杏吧原创

THE countdown has begun. Last week, US vice president Mike Pence announced that NASA is being directed to put astronauts on the moon by 2024 鈥� the final year of president Donald Trump鈥檚 tenure, should he win re-election.

It is an ambitious goal and there are several obstacles to clear before we will see new bootprints in the lunar soil. Most crucially, reaching the moon requires a heavy-lift launch vehicle 鈥� a rocket that can boost 20 to 50 tonnes into low Earth orbit 鈥� to carry a crew, cargo and the vast amounts of fuel for the trip.

NASA doesn鈥檛 have one, but it isn鈥檛 for lack of effort. In 2010, then-president Barack Obama gave the space agency the go-ahead to design and build the Space Launch System (SLS), a heavy-lift rocket to rival the Saturn V rocket that powered the Apollo missions. NASA contracted Boeing in 2012 to build SLS, but the firm has overspent on the project by billions of dollars and run into delay after delay.

As part of his announcement, Pence said that commercial rocket providers, such as SpaceX, could offer an alternative. 鈥淲e鈥檙e not committed to any one contractor,鈥� he said. 鈥淚f our current contractors can鈥檛 meet this objective, then we鈥檒l find ones that will.鈥�

Either way, it is going to be expensive 鈥� and the new mission came with no promise of a politically difficult budget increase. 鈥淚t seems highly unlikely Congress would agree to provide the massive infusion of funds needed to accomplish this goal, whether NASA builds the hardware itself or buys services from commercial companies,鈥� says Marcia Smith at Space and Technology Policy Group, a policy analysis firm in Virginia.

Part of the problem is that SLS was designed to build on older US space efforts. Boeing was required to use parts from both the space shuttle and the defunct Constellation programme from the early 2000s. SLS is also designed to be built in a series of increasingly powerful versions, which will lift 70, 105 and then 130 tonnes into low Earth orbit, so it can be used for both moon and Mars missions.

That sounds like SLS has been trying to do too much, but the delays may actually be due to it doing too little, says Lori Garver, who served as NASA鈥檚 deputy administrator from 2009 to 2013. Despite spending more than $6 billion on SLS, NASA never had a specific destination in mind, she says. 鈥淚t really was put together by the contractors to extend their own contracts.鈥�

The current goal for SLS is to launch the Orion crew capsule on an uncrewed test flight around the moon. That flight has been delayed by two years, and NASA鈥檚 current administrator, Jim Bridenstine, recently said that he would be open to using a instead to hit the planned June 2020 launch date.

Many of the delays could have been avoided. An last year found that Boeing made missteps in construction and staffing that resulted in two-and-a-half years of lost time.

Another problem with SLS is that even when built, it will cost more to operate than upcoming commercial spacecraft.

The only rocket anywhere near being ready to carry enough weight into space for a moon mission is SpaceX鈥檚 Falcon Heavy. This made its maiden launch in February 2018, carrying CEO Elon Musk鈥檚 personal Tesla Roadster car, and its reusable rocket boosters landed back on Earth just a few minutes later. Reports suggest that Falcon Heavy will return to space this month, carrying a communications satellite on its first commercial launch.

Rockets needed

says Falcon Heavy can ferry nearly 64 tonnes into low Earth orbit at a cost of $90 million per ride. Estimates for launching SLS range from $500 million to more than $1 billion, and the ageing rocket design doesn鈥檛 include any reusable parts, increasing costs.

Using proven legacy hardware means SLS should at least be reliable, but it could also be technologically obsolete before it even gets off the launch pad. Its solid rocket boosters use 1970s technology developed for the space shuttle, says Garver.

For these reasons, SLS has been falling out of favour. Its most powerful version, capable of launching the Orion crew capsule, isn鈥檛 in Trump鈥檚 proposed 2020 budget, which calls for cuts to NASA鈥檚 already shrinking funding (see 鈥�Money to burn鈥�).

The budget also sidelines SLS in favour of cheaper commercial rockets for future missions to build a space station circling the moon and send an orbiter to Jupiter鈥檚 moon Europa.

That seems at odds with the directive to land on the moon. To meet the deadline for the uncrewed Orion test flight in 2020 and a crewed lunar orbit mission slated for 2022, corners may need to be cut with SLS. Bridenstine has already proposed skipping a planned engine safety test to accelerate the timetable, but, ironically, this may be just another reason to ditch the rocket.

One concern NASA has about using commercial space-flight companies is that they may follow different safety procedures. 鈥淣ASA鈥檚 had the view that if they鈥檙e doing it, it鈥檚 safer,鈥� says Garver. If NASA is considering cuts to stay on schedule, the argument that it is inherently riskier to use commercial rockets falls apart.

So, if it is cheaper and just as safe to use privately built rockets, why hasn鈥檛 SLS been axed yet? The agency has sunk a lot of money into the system, often at the urging of politicians from states where rocket building creates jobs. 鈥淭he SLS and Orion programmes are, of course, key to the health of our national aerospace supplier base,鈥� said congressman Robert Aderholt at a budget hearing last week.

It is also down to the preference of NASA managers, says Garver. 鈥淚t鈥檚 a combination of a lot of people who either flew in space or are leaders in the space programme,鈥� she says. 鈥淭here鈥檚 a bunch of men who want to focus on building the big rocket.鈥�

They would do well to focus on something much smaller: a craft that can land on the moon and then return to lunar orbit. 鈥淵ou need a lunar lander and you鈥檝e got to demonstrate that it can do some kind of propulsion in lunar orbit,鈥� says Jonathan McDowell at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. 鈥淭hat鈥檚 a whole new spacecraft. It鈥檚 hard to do it in five years.鈥�

The last spacecraft to take off from the moon鈥檚 surface was the Soviet Union鈥檚 uncrewed Luna 24, which returned samples of moon rock to Earth in 1976. NASA hasn鈥檛 done it since 1972, when the final Apollo crew blasted off.

Lockheed Martin, the company that is building the Orion capsule, said in a statement that Pence鈥檚 accelerated timeline is 鈥渁ggressive but achievable鈥� and that it could build a crewed lander based on existing systems used in Orion.

Blue Origin, a space-flight firm headed by Amazon CEO Jeff Bezos, has plans to develop a lunar lander called Blue Moon. However, this is in the concept phase, and is at present intended to transport cargo rather than humans. It could be converted to carry a crew, but Blue Origin doesn鈥檛 yet have a way to launch it. The firm is currently building its own heavy-lift rocket, New Glenn, which will be able to lift 45 tonnes and has a planned first launch of 2021.

Meanwhile, SpaceX is building Starship, an even bigger rocket than its Falcon Heavy. And while some doubt the new launcher could be ready for 2024, Musk thinks it could be. When asked on Twitter if Starship could send humans to the moon in five years,

鈥淣ASA鈥檚 Space Launch System could be obsolete before it even gets off the launch pad鈥�

McDowell says Blue Origin or SpaceX鈥檚 rockets are the best bet for NASA to take humans to the moon. He says either firm could probably send a crewed mission into orbit around the moon within five years, but would need the agency鈥檚 help to fund a lunar lander. Either way, it probably won鈥檛 happen by 2024.

鈥淪paceX talks a good game and it delivers great stuff, but never on the schedule does it say it is going to do it,鈥� says McDowell, while Blue Origin prefers the slow and steady approach.

Garver agrees that commercial rockets are the way forward, adding that even if the 2024 deadline can鈥檛 be met, the latest push towards the moon might be what helps pull the plug on SLS. 鈥淭he fact that NASA has wrapped its head around the possibilities of doing something different than SLS is a good thing,鈥� she says.