Four hours, three days, nine months. At first glance, these numbers seem to tell a simple story about increasing distance. They actually reveal something much more interesting about how space travel works. NASA astronaut Anne McClain spent six hours and four orbits around the Earth traveling to the ISS in December 2018. Half a century ago, the Apollo 8 astronauts spent nearly three days traveling to the Moon. A journey to Mars, on the other hand, is measured in months. The distances are enormous: approximately 400 km to the ISS, 384,400 km to the Moon and tens to hundreds of millions of kilometers to Mars. However, distance alone does not determine travel. In space, getting somewhere means catching something that is already moving. Six hours of chasing a house in the sky McClain left Earth from the Baikonur Cosmodrome in Kazakhstan on December 3, 2018, along with Canadian astronaut David Saint-Jacques and Russian cosmonaut Oleg Kononenko. They took off at 6:31 a.m. EST. At 12:33, after four orbits of the Earth, their Soyuz docked with the ISS. Photo: NASA “It won’t be easy to put this journey into words, but I’ll try,” McClain later said. “Finally I am where I was born.” NASA recorded this comment in its report on her first trip into space. It is noteworthy that her destination was only 400 km above the Earth. A passenger plane could travel 400 km horizontally in less than an hour. Space flight is different in that McClane’s Soyuz could not simply rise to 400 km and stop. The ISS was moving at a speed of about eight kilometers per second. It orbits the Earth approximately once every 90 minutes, completing about 16 orbits per day. The Soyuz would first have to launch itself into orbit and then maneuver until its trajectory and speed allowed it to rendezvous with the station. The astronauts literally chased their destination across the planet. Photo: NASA NASA says the spacecraft will be able to reach the ISS in just four hours. Other missions take longer because the journey depends on launch timing, orbital geometry and rendezvous profile. This brings us to the first lesson in answering the deceptively simple question of how long space travel takes: 400 km in space is not the same journey as 400 km on Earth. Three days until the moon The Moon is almost 1000 times further away than the ISS. Its average distance from Earth is about 384,400 km. A radio message traveling at the speed of light can travel there and back in about 2.5 seconds. It took people about three days to get there. Apollo 8 perhaps provides the best description of what the journey actually looked and felt like. Photo: NASA Frank Borman, Jim Lovell and Bill Anders launched on December 21, 1968, becoming the first people to leave Earth’s orbit and travel to another world. Two hours and 50 minutes after launch, the Saturn V’s third stage fired for the translunar landing, propelling Apollo 8 toward the Moon. As the Earth retreated from them, the view began to change. Halfway to the Moon, Bormann looked out the window at the planet that no longer filled the sky. “It’s a beautiful, beautiful view,” he told Mission Control. Apollo 8 reached the far side of the Moon 69 hours, eight minutes and 16 seconds after launch. The spacecraft then had to do something that a probe simply flying past the Moon wouldn’t: slow down. Its engine burned for four minutes, reducing Apollo 8’s speed enough for lunar gravity to propel it into orbit. This difference explains why the question of how quickly a spacecraft can reach the Moon is different from the question of how long a lunar mission will take. New Horizons, an unmanned probe bound for Pluto, crossed the Moon’s orbit just eight hours and 35 minutes after its launch in 2006. The matter did not stop there. Apollo 8 was to come and stay. “See you on the other side” Just before Apollo 8 reached the Moon, a moment occurred during the journey that clearly illustrates another problem associated with traveling far from Earth. Photo: NASA The spaceship was about to disappear behind the Moon. Radio contact with Mission Control will be lost the moment the crew fires the engine needed to enter lunar orbit. “Apollo 8, one minute to LOS,” spacecraft communicator Jerry Carr told them. “All systems are good. Enjoy your trip, guys.”“See you on the other side,” Lovell replied. There was nothing Houston could do then. Mission control waited 37 minutes and 32 seconds. When telemetry finally began to appear again, Apollo 8 was safely in lunar orbit. Carr asked what the Moon looked like from 60 miles up. “Like dirty beach sand with lots of footprints,” Anders replied. Having traveled nearly 400,000 km, it was the first time an astronaut had described another world. An even bigger surprise came when the team turned away. During the fourth orbit of Apollo 8, Anders saw the Earth appear above the lunar horizon. “The Earth is rising. Wow, this is beautiful,” he said. Photo: NASA He took the photograph that became known as Earthrise. Anders later summed up the strange conclusion of mankind’s first trip to the Moon: “We came all this way to explore the Moon… and most importantly, we discovered the Earth.” Mars changes the meaning of the word “far” There is no astronaut story to open the Mars chapter. This is because there was no one there. Given current mission profiles, NASA estimates that a relatively direct path to Mars will take seven to ten months. The Mars Reconnaissance Orbiter took about seven and a half months; MAVEN took about ten. Even saying how far away Mars is requires qualification. The Earth and Mars are constantly moving around the Sun. At particularly close encounters, they could be about 56 million kilometers apart. When they are on opposite sides of the Sun, the distance can be approximately 401 million kilometers. Therefore, the spacecraft cannot simply be aimed at the location where Mars will appear on launch day. It should intercept where Mars will be in a few months. Imagine throwing a ball to someone running across a field. You are racing ahead of the runner. A mission to Mars does something similar, except that both the point of departure and the destination are racing around the Sun, and the “ball” could take the better part of a year to get there. Useful Earth-Mars launch opportunities occur approximately every 26 months. Why not just go faster? This is where space travel gets counterintuitive. A faster spacecraft is not automatically a better spacecraft. To move faster, you need energy and fuel. More fuel increases the mass that needs to be launched from Earth. And the rapid arrival creates another problem: the spacecraft must somehow slow down. The Mars orbiter must reach the planet at a speed that allows it to enter orbit. The lander must survive reentry and then reach the surface. These requirements limit the trajectories that mission planners can choose. The Moon represents the same compromise. NASA orbital dynamics expert Brent Barbee notes that trajectories designed to save fuel could take weeks or even months rather than days. Sometimes spending time saves fuel. Sometimes fuel consumption saves time. And sometimes gravity lends a helping hand. A spacecraft traveling much further into the solar system can fly past planets, changing its trajectory and speed, without using an equivalent amount of fuel. NASA’s Galileo took just over six years to reach Jupiter; Juno took just under five. On Mars, even conversation has a travel time The distance to Mars becomes most clear when one is trying to send a message home. Radio waves travel at the speed of light. However, at interplanetary distances, even this does not happen instantly. Depending on where Earth and Mars are located, ESA sets the one-way signal delay to between approximately four and 24 minutes. During the landing of NASA’s Curiosity rover, it was 13 minutes and 48 seconds. Photo: NASA Imagine that Curiosity at this moment solves the problem and sends a message to Earth. Dispatchers were unaware of this for almost 14 minutes. If they had immediately realized the problem and relayed the command back immediately, it would have been nearly 14 more minutes before the spacecraft received it. Therefore, ESA pre-loads commands and ensures autonomy for spacecraft operating on Mars. A faster computer or more powerful radio will not eliminate the delay. The signal is already propagating at the fundamental limiting speed of the Universe. For future astronauts, this will fundamentally distinguish Mars from the ISS or even from the Moon. A person on Mars could ask a question to someone on Earth and, at its greatest distance, wait more than 40 minutes to hear the answer. There will be no regular telephone conversation with home. Hours, days, months So how long does it take to travel in space? As for the ISS, think for hours. Anne McClain took six. Regarding the Moon, think about the days. It took Apollo 8 just under three to reach lunar orbit. For Mars, think about months. Considering the trajectories that spacecraft typically fly today, it’s typically between seven and ten. But these numbers are actually the answer to a much more interesting problem. The spaceship does not move between two fixed points on the map. It leaves the moving Earth to meet another moving object. It must carry enough fuel, follow the right orbit, arrive at the right moment and, most importantly, reach its destination at a speed that allows it to do what it came to do. Perhaps Apollo 8 captured this better than any calculation. The three men traveled further from home than humans had ever traveled. They traveled almost 400,000 km to see the Moon up close. And when they got there, what struck them most was the sight of the small blue world that they had left behind. Post navigation The Labor Day travel gear sale includes up to 75% off luggage sets, carry-ons and more. I’ve stayed in the best hotels in the Dolomites – this is where I would really spend my money.