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Earlier today, the U.S. National Science Foundation announced that it is decommissioning the Arecibo radio telescope. The 305 meter dish is famous for the 1974 message for extra-terrestrial intelligence, and its appearance in several science fiction movies. Less glamorously, its radar capabilities have been crucial for tracking near-Earth asteroids.

The facility has fallen to disrepair during the last three years, due to hurricanes, earthquakes, and funding cuts. Parts of the dish have collapsed, and two cables have snapped. Citing safety concerns, the NSF announced the decommissioning of the facility today.

How will the closure of Arecibo affect communications with deep space probes?

Wikipedia claims that there were only two radar astronomy facilities in regular use: Arecibo, and the 70 meter DSS 14 dish at Goldstone. The problem is that DSS 14 is also used by the Deep Space Network, particularly for staying in contact with the Voyagers and New Horizons probes. Indeed, I had already noticed earlier this week that DSN had started talking to Voyager 2 with a pair of 35 m dishes, rather than the 70 m dishes. Have deep space communications already been bumped to smaller dishes, so the larger dishes can track asteroids?

Arecibo damage

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    $\begingroup$ It feels bad upvoting this sad question. $\endgroup$ Nov 20 '20 at 10:50
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    $\begingroup$ The dish is 57 years old and severely damaged. Repair costs will be huge. How many operational years would be possible after a repair? $\endgroup$
    – Uwe
    Nov 20 '20 at 11:36
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    $\begingroup$ @Uwe How expensive is it to maintain a valley-spanning metallic mosquito net, radio equipement in a gondola and a few server rooms, the contents of which are refrreshed regulary? Maybe a cafeteria, too. I guess Puerto Rico also gets some money for rights-of-use. $\endgroup$ Nov 20 '20 at 17:51
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    $\begingroup$ The facility's projected 2021 budget was only $3 million. A GoFundMe campain, anyone? $\endgroup$
    – mustaccio
    Nov 20 '20 at 19:01
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    $\begingroup$ @mustaccio, the problem is that the receiver platform needs a complete rebuild: the cable that broke did so at only 60% of rated load, which raises severe doubts about the safety of the rest of the cables supporting it. $\endgroup$
    – Mark
    Nov 20 '20 at 21:56
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There is some indirect evidence that Arecibo's loss has already impacted communication with some of the deep space probes.

NASA's official statement on Arecibo says this:

NASA’s Goldstone Observatory in California, another planetary radar, recently returned to full operations after successful delivery and testing of a new klystron tube for its high-power transmitter. Radars such as those at Goldstone and Arecibo are used only to characterize known NEOs, not to discover previously unknown asteroids and comets, so NASA’s NEO search efforts are not impacted by the planned decommissioning of Arecibo’s 305m radio telescope.

NASA's FAQ on planetary defense from asteroids does not appear to be updated for Arecibo's loss. However, it does credit Arecibo with ruling out an impact in 2029:

Radar observations can reduce the uncertainly in position of an asteroid from the several thousands of kilometers provided by optical observations to just a few meters. The impact risk posed by a potentially hazardous asteroid can be relatively quickly resolved with radar observations, while it might otherwise remain uncertain for years if only optical observations are available. Such was the case with the asteroid Apophis, discovered in 2004 and initially thought to pose a risk of Earth impact in April 2029. Radar observations made by Arecibo Observatory in 2005 eliminated that possibility of impact.

It also reports that radar analysis of near-Earth objects was 1/3 of Arecibo's budget:

The Near-Earth Object Observations Program provides $4.5 million a year in funding to the Arecibo Observatory in Puerto Rico – about one third of the observatory’s yearly budget – to fund its planetary radar capability. The National Science Foundation is in the process of transitioning the Arecibo Observatory to the University of Central Florida and UCF’s proposed alternative funding avenues while reducing NSF’s own level of support. NASA has agreed to continue to fund planetary radar capability at Arecibo during this transition. NASA also uses planetary radar capability at its own Deep Space Network Goldstone station. However, Goldstone’s radar is not as powerful as Arecibo’s, so it does not have the same range into space, meaning that fewer NEOs could be characterized by radar.

With the loss of Arecibo, Goldstone's DSS-14 now becomes the world's largest and most powerful radar dish. (China's 500 meter FAST dish is larger, but has no transmitter and is purely passive.) Sky and Telescope reports that "Arecibo offered 18 times the sensitivity of other existing facilities, such as NASA's Goldstone receiver." It also states

Arecibo is also irreplaceable for scientists. Even though it’s technically the second-largest radio dish in the world (China’s Five-hundred-meter Aperture Spherical Telescope, or FAST, recently broke the record Arecibo held for decades), the observatory has unique capabilities, among them its radar. “FAST cannot do radar, it’s specifically incapable of doing active observation,” Springman explains. Because of that, FAST can’t take Arecibo’s place in planetary defense by characterizing asteroids and their orbits.

It sounds like a significant amount of Goldstone's DSS-14 time will be shifted away from its Deep Space Network duties, and instead put on Near Earth Object radar duty. The 70 meter dishes of the other two DSN sites may also be similarly conscripted to NEO duty. In particular, the tracking schedule for the Canberra DSN site shows considerable changes:

  • In the past, the large 70 meter DSS-43 dish (the largest in the southern hemisphere) was used almost daily to contact Voyager 1, Voyager 2, or New Horizons. It now has only four contacts scheduled over the next two months: one with Voyager 1, two with Voyager 2, and one with the Solar Parker Probe.
  • Voyager 1 has no other scheduled contacts with Canberra. This is probably because VGR1 is above the ecliptic, but Canberra is in the southern hemisphere.
  • Voyager 2 has 113 more scheduled contacts with Canberra, using the smaller dishes, usually in a pair of dishes.
  • New Horizons has 10 more scheduled contacts with Canberra, using the smaller dishes.

So there is evidence that some of the communications with the farthest-out spacecraft have been bumped down to the smaller dishes.

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  • $\begingroup$ This is too complicated for me to follow (an inadequacy on my part, not the post) but 34 meter dishes have always been capable both transmitting to, and receiving from from the Voyagers. It's been mostly a game of hemispheres and which dishes have old-style S-band transmitters and the associated modulators that can speak to Voyager. See Why does DSN sometimes uses two dishes at the same time to receive Voyager-1? and also Why was Canberra able to listen to Voyager 2 but not talk to it? $\endgroup$
    – uhoh
    Nov 20 '20 at 11:49
  • $\begingroup$ @uhoh: Thanks, I've added links to those two questions in the answer. $\endgroup$
    – DrSheldon
    Nov 20 '20 at 18:27
  • $\begingroup$ @uhoh Transmit has to be done with 74m antenna, not enough power & gain to use a 34m. Table 5-2 and 5-3 of Voyager Telecommunications sets out link budget for transmit/receive. Haven't had a chance to go through all the numbers yet for the 34m antennas lower gain and the increased range (126 vs 48.6 AU in tables) but I see no way the received power at Voyager2 would be above the noise floor. Doc also talks about using arrayed 34m dishes for receive, well worth a read $\endgroup$ Nov 20 '20 at 19:22
  • $\begingroup$ @astrosnapper Then I'm trying to understand the screen capture shown in Why does DSN sometimes uses two dishes at the same time to receive Voyager-1? which indicates bidirectional communication with Voyager 1 from either DSS-25 or DSS-26 (if it were a GIF we could tell). According to this page 24, 25 and 26 are the 34 meter Beam Waveguide Cluster at Goldstone. I'll try to go through the logs and see if I can find more instances. I can check again for that image since I'd noted a timestamp in the post. $\endgroup$
    – uhoh
    Nov 21 '20 at 1:53
  • $\begingroup$ This does not answer the question. While the Arecibo facility was used to track asteroids, it was not used to track or communicate with human-made probes. The Arecibo facility was not a part of the Deep Space Network. $\endgroup$ Nov 23 '20 at 8:47
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It is too late now to think about a repair. The instrument platform crashed into the dish at the first day of December.

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