At midnight Friday, 21 December 2018, NASA and other major federal agencies shut down due to a political dispute over funding President Trump's border wall proposal. Then the Senate adjourned for the Christmas holiday, ensuring that the impasse will endure until Thursday the 27th at the earliest.
You may be asking yourself, "what does a border wall have to do with NASA's funding?" The answer is: nothing. NASA has nothing to do with the wall, border security, or the Department of Homeland Security. Nevertheless, starting this week, roughly 14,500 hardworking men and women of NASA will be sent home without pay, while the remaining 3,000 who are deemed "essential" to the safety of life and hardware will continue to work or be "on call"—also without pay.
Operations of the ISS will largely continue unaffected, as will the New Horizons flyby of 2014 MU69 ("Ultima Thule")—though NASA's public relations team will be absent. John Hopkins' Applied Physics Laboratory, which manages New Horizons for NASA, will still cover and publicize the flyby.
NASA will continue to communicate with and safely operate its uncrewed spacecraft. Science planning and operations may be impacted without the participation of federal scientists, and it is unclear how long regular operations could continue during an extended shutdown.
Missions in development (such as Mars 2020 or the James Webb Space Telescope) can continue if they are being led by contractors. Those contractors (including JPL) have some cash reserves and can continue operations until the cash runs out or they reach an approval checkpoint that requires NASA sign-off. Again, the degree of impact here depends on the duration of the shutdown.
If you are now asking yourself if this is any way to run a space agency, the answer is also: no. No, it is not. Stochastic stops and starts that demoralize and disrupt a workforce are not generally considered to be a best-practice for successful organizations, much less organizations that manage complex multi-year spacecraft design and operations.
Looking beyond the personal disruption, hardship, and stress caused by the lack of paychecks during the holiday season, overall impact to the space agency could be minimal assuming the impasse is resolved before New Years. Generally not a lot of activity occurs over the holiday season in the final week of the year (New Horizons being a notable exception). But if this frequency of shutdowns continues, I fear that we will see more and more NASA employees ask themselves why they put up with such needless disruptions and leave for jobs in the private sector. We know that NASA can get back to work, but how long will the best and the brightest want to work at an agency that continues to get callously tossed into political churn?
Correction: a previous version of this article stated that Caltech had funded operations of JPL during the 2013 shutdown. JPL had enough cash reserves from a forward-funded contract from NASA during that period and did not require additional monetary support. I had received information that Caltech was willing to do this in 2013, but it is not clear if that will happen again in 2018.