I began reading Scientific American at a single-digit age. My first mind-blowing article was Allan R. Sandage's “The Red-Shift” in 1956. SciAm has gone through ups and downs, but in my view, in the past few years it has been better than ever. Bravo to all of you!



The Threat of Satellite Constellations,” by Rebecca Boyle, highlights the truly existential threat the growing number of orbital spacecraft poses to Earth-based astronomy, as well as the lack of political will to even address it. After all, what's a bit of twinkling in the sky compared with the profits to be made from making the Internet available everywhere and to anyone on Earth? What the article misses is the extreme danger of a coronal mass ejection (CME) of the strength of the Carrington Event of 1859, which even then caused complete disruption and fires in telegraph stations. A massive CME's effect on our modern technological civilization would be catastrophic.

What would happen to the thousands of new satellites in low-Earth orbit (LEO)? The CME would destroy their electronics, the satellite constellations would disintegrate, and collisions would inevitably begin to occur, each generating high-velocity fragments. A chain reaction would result, and LEO would become a barrier that would effectively prevent humankind's access to space! It is up to the scientific community to create the political will to address the control of LEO satellite proliferation because it is in the interest of everybody, not just astronomers.

DARRALL CUTTING Forestville, Australia

BOYLE REPLIES: A geomagnetic storm akin to the record-holding Carrington Event, named for the British astronomer who saw the solar flare that preceded the storm, would be catastrophic for objects in orbit, in cislunar space and on Earth. But solar activity needn't be that extreme to have an impact. As my story reports, geomagnetic activity in February 2022 caused 40 newly launched Starlink satellites to lose altitude and burn up in Earth's atmosphere. A more powerful storm could indeed start a chain reaction of failed satellites crashing into one another.

A solar storm is not the only thing that could cause this disastrous phenomenon, which is known as the Kessler syndrome. Short of reducing the number of objects in LEO, the best safeguard against CMEs may be to predict solar activity and ensuing geomagnetic storms with greater accuracy. Better forecasts could provide enough warning for astronauts to seek shelter and for satellite operators to take protective measures. Sun-observing spacecraft launched in the past five years could improve those storm predictions.


Life as We Don't Know It,” by Sarah Scoles, mentions the seemingly unavoidable criterion that all life involves reproduction. But I fail to see why reproduction is a necessity. Surely it is possible for a living entity to be immortal, sentient, distinct from its environment, metabolizing and even moving around without necessarily having to reproduce. There are at least two well-known science-fiction novels about such possible entities: Solaris, by Stanisław Lem (1961), and The Black Cloud, by Fred Hoyle (1957).

ROBERT CAILLIAU Prévessin-Moëns, France

SCOLES REPLIES: It's true that the inclusion of reproduction, or at least the ability to reproduce, is standard for most definitions of life. But that does ignore pesky complications, a classic one being the mule: an animal that everyone can agree is alive but that typically cannot make more of itself, with or without genetic variations. And flame, which can make more of itself, is not alive. On the topic of aging, Earth does actually have organisms that are functionally immortal, such as hydras. All of this is to say that the reproduction criterion isn't black-and-white, and neither is the definition of life. Humans will probably keep searching for versions of both the definition and life beyond Earth for a long time.


The Coming Collapse,” by Douglas Fox [November 2022], describes the impending disintegration of the Thwaites Glacier, which will contribute to sea-level rise. I have questions about what happens after the glacier's demise.

As I remember, ice's displacement of water remains the same when that ice melts. Thus, there will be no further increase in sea-level rise if ice that is already floating detaches from the main glacier. Is that correct? Additionally, the more apocalyptic stories about Thwaites seem to suggest it will fall into the ocean with a big splash. Does it work that way, or does the glacier slide in over the course of days, weeks, months or years?

Fred Brunyate via e-mail

FOX REPLIES: Because an ice shelf is already floating, its melting does not directly contribute to sea-level rise. Once the ice shelf is gone, however, the glaciers behind it accelerate their flow into the ocean—and this does increase sea level. After the collapse of the Larsen A and B and Prince Gustav ice shelves on the Antarctic Peninsula, the flow of the glaciers they had buttressed accelerated by twofold to ninefold. Much of this speedup happened within months. Thwaites's slide into the ocean won't necessarily accelerate that much when its ice shelf finally dies. But even the mere thinning and shrinking of the ice shelf to date have allowed the glacier's flow to accelerate. The nearby Pine Island Glacier has similarly seen a flow speedup of about 70 percent. The Thwaites and Pine Island Glaciers are so huge, compared with the little glaciers on the Antarctic Peninsula, that even their more moderate acceleration will contribute far more to sea-level rise.

The Thwaites Glacier covers an area approximately the size of Florida, and it's not all going to slide into the ocean immediately. So its “full” collapse will take centuries. But the big concern is that once Thwaites loses its ice shelf and its grounding line retreats past a certain point, the glacier will be inherently unstable. We'll have committed future humanity to the full amount of sea-level rise—even if it takes centuries to unfold.


Concrete Cure,” by Mark Fischetti, Nick Bockelman and Wil V. Srubar, should have said that the process of converting lime into clinker involves the lime sintering (fusing), not both sintering and melting. It also should have said that adding mineralizer to the lime would lower its sintering temperature, not its melting temperature.

Striking Back,” by Allison Parshall [Advances, April 2023], should have said the laser consumes about 10 kilowatts, not 10 kilowatts per hour.

In “Imaginary Universe,” by Marc-Olivier Renou, Antonio Acín and Miguel Navascués [April 2023], two of the equations in the opening illustration should have been given as i5 = i and i8 = 1, not i5 = 1 and i8 = √–1. And the box “What Are Imaginary Numbers?” should have said that rational numbers include the integers, not that “rational numbers are the integers.”