June 24, 1998: Observations obtained by the Hubble telescope and
ground-based instruments reveal that Neptune's largest moon,
Triton, seems to have heated up significantly since the Voyager
spacecraft visited it in 1989. Even with the warming, no one is
likely to plan a summer vacation on Triton, which is a bit
smaller than Earth's moon. Since 1989 Triton's temperature has
risen from about 37 on the absolute (Kelvin) temperature scale
(-392 degrees Fahrenheit) to about 39 Kelvin (-389 degrees
Fahrenheit). The scientists are basing a rise in Triton's
surface temperature on the Hubble telescope's detection of an
increase in the moon's atmospheric pressure, which has at least
doubled in bulk since the time of the Voyager encounter. When
Triton passed in front of a star known as "Tr180" in the
constellation Sagittarius, Hubble measured the star's gradual
decrease in brightness. The starlight became fainter as it
traveled through Triton's thicker atmosphere, alerting
astronomers to changes in the moon's air pressu