Shortening the travel time below about six months requires higher delta-v and an exponentially[clarification needed][an exponential function of what?] increasing amount of fuel, and is difficult with chemical rockets. It could be feasible with advanced spacecraft propulsion technologies, some of which have already been tested to varying levels, such as Variable Specific Impulse Magnetoplasma Rocket,[55] and nuclear rockets. In the former case, a trip time of forty days could be attainable,[56] and in the latter, a trip time down to about two weeks.[4] In 2016, a University of California scientist said they could further reduce travel time for an robotic probe to Mars down to "as little as 72 hours" with the use of a "photonic propulsion" system instead of the fuel-based rocket propulsion system.[57]