Stacey Vanek Smith:  It’s day two of the Supreme Court’s health care reform debate.  Today the big court tackles the big question: Can the government force uninsured individuals to buy health insurance? It’s known as the individual mandate.

Marketplace’s Amy Scott has more.

Amy Scott: The individual mandate means most U.S. citizens and legal residents have to have health insurance by 2014 or pay a fine. And here’s why: For insurance to work, you need healthy people to pay into the system to help cover costs for the sick. The Affordable Care Act requires insurance companies to cover everybody, regardless of pre-existing conditions.

Health economist Joel Hay at the University of Southern California says without the mandate, people would buy insurance only when they needed it.

Joel Hay: That means the healthy people will leave the insurance market, and insurance will end up being only the sick paying for the sick, which is a complete unraveling of the health insurance market.

The penalties for opting range from just under $700 a year to 2.5 percent of household income. To encourage people to buy in, the government will offer subsidies on sliding scale.

So, if the court decides that the individual mandate is unconstitutional, what happens to the rest of the law? The court will take on that question tomorrow.

I’m Amy Scott for Marketplace.