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.
A schematic of Dr. Lubin’s laser-based propulsion system proposal. (UCSB)
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. In addition, with the maximum velocity for trips within the Solar System on the order of hundreds to thousands of kilometers per second, relativistic effects can be safely ignored. 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
Humans are fragile indeed. But rating inanimate objects to survive much higher accelerations is a solved problem. If you’ve ever dropped your smartphone from shoulder height and it still functioned after hitting a hard surface, you’ve demonstrated your phone is rated for about a thousand G of acceleration. There exist artillery shells that feature GPS guidance (M982 Excalibur) and laser-guidance optics (M712 Copperhead) that can withstand 10,000s of G on launch.
Even more extreme, I have launched projectile (albeit with no electronics on board) at 10s of *millions* of G (where loads greatly exceed strength of projectile material) but projectiles survives and comes out of the launcher intact, provided the loading is applied carefully.
There is no need to limit accelerations of laser-driven sails (as in Philip Lubin’s concepts) to just 10 Gs.
I am fully aware that the technology exists to create robust payloads capable of withstanding ultra-high G-loads and never stated otherwise in the article. When I talk about “cargo”, I mean the same sort of generic cargo that is sent to the ISS today – food, fuel, tools, equipment, replacement parts, medical supplies, small Christmas gifts from loved ones back home – the sorts of things that can be readily expected to survive my notional 10-G acceleration limit. I can not ever see the need to send such generic cargo on an ultra-high acceleration hyper-express trip to some outpost in the asteroid belt, for example.
Great read, thanks!
Follow up questions:
– what about travel times to nearby solar systems using this hypothetical 1G drive?
– what about the energy requirements, assuming the energy spent is fully dissipated, and not recovered (as would be the case if the payload was in the middle of a giant spring set between the origin and the destination)? Wouldn’t it be a major blocker to common interplanetary and interstellar travel?
> what about travel times to nearby solar systems using this hypothetical 1G drive?
Well, that would be a subject of yet another article 😉 But, in brief, relativistic effects would start to become important with a 1 g ship traveling much beyond the Kuiper Belt and violate an implied assumption of my exercise. For example, traveling to Alpha Centauri in a one-g ship (where the acceleration is measured from the frame of reference of the spacecraft) would reach its target in 3.6 years (again as measured from the frame of reference of the spacecraft).
> what about the energy requirements, assuming the energy spent is fully dissipated, and not recovered (as would be the case if the payload was in the middle of a giant spring set between the origin and the destination)? Wouldn’t it be a major blocker to common interplanetary and interstellar travel?
These questions are outside the scope of what I wanted to discuss in this piece. I was merely performing a simple exercise on the practical lower limits of travel times to the planets and was not interested in getting into the details or relative merits of one-g space travel (hence my insistence on a hypothetical one-g ship)
How would the vehicle decelerate? Wouldn’t it require a laser facility at or near the destination, or propellant/engines capable of sustaining constant 1-g acceleration?
> How would the vehicle decelerate?
For the purpose of this article, I am only considering a hypothetical 1-g spacecraft without specifying any sort of propulsion system including a laser-based system like Dr. Lubin’s. When I was originally considering this problem back in the early 1980s, I was thinking in terms of some sort of fusion or maybe even antimatter-based propulsion system to get the specific impulse required for such a ship. But those questions are outside of the scope of this article.
That being said, laser-based propulsion systems for interplanetary and interstellar spaceflight have been studied for decades. I read my first detailed technical treatment of the subject about 35 years ago and there was a great summary of the state of the technology in the 1988 book ” Starsailing: Solar Sails and Interstellar Travel” which adressed your question. There are a number of different methods available for slowing a laser-propelled light sail available including having another high power laser at the destination or having the spacecraft deploy another reflector to bounce light back towards the original light sail to slow it down. But like I said, these questions are outside the scope of this particular article which focuses instead on the lower practical limits of travel times to the planets.
Fascinating article, thanks for sharing. What about the travel time to Earth’s moon?
Glad you liked the article. To address your question, assuming that my approximations are still reasonably valid, the trip time to the Moon would be about 3.5 hours. In reality, it will likely be a bit longer than this because the hypothetical 1-g ship will not travel a straight line from the Earth to the Moon but a broad spiral instead as it escapes the Earth. But I think a trip time on the order of 4 hours would be about on the money.
Excellent and concise article. I smirked amusement at your click-bait comment. Well played.
What I specifically like is you identify the misconceptions in the wafer-sat concept of payload viability/desirability. There must be a minimum mass needed for communication and sufficient data transmission as Lubin postulates a craft going to Alpha Centauri at 0.26c. I like how you have bracketed upper limits of speed within the solar system as 1g.
This theoretical (laser propulsion) and even development work (Lockheed’s compact fusion) in play that looks interesting for the future of propulsion. The computational prowess we now have could shrink timelines from centuries to decades on possible prototypes and deployment. If a Mars trip (for humans) could be shrunk to 30 or 60 days, logistics and supplies, and even spacecraft design gets much easier.
Enticing to think these scales may be within a half-century’s reach.
If the propulsion laser was in space, wouldn’t the impulse that drives the spacecraft forward also drive the laser backward? Wouldn’t it be better to base the laser on the moon, where it could collect solar energy in an atmosphere-free environment?
How do you intend to decelerate the craft at the other end?
That question is outside the scope of the article which is about the shortest practical trip times to various planets. The claim that some DESTAR-inspired technology can get a payload to Mars in 30 minutes is cited as an example of an absurd claim that has appeared in the headlines.
Using this same 1G and say 3G concept, what would the travel time to Alpha Centuri be?
Sorry. I just read your answer at approximately 3.6 years. That would be amazing indeed