Info above is correct, on Visa waiver program it only has to be valid for your travel dates but check with the airline, Virgin's requirements are the same.
Here is info from US Customs & Border Proction website.
As a general rule, passports must be valid for six months beyond the date the traveler will exit the U.S., however, the U.S. has signed agreements with a number of countries to waive this requirement. When such an agreement is in place, the passport must be valid for the entire period of the visitor's intended stay, but the additional six month validity period is not required
If you are within 9 months of passport expiry you can have upto 9 months added to your new passport, in most cases you might as well renew and get new passport valid for upto 10 years 9 months and beat any increase of fees as well as knowing you have a long valid passport.