For the current 50K Explorer Card Offer the offer details provide:

50,000 Bonus Miles
This new cardmember bonus offer is not available to either (i) current cardmembers of this consumer credit card, or (ii) previous cardmembers of this consumer credit card who received a new cardmember bonus for this consumer credit card within the last 24 months. With this bonus offer you will qualify and receive 50,000 bonus miles after you make Purchases totaling $2,000 or more during the first 3 months from account opening. ("Purchases" do not include balance transfers, cash advances, cash-like charges such as travelers checks, foreign currency, and money orders, any checks that access your account, overdraft advances, interest, unauthorized or fraudulent charges, or fees of any kind, including an annual fee, if applicable.) After qualifying, please allow 6 to 8 weeks for bonus miles to post to your account. To be eligible for this bonus offer, account must be open and not in default at the time of fulfillment. Bonus miles do not count towards Premier status.