Mark Whittington’s Jan. 4 op-ed ( “NASA wants to go back to the Moon the hard way”) questioned NASA’s strategy of building an orbiting lunar space station before establishing a base on the Moon’s surface. His piece concludes with a broad swipe at NASA’s Space Launch System, currently under development, and advocating cancellation of the project in favor of commercial launch providers.

Doing so at this time would put the United States at serious risk of failing to meet its stated objectives in both manned and unmanned space exploration, precisely at a time when other nations are undertaking bold new initiatives in space.

Critics of the SLS have focused on its sticker price and a price-per-pound comparison relative to reusable rockets. As former senator and astronaut Harrison Schmitt noted last year , that argument fails to consider the real requirements of a deep space mission.

Ambitious and meaningful missions to the Moon and farther into space will require significant infrastructure. Proponents of SpaceX argue that four to six Falcon Heavy launches, at a cost of $150-200 million per launch, could get the material into orbit, where it could be assembled and then propelled to the final destination. This adds complexity and risk. The total cost for those multiple launches is $600 million to $1.2 billion. Meanwhile, the single SLS Block IB launch could carry all that material to the final destination for $500 million to $1 billion.

We should also note that NASA’s Orion spacecraft has the fuel capacity and systems redundancy necessary to support a mission to the Moon and beyond and the SLS is the only rocket capable of launching Orion. SpaceX’s Dragon is lighter and could be launched on a Falcon Heavy, but the Dragon is not capable of long-duration missions.

Critics of the SLS have also claimed that Falcon Heavy could send two unmanned spacecraft to outer planets for the price of a single SLS-based mission. What is not stated is that these spacecraft would have to be lighter (and less capable) or would have to use gravity-assist maneuvers to propel them to their targets. This can add as many as seven years to the time needed to get a spacecraft into position to begin its science mission. SLS can loft a large unmanned probe directly to Jupiter or Saturn without the need for gravity assist.

Finally, Whittington argues that the SLS will fly only a few times before privately funded launch vehicles render it “obsolete.” However, his timelines assume that SpaceX and Blue Origin remain on schedule and on plan — and that’s a big “if.” Private launch providers are subject to the same technical challenges, delays, and unanticipated cost increases as NASA. For example, SpaceX announced that the Falcon Heavy would fly in 2013. Then, the launch date slipped to spring 2016, then late 2016. Then, it was 2017. The first (and, to date, only) Falcon Heavy launch was February 2018. The delays in developing the Falcon Heavy led to it being outdated before it ever flew. The day before Falcon Heavy’s first flight, SpaceX announced that it would not attempt to certify the Heavy for human spaceflight.

SpaceX still has yet to man-rate its Crewed Dragon spacecraft and the current-generation Falcon 9 rocket. Meanwhile, the company’s website still headlines an article announcing that SpaceX would send a privately crewed Dragon beyond the Moon in 2018. It made for great press, but obviously, the company did not deliver on that bold announcement. This mission might not fly until after SLS is operational.

Finally, we need multiple launch providers. As former astronaut Thomas Jones notes , NASA must appoint a presidential commission to investigate any failure involving a space vehicle carrying humans under federal contract. Even if the crew is unharmed, the investigation and corrective actions could ground a launch vehicle for years. That happened with the Space Shuttle after the Challenger and Columbia accidents. We simply can’t rely on a sole provider.

Visionaries like Elon Musk challenge the status quo and inspire the public to support spaceflight. But we also need reliable and sustainable launch services to propel us into the next phase of space exploration. Canceling SLS before any other launch providers have demonstrated equivalent capabilities would be irresponsible.

Jonathan Ward is a space historian and co-author of Bringing Columbia Home: The Untold Story of a Lost Space Shuttle and Her Crew.