Mars has received considerable attention in the past few decades, thanks to the many robotic missions exploring it to learn more about its past. NASA and China plan to send astronauts/taikonauts there in the coming decades, and commercial space companies like SpaceX hope to send passengers there sooner. This presents several significant challenges, one of the greatest being the lengthy transit times involved. Using conventional propulsion and low-energy trajectories, it takes six to nine months for crewed spacecraft to reach Mars.

These durations complicate mission design and technology requirements and raise health and safety concerns since crews will be exposed to extended periods in microgravity and heightened exposure to cosmic radiation. Traditionally, mission designers have recommended nuclear-electric or nuclear-thermal propulsion (NEP/NTP), which could shorten trips to just 3 months. In a recent study, a UCSB physics researcher identified two trajectories that could reduce transits to Mars using the Starship to between 90 and 104 days.

The study was authored by Jack Kingdon, a graduate student researcher in the Physics Department at the University of California, Santa Barbara (UCSB). He is also a member of the UCSB Weld Lab, an experimental ultracold atomic physics group that uses quantum degenerate gases to explore quantum mechanical phenomena. The paper describing his work was published in Scientific Reports on May 22, 2025.

Nuclear propulsion

Per NASA’s Moon to Mars mission architecture, the need for safer, more rapid transportation is paramount. Many proposals have been made for nuclear propulsion to reduce transits to Mars to 90 days. These include NASA’s Cold War era concept, the Nuclear Engine for Rocket Vehicle Application (NERVA), and modern-day concepts like the Demonstration Rocket for Agile Cislunar Operations (DRACO)—being developed by NASA and DARPA—and the high-power electric plasma variable specific impulse magnetoplasma rocket (VASIMR) proposed by Ad Astra.

Since its inception, research into nuclear propulsion has generally fallen into one of two camps: nuclear-thermal and nuclear-electric propulsion (NTP/NEP). The former relies on a nuclear reactor to heat hydrogen propellant, turning it into a hot plasma that is channeled to generate thrust, while the latter relies on a nuclear reactor to power a Hall-Effect engine. These concepts offer high acceleration (delta-v) and steady specific impulse (Isp), respectively, and using them together in the form of bimodal propulsion combines the benefits of both.

Many researchers consider the technology the only means to reduce transit times to the point that a mission will fall within NASA’s career radiation limit of ~600 millisieverts (mSv). Kingdon’s study challenges this prevailing assumption and advances the theory that a 90-day transfer can be achieved using conventional propulsion. This mission architecture could be realized while space agencies and commercial space entities wait for more advanced concepts to be developed. As Kingdon told Universe Today via email:

“This proposal’s main advantage is that it only uses technology that exists or is close to existing. VASIMIR & NEP are very far from existing (for real missions in space), primarily as they all require giant in-space nuclear reactors which will be technically tough and politically even tougher to develop. NTP is almost certainly more expensive than chemical, even though the tech does exist, and it does not offer significant advantages.”

Mission outline

As outlined on its website, conference presentations, and user manual, the SpaceX mission architecture consists of six Starships traveling to Mars. Four of these spacecraft will haul 400 metric tons (440 U.S. tons) of cargo while two will transport 200 passengers. Based on the Block 2 design, which has a 1,500 metric ton (1,650 U.S. ton) propellant capacity, the crewed Starships will require 15 tankers to fully refuel in low Earth orbit (LEO). The cargo ships would require only four, since they would be sent on longer low-energy trajectories.

Once the flotilla arrives at Mars, the Starships will refuel using propellant created in situ using local carbon dioxide and water ice. When the return window approaches, one of the crew ships and 3–4 cargo ships will refuel and then launch into a low Mars orbit (LMO). The cargo ships will then transfer the majority of their propellant to the crew ship and return to the surface of Mars. The crew ship would then depart for Earth, and the process could be repeated for the other crew ship.

Kingdon calculated multiple trajectories using a Lambert Solver, which produces the shortest elliptical arc in two-body problem equations (aka Lambert’s problem). The first would depart Earth on April 30th, 2033, taking advantage of the 26-month periodic alignment between Earth and Mars. The transit would last 90 days, with the crew returning to Earth after another 90-day transit by July 2nd, 2035. The second would depart Earth on July 15th, 2035, and return to Earth after a 104-day transit on December 5th, 2037.

As Kingdon explained, the former trajectory is the most likely to succeed:

“The optimal trajectory is the 2033 trajectory—it has the lowest fuel requirements for the fastest transit time. A note that may not be obvious to the layreader is that Starship can very easily reach Mars in ~3 months—in fact, it can in any launch window, over a fairly wide range of trajectories. However, Starship may impact the Martian atmosphere too fast (although we do not know, and likely SpaceX don’t either actually how fast Starship can hit the Martian atmosphere and survive). The trajectories discussed are ones that I am confident Starship will survive.”

Challenges remain

This study not only offers reduced transits to Mars but also addresses a key issue identified in the SpaceX mission architecture. This is the problem of the Starship’s mass budget, which was identified in a previous study by a team of engineers from the German Aerospace Center (DLR), the University of Bremen, and the Chair of Space Systems at the Technical University of Dresden. After conducting a trajectory optimization, they found that the current plans did not yield a return flight opportunity due to a too large system mass.

In short, they found that once refueled on the surface, the Starship would not have sufficient thrust to achieve escape velocity and a trans-Earth-injection (TEI) maneuver. The addition of additional tankers to refuel in LEO addresses this issue by allowing the Starship to top up before breaking orbit from Mars. Nevertheless, Kingdon acknowledges that there are still challenges that must be overcome before 90-day transits will be possible:

“There are two major challenges with this architecture, and those problems are also inherent to the current SpaceX Mars mission plan. Starship as a system must work—the failures on flight 7,8 & 9 must be overcome, and improvements to vehicle performance must be made, along with development of life support systems and orbital refueling, but these are all planned.”

Another major challenge is the prospect of building refueling stations on Mars’ surface. According to SpaceX’s plan, propellant would be manufactured using a Sabatier reactor, where methane and oxygen are produced via a chemical reaction between hydrogen and carbon dioxide. No one has ever attempted to manufacture cryogenic propellants on another planet, and this presents all manner of unknowns.

“[T]his will be a tough problem, but again likely less hard of a problem than catching a 70m-tall skyscraper with giant mechanical arms,” said Kingdon. “If SpaceX gets close to its intended near-term performance goals for Starship, this architecture is feasible.”