Speaking of Hubble...

Archive: December 2011

The Lowdown on Orbits

December 21, 2011 by Frank Summers
Hubble floats 350 miles (560 km) above Earth.

Hubble floats 350 miles (560 km) above Earth.

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.

A Twilight Eclipse

December 15, 2011 by Frank Summers
Lunar eclipse December 10, 2011, credit: Michael R Perry

Lunar eclipse, Dec. 10, 2011 CREDIT: Michael R. Perry

Because the Hubble Space Telescope produces so many spectacular images of the universe, it can be easy to over-generalize and assume that Hubble is the best telescope for any astronomical observation. Hubble does have some major restrictions: it can’t observe the Sun, track fast-moving objects, or detect x-ray light or radio wavelengths. In addition, there are some celestial observations where there is just no substitute for the experience of seeing it with your own eyes.

I had the fortunate experience of being a guest lecturer on a cruise ship during the Dec. 10, 2011 total lunar eclipse. During the prior week, I gave a presentation on eclipses and ran some evening star-gazing sessions. The skies in the open ocean were beautiful, although the light of the nearly full moon masked a few celestial highlights from our view.

The lunar eclipse began at 3:33 a.m. local time. As you might expect, I was alone on deck. For the first  hour or so, the Moon passed into the fainter penumbra of Earth’s shadow, and it was unimpressive. But as the Moon entered the darker umbra, and slowly passengers joined me, the observing grew into an event. The window shade of Earth’s shadow descending across the Moon piqued curiosity and excitement. And, as the Moon darkened, many stars that had been unobservable on previous nights appeared.

Around 5:45 a.m., the eastern horizon began to brighten with the pre-dawn glimmers of sunrise. The final sequence was a competition between the Sun and Moon for our attention, twilight versus eclipse. When the lunar eclipse became total, just after 6 a.m., the crowd of 40-50 greeted it with heartfelt applause for the celestial showcase.

This experience was truly wonderful and one that Hubble just could not have captured. While I encourage you to explore HubbleSite for all the images and discoveries contained therein, I also hope that it inspires you to use your eyes, or a pair of binoculars, and do a little exploring for yourself.