AP Explainer: What Will Happen in the Partial Government Shutdown

A look at the impact of the partial government shutdown:

What's open and what's closed?

Social Security checks will go out and troops will remain at their posts. Doctors and hospitals will receive their Medicare and Medicaid reimbursements. The U.S. Postal Service, busy delivering packages for the holiday season, is an independent agency and won't be affected. Passport services, which are funded by fees and not government spending, will also continue.

Virtually every essential government agency, including the FBI, the Border Patrol and the Coast Guard, will remain open. Transportation Security Administration officers will staff airport checkpoints.

The air traffic control system, food inspection, Medicare, veterans' health care and many other essential government programs will run as usual. The Federal Emergency Management Agency can continue to respond to disasters.

Nearly 90 percent of the Department of Homeland Security's 240,000 employees will be at work because they're considered essential.

Special counsel Robert Mueller's office, which is investigating potential ties between the Trump campaign and Russia, is unaffected by a shutdown.

But hundreds of thousands of federal workers will be forced off the job, and some services will go dark.

In the past, the vast majority of national parks were closed to visitors and campers, but beginning with the last government shutdown, in January, the Interior Department has tried to make parks as accessible as possible despite bare-bones staffing levels. Some are staying open thanks to funding from states and charitable groups.

In Washington, the museums along the National Mall will remain open at least through Jan. 1, but Smithsonian officials said they will reevaluate the situation if the shutdown continues into the new year. The Washington Monument is closed for repairs.

Arizona and Utah officials put in place plans to keep open Grand Canyon, Zion, Arches and Bryce Canyon and Zion national parks.

At the Statue of Liberty and Ellis Island, it was business as usual, thanks to funding from New York state.

Federal workers still get paid — eventually

While they can be kept on the job, federal workers won't be paid for days worked during the lapse in funding. In the past, however, they have been repaid retroactively even if they were ordered to stay home. White House officials said that would be the case again.

The Senate already has passed legislation ensuring that workers will receive back pay. The House seems sure to follow suit.

Federal employees already were granted an extra day of vacation on Monday, Christmas Eve, under an executive order Trump signed this past week.

Federal workers are exempted from furloughs if their jobs are national security-related or if they perform essential activities that "protect life and property."

According to a report by Democrats on the Senate Appropriations Committee, more than 420,000 federal employees deemed essential will continue to work without pay, including about 41,000 law enforcement and corrections officers. The Homeland Security employees who will keep working include about 150,000 from the Coast Guard, TSA and Customs and Border Protection.

More than 380,000 employees will be furloughed — including nearly all from NASA and Housing and Urban Development and 41,000 from the Commerce Department. About 16,000 National Park Service employees — 80 percent of the agency's workforce — will be furloughed.

Also among those who will furloughed: 52,000 staffers at the Internal Revenue Service, slowing analysis and collection of hundreds of thousands of tax returns and audits.

Shutdowns happen

Shutdowns happened every year when Jimmy Carter was president, averaging 11 days each. During President Ronald Reagan's two terms, there were six shutdowns, typically just one or two days apiece.

Before a three-day lapse in January, caused by Democrats' insistence that any budget measure come with protections for young immigrants known as "Dreamers," the most recent significant shutdown was a 16-day partial shuttering of the government in 2013.

That one came as tea party conservatives tried to block implementation of President Barack Obama's health care law. The government also shut down for a few hours last February amid a partisan dispute over deficit spending.