Near the beginning of most of my talks about the Hubble Space Telescope, I remind the audience about Hubble’s location in space. I emphasize not how far away Hubble is, but that Hubble’s orbit is much smaller than most people imagine.
Earth’s atmosphere is traditionally considered to extend to 60 miles (100 km) above the surface. In truth, the thermosphere and then exosphere layers extend beyond this height to thousands of miles, but they are extremely low-density layers.
Because the main objective is to get Hubble above the blurring effects of the atmosphere, its 350-mile-high (560 km) orbit does the job.
But, compared to a planet 7,920 miles (12,740 km) in diameter, Hubble’s orbit is not that impressive. Note that communications satellites in geosynchronous orbit are about 65 times higher than Hubble.
I was reminded of this topic when, on Dec. 12, 2011 the Dawn spacecraft reached its lowest-level orbit around the solar system’s second-largest asteroid, Vesta. Dawn’s low, altitude-mapping orbit averages only 130 miles (210 km) above the surface. Though Vesta is comparatively small at 330 miles (530 km) in diameter, it is still cool to note that Dawn is closer to Vesta than Hubble is to Earth.
While we are accustomed to weather satellites studying our planet in detail, we now have a satellite orbiting and observing an asteroid. Dawn will do so for about 10 weeks before migrating back to a higher orbit.
Then, in July 2012, Dawn will depart Vesta en route to the solar system’s largest asteroid, Ceres, for a 2015 arrival. The images and discoveries from this mission are and will continue to be spectacular. We are truly examining the asteroid belt in an up-close and personal way.