Mars researchers are bracing for “six minutes from hell” tonight when the first of twin rovers slams into the top of the planet’s atmosphere at 12,000 mph and begins its perilous descent to the surface. The fate of the $410 million Spirit rover, which has traveled 300 million miles in seven months to reach this point, hangs on those 360 seconds. Spirit will arrive at 9:35 p.m. MST as it attempts to become the fourth spacecraft to land successfully on Mars. The condition of the British-built Beagle 2, which hasn’t uttered a peep since its scheduled arrival on Christmas Eve, is still unknown.