The nascent satellite radio industry has entered a critical phase, with both XM Radio and Sirius Satellite Radio posting losses for the fourth quarter. Combined, the two companies have yet to sign up a half-million subscribers.Sirius is in the worse shape of the two, suffering heavy startup costs and sluggish subscription activity. For the fourth quarter, the company posted a net loss of $122.1 million, compared with a loss of $72.7 million for the same period a year earlier. Revenue for 2002 totaled $685,000.
Sirius went live in July, and ended the year with fewer than 30,000…