There have been media reports about a propulsion technology under study that would allow Mars to be reached in just three days instead of the multi-month transit times typical with today’s propulsion systems. There were even a smattering of headlines claiming that travel times to Mars as short as 30 minutes were even theoretically possible. Unfortunately, a closer look at these sensational headlines showed that this is just another case of media hype designed to generate web page traffic (i.e. “click bait”).

The origin of these claims was the latest paper in a series by Philip Lubin (University of California – Santa Barbara) examining the feasibility of laser-based propulsion for interplanetary and interstellar exploration. In a paper submitted to the Journal of the British Interplanetary Society, Lubin describes his DE-STAR (Directed Energy System for Targeting of Asteroids and ExplorRation) concept where a laser in Earth orbit with a power rated at tens of gigawatts could propel a miniature “wafersat” with a mass on the order of a gram fitted with a light sail just a meter across to speeds as great as 26% of the speed of light (or 0.26c) in as little as ten minutes. At such high velocities, the distance to Mars could be covered in just a half an hour.

While an intriguing concept, unfortunately it could not be used to send large payloads to Mars at those velocities. Calculations suggest that a larger 100-kilogram spacecraft would take up to three days to reach Mars while a still larger version capable of carrying passengers could take at least a month. But even if somehow the technology could be scaled and adapted to send large spacecraft to Mars in less than an hour, accelerating to 0.26c in ten minutes to achieve a 30 minute transit time would subject the payload to loads of 13,000 g – far too high for humans or most payloads to survive intact. Although this particular laser-based propulsion method (or indeed any foreseeable propulsion technology) will not get a crew to Mars in less than an hour, it did remind me of a problem I addressed out of curiosity as a physics undergrad over three decades ago: what is the shortest practical travel time to the planets?

To tackle this problem, I assumed a hypothetical propulsion system which could accelerate a large ship along with its passengers and cargo at a constant 9.8 meters/second2. This “one-g” ship would recreate the acceleration due to gravity on the surface of the Earth without the deleterious effects of long-term weightlessness on human physiology or the complexity of spinning the ship to create artificial gravity. Such a ship could accelerate for about half the journey to its target planet then turn to decelerate for the second half of the trip to enter orbit around its destination. Since such an acceleration level is something like three orders of magnitude higher than that due to the Sun’s gravity even among the planets of the inner solar system, the trajectory of such a spacecraft can be comfortably approximated as a straight line after it escaped the Earth – a process that itself would take only a few tens of minutes. As a result, freshman-level physics of linear motion would be sufficient to estimate the travel times to the planets.

My estimates of the travel times from Earth to the seven other planets for a hypothetical one-g spacecraft are tabulated below. Also listed are the dwarf planets Ceres and Pluto which I included to provide examples of typical targets in the asteroid and Kuiper belts, respectively. The assumption is that this hypothetical spacecraft travels in a straight line from the Earth to its target speeding up for half the trip then slowing down for the other half. As a practical matter, the travel times when the targets are at its farthest distance from the Earth on the opposite side of the Sun from us could be a bit longer than indicated to avoid travelling too close to the Sun during the journey. The actual effects of this deviation from the assumed straight-line trajectory would be determined by the design details of the hypothetical one-g spacecraft.

Approximate One-Way Travel Times for One-g Trip

Name Distance (million km) Travel Time (Days) Mercury 77 to 220 2.1 to 3.5 Venus 38 to 261 1.4 to 3.8 Mars 55 to 401 1.7 to 4.7 Ceres 233 to 595 3.6 to 5.7 Jupiter 590 to 970 5.7 to 7.3 Saturn 1,200 to 1,700 8.1 to 9.6 Uranus 2,600 to 3,200 12 to 13 Neptune 4,300 to 4,700 15 to 16 Pluto 4,300 to 7,700 15 to 21

As can be seen, any inner solar system target can be reached within a few days with this hypothetical one-g ship. In fact, the planets Venus and Mars could be reached in as little as about 35 and 41 hours, respectively. Even far off Pluto could be reached in only a couple of weeks – much more quickly than the nine-year travel time of our fastest spacecraft launched to date, New Horizons.

In principle, these various targets could be reached even more quickly by increasing the acceleration rates. But as a practical matter, not too much time could be shaved off of these travel times since they are inversely proportional to the square root of the acceleration. Assuming that a passenger could safely endure long periods of 3-g acceleration, for example, the minimum trip time to Mars would only be cut down to about 24 hours. If high priority cargo were robust enough to withstand long periods of 10-g loads, the transit times would be cut to about 32% of those listed, for example.

While one could envision ultra-short transit times to the planets using durable spacecraft like those proposed by Lubin and others, in the end the practical lower limit for trips to the planets for most cargo will be on the order of a couple of tens of hours to a week. More fragile “cargo” like human passengers will have to endure longer trip times measured in days to a couple of weeks. Still, the ability to move throughout the solar system so quickly would open up a whole universe of opportunities for humanity if the required propulsion technology were to be developed.

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Related Video

Here is a video of Dr. Lubin’s presentation about the Directed EnErgy Propulsion for Interstellar exploratioN or DEEP-IN (a variation of his DE-STAR proposal) project given on October 28, 2015 at the NASA NIAC Fall Symposium in Seattle, WA.

General References

Philip Lubin, “A Roadmap to Interstellar Flight”, submitted to Journal of the British Interplanetary Society, April 2015 [Submitted Draft]

Philip Lubin et al., “Directed Energy for Relativistic Propulsion and Interstellar Travel”, Journal of the British Interplanetary Society, Vol. 68, No. 5/6, pp. 172-182, May/June 2015