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Tail of two comets

July 1, 2014 Comments off

It should be tale, but with apologies to Mr. Dickens I couldn’t resist. The two comets in question are 67P/Churyumov-Gerasimenko and C/2013 A1 (Siding Spring). Neither of these comets will come close to the Earth or be easily visible in telescopes, but they will be in the news starting next month.

Comet 67P/Churyumov-Gerasimenko

Comet 67P/Churyumov-Gerasimenko or Comet CG for short is a member of the Jupiter family of comets, more than likely captured into their current orbits by the king of planets. It is the target of European Space Agency’s (ESA) Rosetta Mission to orbit the comet for 17 months and deposit the Philae lander on the surface of the nucleus.

Rosetta was launched on March 2, 2004 on a long and complicated trajectory utilizing four planetary encounters to boost its orbit so it could match the comet, whose aphelion is near Jupiter’s orbit. Rosetta passed the earth a year after launch, then Mars in 2007, Earth again later that year and finally Earth two years later in 2009. On the way it entered the asteroid belt twice and passed by two asteroids: 2767 Šteins, 5 km in diameter, in 2008 and 21 Lutetia, 121 km along its longest dimension, in 2010.

Rosetta is solar-powered with enormous solar panels, yet it could not maintain it normal operations load when farther than 4.5 Astronomical Units (AU) from the Sun so controllers put the spacecraft in hibernation in May of 2011 with instructions to wake up and phone home on January 20, 2014. The signal came about a half hour late, but Rosetta woke up after 31 months in hibernation.

Rosetta is as of this writing (late June 2014) matching orbits with the comet. It’s out in front of the comet, and after two long rocket burns is slowing itself with respect to the sun and approaching the comet from its sun-ward side. Mission planners hoped to reach the comet before it becomes active, but the comet has surprised everyone by becoming active early. However the activity has stopped, as of mid June.

After four short thruster burns in July the velocity with respect to the comet will be down to 7.9 meters/second or 26 feet/second. And 4126 kilometers or 2,563 miles to go. Orbiting a comet nucleus only 2 X 3 miles across will be hard. The spacecraft’s orbital velocity in relation to it will be centimeters or inches per second. That will happen in August. A landing site for the Philae lander will be found as Rosetta spirals even closer to the comet.

With a landing site chosen the Philae lander will settle down on the surface of the nucleus in November by firing harpoons into the comet when it touches down to anchor it. The Philae lander, weighing approximately 220 pounds on Earth contains 10 instruments weighing 46 pounds which include cameras, organic molecule detector, isotopic ratio detector, magnetometer and plasma monitor, subsurface drill, and more.

The Rosetta orbiter will stay with the comet through the comet’s perihelion in December 2015, just outside the Earth’s orbit at 1.2 AU.

C/2013 A1 (Siding Spring)

Siding Spring was the first comet discovered in 2013. It raised headlines then because it could possibly crash into Mars in October 2014. The orbit has been refined, so the comet will miss by 83,000 miles or 134,000 km. On October 19th. The nucleus of the comet is estimated at somewhat less than a half mile in diameter. The comet’s coma or head has been measured to be 12.000 miles or 19,300 km across, though it’s sure to increase as it approaches closer to the sun. Hydrogen gas from the comet’s head or coma will possibly affect Mars’ upper atmosphere for a short time, increasing atmospheric drag on the five satellites then to be in orbit of Mars: NASA’s Odyssey, Mars Reconnaissance Orbiter, and Maven; ESA’s Mars Express, and India’s Mars Orbiter Mission or MOM.

The amount and spread of meteoric dust that would affect the satellites is unknown. Being a very long period comet, and possibly a first time visitor to the inner solar system, it may have very little dust and debris to menace the orbital armada now circling Mars. I have seen what ESA scientists are planning for their Mars Express satellite. They will, or have been tweaking their satellite’s orbit to be behind the planet when the peak of the meteoric material is expected. And since you can’t hide behind the planet forever, have looked at the design of the spacecraft, and decided to face the incoming meteoric stream with their antenna first. NASA has similar plans to duck their orbital assets behind Mars.

How do you change an orbit to duck behind a planet at a specific time? It’s the same technique for two space craft to rendezvous. One doesn’t want to change altitude too much. So to speed up a bit, drop into a slightly closer orbit of the planet. You do this by slowing down. It’s not quite intuitive. And the opposite is true to slow down. When the desired distance along the orbit is achieved reverse the process to get back to your original orbital altitude.

The Mars rovers Opportunity and Curiosity will be protected by Mars’ atmosphere. But could see some nice meteors, meteor shower or meteor storm. Curiosity, being nuclear powered can operate at night. I’m not sure if Opportunity has the power reserve.

This was originally printed in the Stellar Sentinel the July 2014 newsletter of the Grand Traverse Astronomical Society.

Addendum

Links to my sources and more information:

ESA’s Rosetta mission

NASA’s Rosetta website

Comet Siding Spring Wikipedia page

NASA’s Comet Siding Spring at Mars site

ESA’s Mars Express blog posts on Comet Siding Spring

 

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07/01/2014 – Ephemeris – July 2014 Preview

July 1, 2014 Comments off

Ephemeris for Tuesday, July 1st.  Today the Sun will be up for 15 hours and 30 minutes, setting at 9:31.   The Moon, half way from new to first quarter, will set at 11:42 this evening.  Tomorrow the sun will rise at 6:01.

Lets preview July’s skies. The sun, having reached its northern solstice, is beginning to slide southward again, at first imperceptibly, then with greater speed.  The daylight hours will decrease from 15 hours and 30 minutes today to 14 hours 41 minutes at month’s end.  The daylight hours will be slightly shorter south of Interlochen, and slightly longer to the north.  The altitude of the sun at local noon, when the sun is due south will decrease from 68 degrees today to 63 degrees at month’s end.  The sun will be a degree lower in the Straits area.  Despite the warmth, the earth will reach its greatest distance from the sun on Thursday the 3rd.  The planets visible this month in the evening are Mars and Saturn.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

July 2014 Star Chart

July star chart for 11 p.m. on July 15, 2014. Created by my LookingUp program.

Astronomical twilight ends at 11:58 p.m. on July 1st, decreasing to 11:13 on the 31st.

Add a half hour to every week before the 15th and subtract and hour for every week after the 15th.

For a list of constellation names to go with the abbreviations click here.

Also shown is the Summer Triangle in red. Clockwise from the top star is Deneb in Cygnus, Vega in  Lyra and Altair in Aquila.

The green pointers from the Big Dipper are:

  • Pointer stars at the front of the bowl of the Big Dipper point to Polaris the North Star.
  • The leaky bowl drips on the back of Leo the lion.
  • The arc of the dipper’s handle points to Arcturus.
  • The straighten the ark to a spike to point to Spica.