That’s one small change in an asteroid’s orbit, one giant leap for humanity.

On September 26, 2022, the Double Asteroid Redirection Test (DART) spacecraft ended its existence in spectacular fashion by smashing into the asteroid Dimorphos (S&T: Jan. 2023, p. 8). Many of us watched spellbound as DART’s cameras fixed upon the approaching target. At first, we saw only Dimorphos’s larger orbital companion, 65803 Didymos, as a bright pixel, but soon it resolved into two objects. In the final minutes, the target suddenly loomed, filling the screen with a final, brief, fantastically detailed glimpse of its surface (see video at

The approach reminded me of my student days witnessing the Voyager encounters. The monitors at JPL would show a distant target growing gradually for days, followed by a few hours of quickening expansion to a differentiated object that filled the screen. I’d pretend the monitors were windows and we were flying past newly seen worlds. 

I had that sensation again with the New Horizons spacecraft at Pluto in 2015, and now again with DART. Except that those other encounters were flybys, not crash-intos. And instead of days, this event took place within an hour. You could almost hear the swooping Star Trek alert sirens and the command to “Brace for impact!”

The impact shortened the roughly 12-hour orbit of Dimorphos around Didymos by about 32 minutes (S&T: Feb. 2023, p. 9). Such a small change might sound inconsequential. But arguably it represents a significant turning point in the history of our planet, our biosphere, and our solar system. The influence of the Anthropocene Epoch, marked by humans as a geological force, has begun to extend off-planet.

I don’t know who first said that the difference between us and the dinosaurs is that they lacked a space program, but this joke is both funny and profound. Our solar system now has a new kind of planet — one that can begin to defend itself. The knowledge gained will help us forestall dangerous cosmic impacts that, if we waited long enough and did nothing, would inevitably cause regional destruction and, eventually, new mass extinctions on Earth. We’re no longer helpless against this threat, and so, assuming we keep our act together and retain a technologically advancing global civilization, our biosphere and our planet need never again suffer such catastrophes.

Before we get too cocky, though, remember that dinosaurs walked the planet for 180 million years. Humans have been here for a few percent of that. And our burst of technical ingenuity has created new, self-imposed threats. Need I enumerate them? We certainly are gaining skills that could allow us to survive while the big lizards perished. But whether our wisdom keeps pace with our prowess and allows us to create the kind of global society that can harness that cleverness in the service of dinosaur-scale longevity is still TBD.

Back in the 1990s, Carl Sagan warned against messing with asteroids, lest someone get the crazy idea to direct one towards Earth. But in a world armed with thousands of nuclear weapons, that’s the least of our worries. At least now we’re taking baby steps toward protecting ourselves against a truly long-term, planetary-scale threat.

We, the adolescent technological species from Earth, have started, ever so slightly, to rearrange the solar system. Hopefully this is part of growing up. If so, then someday we may save not just our species but our biosphere.

This article originally appeared in print in the March 2023 issue of Sky & Telescope. Subscribe to Sky & Telescope.


Image of Robert-LaPorta


December 15, 2022 at 9:51 am

"remember that dinosaurs walked the planet for 180 million years. Humans have been here for a few percent of that."

And all during that 180 million years it appears that no dinosaur developed intelligence. While life in the universe may possibly be common (or perhaps rare) it appears that intelligence may be many orders of magnitude rarer.

You must be logged in to post a comment.

Image of Anthony Barreiro

Anthony Barreiro

December 15, 2022 at 8:54 pm

DART is a historic technological accomplishment. But it doesn't make me more optimistic about the future of humanity or life on Earth. NASA has now demonstrated that we can change the momentum of an asteroid. Meanwhile, human beings are recklessly melting the icecaps, acidifying the oceans, and causing the sixth mass extinction of life on Earth. In the immortal words of Pogo, "We have met the enemy, and he is us."

By the way, dinosaurs comprised two orders of reptiles. Homo sapiens are only one species. The earliest known fossil in our class Primates is 57 million years old.

You must be logged in to post a comment.

Image of Anthony Barreiro

Anthony Barreiro

December 16, 2022 at 12:52 am

Whoops, Homo sapiens are in the order Primates. Our class is Mammalia. Sometimes I'm not so sapient.

You must be logged in to post a comment.

You must be logged in to post a comment.