At least seven people were killed by a mysterious explosion in northern Russia, and U.S. officials believe it happened during the test of a prototype for a nuclear-propelled cruise missile. President Vladimir V. Putin of Russia has hailed the weapon as the centerpiece of Moscow’s arms race with the United States — but what will this mean for an arms race that both countries want to win? Guest: David E. Sanger, a national security correspondent for The New York Times. For more information on today’s episode, visit nytimes.com/thedaily.

Background reading:

• Intelligence officials suspect the blast involved a prototype known as Skyfall, a missile that Mr. Putin has boasted can reach any corner of the earth and evade American missile defenses.
• As the death toll has risen from the explosion, Russia’s silence and contradictory accounts have conjured dark memories of Chernobyl.