will ultimately be determined by available funding and
mission requirements, NASA is working towards
flying at least one crewed mission per year.
Into the Proving Ground: Solar Electric
Propulsion and the Asteroid Redirect
Mission
Solar electric propulsion (SEP) uses energy from
the sun to accelerate ionized propellant to very high
speeds. Compared to chemical propulsion, electric
propulsion provides very low levels of thrust;
however, it is incredibly efficient and can provide
thrust continuously for months or years, allowing
more mass to be transported with far less propellant.
These systems are an order of magnitude more
efficient than chemical propulsion systems, with
a specific impulse (Isp) from 2,000-3,000 seconds
compared to 200-500 seconds. SEP systems are
also very resilient and, if refueled, could provide
a reusable, in-space transportation infrastructure.
Adding a SEP capability to Orion and SLS provides a robust transportation
infrastructure for human missions to support the journey to Mars.
High-power SEP is a key enabler for NASA’s pioneering strategy, allowing
NASA to pre-position infrastructure and resources while reducing the surge
of launches. This could reduce the costs estimated in previous human Mars
mission studies. Pre-positioning supplies months or years ahead of crew,
rather than aggregating all necessary equipment and supplies in Earth orbit,
is often called a split mission. With several tons of xenon (Xe) propellant
and solar arrays capable of generating 40 kilowatts (kW), an early SEP
vehicle could efficiently position several tons of cargo throughout the solar
system. A more advanced SEP system with additional power and propellant
could deliver landers, habitats, and supplies to Mars orbit.
NASA is considering several mission architectures that evolve from a 40 kW
SEP vehicle. In an approach that leverages both SEP and chemical propulsion,
a Mars SEP cargo vehicle would transport chemical return stages, habitats,
and landers to Mars orbit, while crew travel separately. For this approach, the
crew would rendezvous with the pre-positioned assets in Mars orbit including
a surface lander and, on the return trip to Earth, chemical departure stages. In
an alternative approach, NASA would use a hybrid SEP vehicle, supplemented
with small chemical engines for strategic high-thrust maneuvers, to reduce trip
times. This vehicle would still pre-position landers, habitats, and equipment
in a cargo mode; however, for crew transit, the vehicle would contain enough
propellant for a round trip and would not require chemical return stages. Both
approaches are being studied as the Mars transportation architecture evolves.
20
SEP enables nearterm missions while
proving capabilities
for a reusable deep
space transportation
system