I was reading the “Science & Vie” magazine when I came across a question from one of the readers: “Does the Moon have satellites?” At first I considered this quite a silly question, but then realized that we have placed artificial satellites around the Moon. So why could there not be natural ones?
The Moon is not billiard-ball smooth gravitationally. It’s heavily scared surface due to past asteroid and comet impacts have affected the local density of the surface crust, and therefore the local gravity field varies across the surface of the Moon.
Map of gravity acceleration values over the entire surface of Earth’s Moon. Lunar Gravity Model 2011
One of the famous effects of these local gravitational variations is the Apollo 11 landing, where Neil Armstrong had to take manual control to land, some 5km down range where the navigational computer was targeting.
Another factor is that any satellite around the Moon would also be under the influence of the Earth and the Sun. Any asteroid captured by the Moon would quickly be ejected due to all these influences. Now there are more favorable orbital angles: 27, 50, 76 and 86 degrees from the Moon’s equator. But it would still be a highly unstable orbit. All spacecraft that are placed in orbit around the Moon need to use up propellant to maintain orbit over time. And when propellant is about to run out, most space agencies elect to purposely crash the satellite to obtain additional science data. One recent example is NASA’s LADEE moon orbiter crashing on the Moon on April 18th, 2014.
In conclusion, no our Moon does not have any natural satellites, and if by chance it would capture a wandering asteroid, most experts believe it would only survive a century at most before impacting the lunar surface or getting flung out of orbit.
In my previous post I’ve mentions that coment 67P/Churyumov–Gerasimenko is currently between the orbits of Jupiter and Mars, on a trip towards the Sun. While some comets take decades to become visible again this one has an orbital period of 6.44 years, therefore a frequent visitor. That was one of the selection criteria for the target comet: short orbital period such that it did not take too much fuel or planetary gravity assist to intercept.
On August 13th it will be at it’s closest position to the sun (perihelion), therefore brightest and a good time to observe. Afterwards it will be swinging back out towards Jupiter on its elongated orbit. For people in the Northern Hemisphere, the best time to observe comet 67P will be after this August date. Below is a chart showing that the comet will be visible in the early morning starting in June 2015, and will be visible at higher altitudes in the sky throughout the following months.
Comet 67P visibility for around 45 Latitude N.
Below is a chart (click to enlarge) showing the position of 67P until November 15th.
Comet 67P/Churyumov–Gerasimenko sky chart for Nov 2014 to Nov 201
A good photographic opportunity will be August 8th when comet 67P will pass right under open cluster M35.
Comet 67P passing under Open Cluster M35
Graphics generated with C2A Planetarium Software
First of all I want to congratulate the Rosetta team on their successful landing of Philae on the comet surface. When you consider that the spacecraft was launched 10 years ago, it was essentially designed and assembled with 15-year-old technology. Back then, digital cameras were just entering the market and the Palm III PDA was the mobile device everyone wanted. In fact, much of the code running on Rosetta and Philae was developed after the aircraft was launched.
Presently 67P is located between Jupiter and Mars, on its swing towards the sun. To give you an idea how far out it’s located in the solar system, it takes over 28 minutes for radio communications from Rosetta to reach Earth. Therefore if mission control sends a comment to Rosetta, the results are only known one hour later!
Location of comet 67P/Churyumov–Gerasimenko on November 13th, 2014 (Credit: ESA)
Starting in May 2015 67P will become visible to observers in the Northern Hemisphere, and will gradually brighten until achieving perihelion on August 13, 2015. Because it does not venture very close to the sun, past observations indicate that it will only reach magnitude 11 at best; a challenge to backyard telescopes.
Better slew your observation by 9 degrees north, that’s where comet 141P/Machholz will be in the same constellation (Gemini) and at magnitude 8; a slightly brighter target. And if you have no luck observing either comets or capturing them on photo, open cluster M35 is in the area.
Until then, you can follow Rosetta and Philae’s adventure on their blog: blogs.esa.int/rosetta/