The Artemis II Distance Record in Cosmic Perspective
The Artemis II spacecraft carried four humans to a distance record of 406,771 (~0.4 million) kilometers away from Earth. How significant is this accomplishment in a cosmic perspective?
To gauge this feat, consider the cosmic ladder in distance jumps of roughly a factor of 10,000 per step.
At a distance 10,000 bigger than the Artemis II record, lies the farthest planet in the Solar System, Neptune, of about 4.5 billion (4.5x10⁹) kilometers.
Another factor of 10,000 brings us to the distance of the nearest star, Proxima Centauri, of about 40 trillion (4x10^{13}) kilometers.
The diameter of the orbit of the Sun around the Milky-Way center is another factor of 10,000 up, of order 500 quadrillion (5x10^{17}) kilometers.
The final step of 10,000 brings us to the largest cosmic scale over which the distribution of galaxies is clustered, of order 4.6 sextillion (4.6x10^{21}) kilometers. This is the scale of the so-called Baryonic Acoustic Oscillations (BAO), which serve as the largest yardstick for measuring the geometry of the Universe.
The radius of the observable universe, the so-called cosmic particle horizon today, is approximately 100 times larger than the BAO scale, of order 440 sextillion (4.4x10^{23}) kilometers. We cannot observe what lies beyond that distance.
This sets a limit to how far light had traveled since the Big Bang. But the limit on how far we can travel in the future is much smaller because of the accelerated expansion of the Universe.
In order for a spacecraft to catch up with a distant galaxy, it must move faster than the recession speed of that galaxy. But given the accelerated expansion of the Universe, the task is even more daunting. The cosmic expansion is expected to be exponential in the future. This implies that irrespective of how fast we launch and how long we wait, a spacecraft would never catch up with galaxies beyond a certain distance from us. This is because distant galaxies will eventually be separated from us faster than light as a result of accelerated cosmic expansion.
A spacecraft launched out of our galaxy at some speed could only reach a galaxy that is currently receding from us at a cosmic speed of less than half the spacecraft’s speed. This introduces the concept of a cosmic horizon for any launch speed, akin to prison walls for our travel ambitions.
What would be the realistic expectations for future propulsion schemes that do better than chemical rockets? As I showed in a paper with my former postdoc, Manasvi Lingam, an ambitious space program could use light sails or electric sails to exceed the escape speed from the Milky Way. A spacecraft moving a hundred times faster than the speed of the five chemical rockets we sent so far to interstellar space could reach intergalactic space with a speed of 1000 kilometers per second, 0.3% of the speed of light. At that speed, it could catch up with galaxies that are currently within 5% of the BAO distance scale from us. But this spacecraft will never catch up with galaxies farther away, irrespective of how long we wait.
The center of the nearest cluster of galaxies, the Virgo cluster, is about 15% of the BAO distance scale. Reaching beyond this distance requires spacecraft that move faster than a percent of the speed of light or 3,000 kilometers per second. Humanity’s most ambitious space travel initiative: Starshot — which I have the privilege of leading, aims to reach a speed that is an order of magnitude larger, above a tenth of the speed of light. This initiative envisions shining a powerful 100-gigawatt laser for a few minutes on a meter-size, gram-mass light sail. A Starshot probe could reach in the future galaxies that are out to the BAO distance scale.
The cosmic horizon for intergalactic travel will include fewer destinations in our future because distant galaxies will keep accelerating away from us. We need to get our act together if we wish to reach them. As I had shown in a 2001 paper, once the Universe will age by a factor of ten — even a spacecraft moving at the speed of light will not be able to catch up with any galaxy beyond our own.
All in all, we have a lot to aspire to in terms of our ambitions for space exploration. Long trips will likely be guided by robots with artificial intelligence (AI) rather than by humans with natural intelligence. The long travel time for communication signals, amounting to tens of thousands of years across the Milky-Way galaxy alone, will make real time communication futile.
Launching interstellar mission with AI robots would feel like sending kids away from home to the world at large. It is unrealistic to expect frequent reports from them about their whereabouts. All we can hope for is that they will represent us well and spread our qualities of curiosity, integrity and intelligence to the Universe. Given that these AI robots will likely pass the Turing Test, they do not need to be made of flesh and blood in order to serve as effective ambassadors of humanity.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics (2005–2026), and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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