According to Colwell

This year was my first exposure to what is apparently an annual e-mail hoax regarding Mars appearing to be as large as the Moon in the sky. Since I’m teaching an introductory astronomy course this semester it might make for a good teaching moment. One student received the e-mail from her mother, and my wife got it from a friend. Physically, Mars is about twice as large as the Moon and a whole heck of a lot further away. Rather than looking up numbers in a textbook (or wikipedia), the layperson can figure this out with simple quantitative reasoning. The Moon orbits the Earth and Mars orbits the Sun. This tells us that Mars must be much further from the Earth. Earth’s gravity keeps the Moon in orbit because it is close enough to the Earth for Earth’s gravity to be more important than the Sun’s. For Mars to appear as large as the Moon, it would need to be as close to the Earth as the Moon. The timescale for the motion of Mars is roughly a year (a martian year is about two Earth years, but we don’t need to worry about precision to a factor of two, or even a factor of ten, for this exercise). Thus, the idea that one day Mars is its normal pinpoint size and the next day it is huge contradicts the actual time it takes for Mars and the Earth to move relative to each other. On the one hand I find it discouraging that people are unfamiliar enough with the appearance of the sky to give hoaxes like this any credibility. On the other, maybe there is some value in it: if it makes someone look up at night for a few minutes, then next time they’ll know to send this and similar e-mails straight to the junk folder.

There is an astronomical event worth noting this week, however. In the early hours of Tuesday there will be a total lunar eclipse. It will be more easily visible from the western United States than the in the East. This chart gives a map of the times and how much is visible from different parts of the world. A lunar eclipse occurs when the Moon passes through the shadow of the Earth. Because the Earth has an atmosphere, the Moon does not become completely dark: sunlight is refracted through the Earth’s atmosphere causing it to be deflected and still strike the Moon. Red light is more strongly refracted than blue, so the full Moon becomes reddish in color during the eclipse. Lunar eclipses are one way that the ancients knew the Earth was round: they realized they were seeing the shadow of the Earth, and the shadow had a round edge. The only object that always casts a round shadow is a sphere. Lunar eclipses occur about every six months instead of every month because the Moon’s orbit is not in the same plane as the Earth’s orbit around the Sun, so only twice each year do the three objects roughly line up.

It was thirty years ago today that the Voyager 2 spacecraft was launched from Florida for a grand tour of the outer solar system. (Voyager 1 was launched two weeks later; the details of their trajectories led Voyager 2 on a slower path to the outer solar system, so while launching later, Voyager 1 actually got to Jupiter and Saturn first.) At the time of launch it was not known if there would be funding for Voyager 2 to continue to Uranus and Neptune and become the only spacecraft (still) to explore those planets at close range. I have a special fondness for the mission because the data returned from the flyby of the outermost two planets in the solar system formed the core of my Ph.D. dissertation.

Voyager 2 is about 12 light-hours from Earth and is operating with about 300 Watts of power from its radio-isotope thermoelectric generators (RTGs). Its scientific mission continues with measurements of the magnetic field and charged particle densities as the spacecraft drifts across the boundary between the heliosphere and interstellar space. This is as good a definition as any for the edge of the solar system - the location where the background plasma is no longer dominated by streaming solar wind electrons and protons from the Sun but by the interstellar medium. There are comets gravitationally bound to the Sun much further away than Voyager 2 right now, but you’d have to go a fair way toward the nearest star to pass the Oort cloud of comets.

Cassini colleague Matt Hedman and other members of the imaging team have published a paper identifying an intriguing arc in Saturn’s G ring as the source of the particles in the rest of this faint dusty ring. Most dust rings in the solar system are associated with small moons. Enceladus’s geysers are responsible for most of the particles in the E ring. There are faint dust rings at Jupiter where the dust particles have orbits that clearly link them to individual moons. The moons produce the rings not through geologic activity like Enceladus, but as giant cosmic targets for interplanetary micrometeoroids which constantly pepper the moons at high speeds and knock off tiny bits of ice and rock. In the case of the G ring, Hedman found that there was a significantly brighter arc within the ring that is locked in a 7:6 corotation resonance with the moon Mimas. Apparently the particles in this G ring arc are knocking into each other and also being bombarded by meteoroids to act as a swarm of so-called “parent bodies” for the rest of the G ring (try calling your own parents that - it’s fun!), rather than an individual large satellite parent body. There’s even the chance that some of these mini-moonlets in the arc might be observed when Cassini flies particularly close to the G ring arc during Cassini’s extended mission, now in full-scale planning. I have a special interest in this study because I developed models of dust-ring production from swarms of parent bodies in the ring systems of Uranus and Neptune for my Ph.D. dissertation. The G ring appears to be similar, now, to the Adams ring of Neptune, the first ring known to have a distinct and long-lived arc, though the G ring is far fainter overall than the Adams ring.