Bakersfield Night Sky – September 20, 2014
By Nick Strobel
[The first part about how much the Sun would be covered by the planets as seen from Pluto was not printed in newspaper two weeks ago due to space limitations (no pun intended). It was in the article posted on the Planetarium's website.]
In my previous column I wrote about the scale model I pace out with my students on the first day of the semester. In the scale model, the Sun is a yellow nerf basketball 6.5 inches across, the Earth is a large grain of sand about 18 meters from the Sun, and Jupiter is the size of a dime about 92 meters from the Sun. Dwarf planet Pluto is about 700 meters away, so from Pluto the Sun ball would appear very small but the planets would be even smaller---mere dots too small to see. The planets are very small compared to the distances between them. It got me to wondering how much of the Sun would the planets block as seen from Pluto, so I set out to calculate it one afternoon. Now remember that I'm an astronomy teacher, so exercises like this I actually consider "fun".
What would be the absolute maximum that the planets could cover up the Sun as seen from Pluto if they were all approximately in the same plane and lined up such that one planet does not block another but still would be in front of the Sun? Let's ignore the fact that Pluto's orbit is significantly out of alignment with the planets and the planets don't ever line up to pass in front of the Sun at the same time. Let's assume that we're observing from Pluto's closest distance to the Sun and the planets are at their farthest distance from the Sun in their slightly elliptical orbits around the Sun. This, of course, never happens either because the planet orbit ellipses are not no neatly arranged as that. However, with this setup the planets would be closest to Pluto, so they would appear the largest they ever could.
We do have to ignore Neptune in this experiment since Pluto's orbit does cross inside of Neptune's orbit. If we don't ignore Neptune, then Neptune would be right on top of Pluto, swallowing it up whole. However, Neptune and Pluto are always positioned so that Neptune is very far from Pluto when Pluto is crossing inside Neptune's orbit. Furthermore, Neptune is never going to come in between Pluto and the Sun. For the rest of the planets in this artificial scenario, all of them are covering up a tiny part of the Sun but not blocking another planet, so the observer at Pluto sees seven dots in front of the Sun. It turns out that the seven planets (Mercury to Uranus) would block just 4.4 percent of the area of the Sun's disk. Pretty small but detectable!
Looking at the planets from the other stars, the planets would block a much smaller fraction of the Sun's surface. From 10 light years away (still very much in the Sun's "back yard" compared to the rest of the Milky Way), all of the planets including Neptune added together would block less than 2% of the Sun's surface. The situation is reversed when we're looking for exoplanets crossing in front of their star as seen from near Earth and real exoplanets are not going to be all nicely lined up to transit their star at the same time.
The Kepler space telescope continues searching for exoplanets by looking for those that cross in front of their star, blocking their light. It has the ability to see things smaller than the Earth crossing in front of a star the size of the Sun. An Earth-sized exoplanet crossing in front of a Sun-size star covers up just 0.0084% (mind all the zeroes) of the star. That is like seeing a small moth fly in front of a searchlight from over a mile away! Kepler is no longer looking for exoplanets in the direction of Cygnus because of a hardware malfunction. Instead, it is using a very clever scheme that incorporates the pressure of sunlight to point the space telescope at star systems that are in roughly the plane of our solar system.
Tomorrow afternoon the MAVEN spacecraft arrives at Mars after a ten-month journey. It will help us figure out how Mars lost most of its atmosphere. Its atmosphere is now so thin that the air pressure at the surface is too low for liquid water to exist. Last week, the Curiosity rover reached the large mountain in the middle of Gale Crater. The mountain is called Mt Sharp and it is made of layers of sedimentary rock deposited over hundreds of millions of years in Mars' early history. The layers at the bottom of the mountain are the oldest layers from a time we think when Mars would have been nice enough for life to form. It will go up Mt Sharp drilling into the rocks, taking microscope images of the rock grains, and using other instruments to explore Mars's geologic history as it moves upward to younger rocks. We already know that Mars did have a nice enough environment long ago for life to get started. With the climb up Mt Sharp, we will be able to see how the habitability changed over time. The record is right there in the rocks if we know how to read it. Later missions to Mars will look for signs of past life---Curiosity does not have the tools for that.
The United States is not the only country exploring Mars. The European Space Agency has sent spacecraft to Mars in years past as has the former Soviet Union, Russia and Japan. The European Space Agency began by building instruments for American and Soviet, later Russian, spacecraft heading to Mars. In 2003 it launched the Mars Express mission that consisted of two parts: an orbiter and a lander called Beagle 2. Beagle 2 had a suite of instruments designed to look for signs of life on Mars, past or present. Unfortunately, the Beagle 2 did not survive the landing. However, the orbiter continues to do detailed study of Mars's surface from high above Mars including mapping the minerals on the surface and using ground-penetrating radar to study the permafrost just below the surface. It also studies the martian atmosphere circulation. See http://sci.esa.int/mars-express/ for more about Mars Express. On September 24th, India will join the Mars club when its Mars Orbiter Mission (MOM) spacecraft arrives at Mars. MOM will study the composition of Mars's surface and atmosphere from orbit, including sniffing the atmosphere for methane.
The European Space Agency's Rosetta mission to Comet 67P/Churyumovâ€“Gerasimenko continues on schedule. Like the ESA Mars Express mission, the Rosetta mission consists of an orbiter and a lander. It has been orbiting the comet since August 6th spiraling inward to now just 30 kilometers above the surface of the comet. One of the key tasks in the past few weeks has been to identify possible landing locations for the Philae lander and earlier this week ESA announced where they were going to land the Philae craft in November. Comet CG is made of two oblong chunks: a smaller head and a larger body connected by a "neck" of rock and ice. The landing site will be on the head piece. The Rosetta mission is the first mission to not only orbit a comet but also to land on a comet. It will follow Comet CG through the various stages of activity from its current cold, dead state to its warmer active state as it approaches the Sun. Comet CG will get to within 185 million miles from the Sun (that's further out from the Sun that we are) in August 2015 and Rosetta is going to be with the comet all the way.
Comets are frozen relics from the beginning of the solar system. Unlike the planets, comets have not changed since they first formed 4.6 billion years ago. Planets have all sorts of geologic and atmospheric activity to totally erase their birth records. Comets have not, so studying them tells us what the chunks that formed the planets long ago were like.
In the evening sky Mars will pass just north of the red heart of Scorpius, the supergiant star Antares in the last half of this week. Next Saturday a thin Waxing Crescent Moon will be next to Saturn low in the west in the early evening and the following Monday, a fatter crescent Moon will be above Mars and Antares.
Finally, two dates to put on your calendar. The first is September 22nd at 7:29 PM which marks the official start of the season of fall with the autumnal equinox. At that moment in time, the Sun is right on the celestial equator. The second date is September 27th when I will be on a panel of science-trained people who are also from various religious faiths. For this annual tribute to Mahatma Gandhi, speakers will give their opening remarks on the topic "Are Science and Religion Compatible?" from the perspective of Judaism, Islam, Hinduism, and Christianity followed by Q/A with the audience. Details: Saturday, September 27 from 10 am to 12:30 pm in the Student Union Building at CSUB.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com