Bakersfield Night Sky – November 15, 2014
by Nick Strobel
The Philae Lander has landed successfully on Comet 67P/Churyumov-Gerasimenko. This was an insanely difficult landing as I'll describe further below. Part of the landing sequence did not work as planned, so Philae bounced off its original intended target location, traveled about two hours in a kilometer-long arc in the very weak gravity field of the comet to its current location on the side of a cliff or crater wall. As I write this on Thursday afternoon, the precise location had not been determined but it looks like Philae is positioned vertically with two feet resting on a surface and the third foot "dangling" out in space. Unfortunately, it is not placed well for the solar panels on the lander to capture enough sunlight to recharge its battery, so the scientists are scrambling to get as much data from Philae's science instruments and cameras as they can before the battery runs down.
Even though the landing was partially successful, this "double-landing" was a far more successful venture than anyone had a right to expect. Let me explain. This is the FIRST landing on a comet. Both the Rosetta orbiter and Philae lander were launched ten years ago without any knowledge of what the comet looked like up close and before any of our other major comet missions had reached their targets such as StarDust in 2004 or Deep Impact in 2005.
The Rosetta/Philae technology took years to develop, so it was in the finishing stages of development when the Deep Space 1 spacecraft whizzed by Comet Borrelly in 2001. The only other close-up investigation of a comet was the fleet of craft that visited Comet Halley in 1986 with only the European Space Agency's Giotto spacecraft getting close enough to image the comet nucleus while it flew past the nucleus at 68 kilometers/second (152,000 mph).
All of the previous comet missions were flybys. In its November 3rd ScienceCast, NASA classifies space missions in three tiers of challenge: "difficult", "more difficult", and "ridiculously difficult". Flybys are "difficult". Orbiter missions are "more difficult" because the craft not only has to arrive at the object's position, it has to brake in just the right way to go into orbit around the object (and not burn up in its upper atmosphere or get slingshotted out into deep space). Lander missions are "ridiculously difficult" as witnessed by all the stages of descent of the Curiosity rover on Mars two years ago (the "seven minutes of terror").
Not to take away from the accomplishments of the Curiosity rover landing team but the Philae lander probably had an even more difficult task. Comet 67P/CG is not the oblong, rounded, lazily spinning object scientists and engineers envisioned when Rosetta and Philae were built and like the other comets visited by spacecraft. No, that would be too easy. Comet 67P/CG is a contact binary that wobbles about every 12.4 hours. It looks like two big chunks, each less than 2 miles across, that hit each other at a slow enough speed to stick to each other. The two chunks are connected by a neck of smoother material. It turns out that the neck looks smooth because there are jets of gas and dust shooting out of it that would erase any irregularities such as craters.
Those jets were not expected this far out from the Sun (almost 300 million miles). If Philae flew through a jet on its way down to the nucleus, it could have been deflected out to space. Jets also can change the rotation direction and speed of a comet just like firing a rocket thruster. Philae was essentially "dropped" from Rosetta at a distance of 14 miles and coasted for 7.5 hours to its original landing spot at the "front end" of the smaller chunk. Philae has only one small thruster and that was supposed to fire only as Philae made contact with the ground. If the Rosetta teams didn't time the release from the Rosetta orbiter just right or if the comet changed its wobble in the wrong way, Philae would have missed the comet. The comet is much too low in mass to have enough gravity to pull Philae in.
The comet has approximately 100,000 times less gravity than Earth does, so Philae was to use two harpoons and drills on each of its three feet to anchor itself to the comet's surface. A rocket thruster at the top of the lander was supposed to fire, pushing Philae downward as it made contact with the surface to prevent the lander from rebounding off the surface. The Rosetta team found out before Philae was released from the Rosetta orbiter that there was a problem with the rocket thruster but they gave the go-ahead with the hope that the harpoons would still be able to find a solid enough patch to which to attach.
The lander had to take care of all of the landing steps by itself without help from mission control because it did the landing while over 316 million miles away from Earth, so radio waves take over 28 minutes to travel between the comet and Earth. It turns out that the harpoons did not fire upon landing which may have been good luck in one sense. Firing the harpoons would cause a recoil that the defective rocket thruster was supposed to counter. However, without the harpoons anchoring Philae onto the surface, it did have a gentle bounce off the original landing location and floated for two hours over to another location.
The science instruments and cameras on Philae are working properly. That is a testament to quality engineering. They've been traveling for hundreds of millions of miles for over ten years of harsh radiation and temperature swings of outer space. The landing, however successful, was definitely, "ridiculously difficult"!
The Rosetta orbiter has plenty of science instruments to study the comet through its various stages of activity as it approaches its closest distance to the Sun in August 2015. At closest approach to the Sun (perihelion), the comet will still be outside the orbit of the Earth at a distance of 1.2 AU. The mission is designed to follow the comet for the few months following perihelion to track the waning of activity as the comet heads back out to its farthest distance, slightly greater than Jupiter's distance from the Sun. For more about Rosetta, see either the JPL site at http://rosetta.jpl.nasa.gov or the primary mission site at http://rosetta.esa.int .
Another comet-related event is the Leonid meteor shower that peaks on the mornings of November 17th and 18th. Meteor showers are the result of the Earth plowing through the dust trail left behind a comet as the comet sheds material when it nears the Sun. The dust trail spreads out along the comet's orbit so the few nights before and after a shower's peak will still bring comet bits hurtling through the upper atmosphere at speeds far faster than a bullet. The Leonids are from Comet 55P/Tempel-Tuttle and its dust particles hit our atmosphere at 71 km/sec (almost 160,000 mph). Good thing there's several tens of miles of atmosphere between us and them!
The dust trail is densest near the comet's nucleus. Comet 55P/Tempel-Tuttle has an orbital period of 33 years so every 33 years we get an especially impressive display for the Leonids. However, the last perihelion passage of the Comet Tempel-Tuttle was in 1998, so we'll need to wait until 2031 for another meteor storm. For this year's shower you'll see about 6 to 10 meteors per hour.
The Moon will be in a Waning Crescent phase so its light won't wash out the meteors and it won't rise until 2 AM on the 17th and 3 AM on the 18th. Jupiter will be well up in the eastern sky by then, just to the right of the Sickle (backward question mark) part of Leo (see the attached star chart below). When you trace the Leonid meteor streaks backward, you'll see that they appear to intercept under the crook of Leo's sickle (at the head of the lion), hence the name of the shower. Jupiter is at a 90-degree angle with respect to the Sun, so a view through a telescope will show the biggest distance between the large moons and their shadows on the cloud tops of Jupiter. The Moon will be at New Phase next Saturday, but, alas, no solar eclipse this time.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning Astronomy Notes website at www.astronomynotes.com