Today the Cassini spacecraft made a close approach and dive through the plume of icy spray from Enceladus, Saturn’s sixth largest moon. What surprised me was the low altitude flyby: just 48km from the moon’s surface.
OK, by aircraft standard a 48km altitude is still way up there as commercial aircraft operate at an altitude of 10km, and even the famed U2 and SR-71 spy planes designed to fly above surface or air launched missiles top out at 20 and 25km altitude respectively. But for a multi-billion dollar spacecraft this is quite low due to the high risk. Low Lunar Orbit used during the Apollo missions were at a 100km altitude, and all the hype on New Horizons Pluto flyby, it was at a distant 12,500km pass. OK ESA’s Rosetta spacecraft was maneuvered down to 29km around comet 69P, but has since moved out to a safer 300km orbit.
Looking forward to seeing what comes out of Cassini’s E-21 flyby.
WithNew Horizons‘ flyby of Pluto and all the great images the spacecraft has been returning I’ve wondered: Is it possible to observe Pluto from one’s backyard? Personally I’ve never bothered trying for Neptune and beyond as I knew my small 80mm aperture telescope would not be up to the task. Nevertheless I looked up Pluto’s apparent magnitude and found that it varies between 13.6 and 16.3 due to its elliptic orbit around the Sun. It’s last closest approach (perihelion) was September 1989, and unfortunately Pluto is currently distancing itself for its 248 year journey around the Sun therefore slowly dimming, sitting right now at apparent magnitude 14.
Pluto as viewed by New Horizons during flyby (14 July 2015) – NASA
What size of telescope does it take to observe an apparent magnitude 14 object? Based on the theoretical limits it should be possible to make visual observation with a 10in aperture telescope, but most would say you need a 12in if you plan to observe with an ocular. Of course, equipped long exposure cameras you can have a smaller telescope, but a high focal length would be preferred to reduce to better pick it out from the background of stars. And I’ve managed to pick up mag 14 stars in my photos with the Skywatcher 80ED with 60sec exposures. Therefore Pluto should be accessible to backyard astronomy. Note that at Pluto’s size and distance it shows up as a light point and not a sphere like the other planets.
Up to the challenge? Middle of November will be a great opportunity to locate Pluto as it will swing within 1deg of Ksi 2 Sagittarius, a magnitude 3.5 star. In the June edition,Sky & Telescope created a great star-chart to locate Pluto until December 2015. Good Luck!
Click on image for Sky & Telescope Pluto 2015 Sky Chart
Welcome to a journey into our Universe with Dr Dave, amateur astronomer and astrophotographer for over 40 years. Astro-imaging, image processing, space science, solar astronomy and public outreach are some of the stops in this journey!