(Second in a series)
Now the Moon is back in America’s sights again, just as it was from my vantage point in a shuttle cabin in the 1990s and 2000s. The Artemis Program aims to take NASA astronauts and their international partners back to the Moon, using a four-person, deep-space craft called Orion, capable of cislunar flight durations of about a month. Orion is the command ship for Artemis, and has flown two orbital test flights. The most recent, Artemis I in 2022, took an unpiloted Orion into lunar orbit on a nearly month-long mission. The views from Artemis I, nearly 38,000 miles beyond the Moon, showed us the Earth-Moon system from the perspective of future lunar explorers.
Why Go Back?
The US is not returning to the Moon just to get there before the Chinese communists do (although that would be a positive development), or to try to “redo” Apollo 11’s achievement–though that is all the CCP has to do to claim victory in the current space race. Instead, NASA’s goal is to use human and robotic skills on the Moon to tap resources and enable a long-term human presence there. In the process, the US and its partners hope to establish, under the Artemis Accords, Western democratic standards for scientific and commercial activity on the Moon. Those lunar explorers will use local resources, such as water, to decrease dependence on Earth for supplies, and gain off-planet experience to help with the long-term exploration of the nearby asteroids and Mars.
A good primer on why we should return permanently to the Moon is planetary scientist Paul Spudis’ book, “The Value of the Moon.” (Spudis died in 2018). Paul held—and I agree–that the Moon deserves our attention because: 1. It is close by; 2. It is interesting scientifically; and 3. It is useful.
We’re so familiar with the Moon’s disk hovering in the day- and night-time sky that we tend to overlook the obvious: its prominence is due to its proximity. In solar system terms, the Moon is close. Its average distance is some 382,500 km (240,000 miles), and it takes a spaceship only three days or so to reach the Moon’s neighborhood. By contrast, Mars is on average some 225 million km from Earth, about 588 times farther away than the Moon. Travel times to Mars are generally seven to nine months on a fuel-efficient trajectory. So if we want to explore a planet and learn how to use its resources, it’s much easier to operate at the Moon.
We can get there pretty quickly, and if there’s trouble, get back in under three days. And the moon is accessible most of the time; we can launch from Earth almost any day of the month, depending on the final orbit that we would like to attain. To get to Mars with a fuel-efficient trajectory, we can only launch about every 26 months, when the planets line up correctly. So, the moon is close, and that advantage is something that we should exploit.
Next: The Right Lunar Orbit
Thanks for mentioning those nearby asteroids as human spaceflight destinations, Tom. As you know, they’re the interplanetary steppingstones to Mars.
Oh, and I believe propellant-efficient opportunities to reach Mars from Earth (and vice-versa) only arise once every 26 months on average.
Permit me to elaborate on your remark about being able to launch from the Earth to access the Moon “almost any day of the month”. While that’s true theoretically, the SLS Block I launch vehicle does not fulfill that promise for NASA in the near-term. According to the calendar of viable Artemis II launch dates in February through April 2026 (ref. https://universemagazine.com/en/humanity-returns-to-the-moon-artemis-ii-mission-flight-plan/), only 16 of 89 days (18%) are viable. Although viability constraints for Artemis II launch are myriad and often obscure to the public, a major one is the performance mismatch between the SLS Core Stage and the Block I second stage. This imposes a pre-trans-lunar injection elliptical orbit on the mission that can lead to lunar intercept only a few days each month. The Core Stage was designed to optimally operate with an Exploration Upper Stage that keeps getting deferred due to NASA’s lean budget.
Thanks, Dan, for a lucid explanation of the SLS’s lunar limitations. Please keep me straight on future discussions, especially on lunar orbits like the rectilinear halo orbit. I’ve argued in the press recently that NASA needs more resources for Artemis as we near an eventual landing mission.
Thank you, Tom, excellent article.
Another problem with SLS was using the anemic ESA service module which is incapable of getting too and from low lunar orbit.
That necessitated overly heavy and complex landers and complex orbits.
The SLS rocket was saved by Congress when Obama canceled the return to the Moon in 2010. But the Altair lander wasn’t saved incurring future delays.
Now its a race between SpaceX and Blue Origin for a lander in a couple years or longer that offers China the opportunity to beat us and declare the territorial claim as they’ve promised.
Everything is coming together though. Lets go!