at a value of minus 70 Hz is the reflected signal (or “echo”) from 2021 PJ1; the other, …”

    minus 70 Hz ? What is – 70 Hz ?

    • Ron S. September 13, 2021, 17:10

      It’s right there in the axis label: Doppler shift. Since it’s negative the relative velocity is away from Earth.

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