A new card that Americans can use to cross the Mexican and Canadian borders may be issued as early as April at less than half the cost of a passport, the U.S. State Department said Monday.

Like the passport, it will contain a computer chip that can be read remotely by scanners, which critics say raises privacy and security issues. The State Department says the chip does not pose problems in these areas.

The passport card is the U.S. government's latest attempt to tighten border security.

Currently, Americans returning to the U.S. by air must present a passport at U.S. border checkpoints, but those returning by land or sea from Mexico, Canada, the Caribbean and Bermuda do not need one.

By summer 2009, under a deadline that Congress recently extended by a year, most land and sea travelers will need to present a passport, the new passport card or other high-security documents (still under development) to cross these borders.

Details are still being worked out, and there are exceptions to the proposed rules. But here are some questions and answers, based on a final rule published Monday in the Federal Register and on interviews with government officials:

Question: When will the passport card be available?

Answer: "Frankly, we're aiming for . . . sometime in April and May," Ann Barrett, deputy assistant secretary for passport services at the State Department, said in an interview Monday. She said the department hopes to award the contract to make the cards in the next few weeks and to allow Americans to apply for them a month or two in advance.

Q: Will you be able to use it everywhere?

A: No. Unlike a passport, the passport card will be accepted only for land and sea crossings into the U.S. from Mexico, Canada, the Caribbean and Bermuda. Air passengers will not be able to use it; they need a passport.

"Basically, the passport card is a limited-use passport," Barrett said, adding that it's intended mainly for "the border population that does frequent cross-border travel."

Q: What will the card cost?

A: The card will cost $45 for adults and $35 for children younger than 16; renewals will costs $20 for adults and $10 for children. New passports, by comparison, cost $97 for adults and $82 for children; renewals cost $67 for adults and $52 for children.

Q: How long will the card be valid?

A: Like a passport, the card will be valid for 10 years for adults and five years for minors. Then you must apply to renew it.

Q: What it will look like?

A: Much like a driver's license. It will have a photo of you and printed information similar to what is on a passport: name, date and place of birth, gender and the card's issuance and expiration dates. A computer chip will be embedded in it.

Q: What will be on the computer chip, and how will it work?

A: The chip will contain only one piece of data: what the State Department calls "a unique identifying number that has meaning only inside the secure CBP [U.S. Customs and Border Protection] computer system." Border officials will use radio frequency identification, or RFID, scanners to "read" the chip at checkpoints, just as they do with chips on new passports.