Today’s is a guest post, brought to you by my father, engineer, physicist, computer programmer, and Lego, Mars, and animation enthusiast: Keith Enevoldsen. If you’re not caught by the title of the post, scroll down and look at the pictures… then scroll back up and read the rest of the article because it is fascinating stuff! (P.S. He has a website full of awesome science stuff too!)

A teaser for what you have to look forward to…

~ A l i c e !

The Rocker-Bogie

All the Mars rovers have six wheels and use a rocker-bogie suspension system to drive smoothly over bumpy ground. The rocker-bogies are easy to see in pictures of the rovers (see pictures below). There is one rocker-bogie assembly on each side of the rover. The rocker is the larger link that connects to the rover body (the chassis) in the middle (at the rocker pivot), has a wheel on the front, and connects to the bogie in the back. The bogie is the smaller link that connects to the rocker in the middle (at the bogie pivot), and has wheels at both ends. Each of the six wheels has its own motor.

The Differential

It is not so easy to see and understand how the rocker-bogie mechanism keeps the body level. What prevents the rover body from tipping all the way forward or backward around the rocker pivots? If you build a model rover and you attach the rockers to the body with an axle or two pivot pins, the body will tip forward or backward until it hits the ground! In the real rovers the two rockers connect to each other and to the body through a mechanism called a differential. The differential is what keeps the body level. Relative to the body, when one rocker goes up, the other rocker goes down. Relative to the ground, the body angle is halfway between the angles of the two rockers. That’s cool, but how does it work? The different rovers use different mechanisms: a differential gearbox or a differential bar.

Differential Gearbox

The Mars Pathfinder (Sojourner) and Mars Exploration Rovers (Spirit and Opportunity) use differential gearboxes.

MER’s Differential
Image and Model © 2012 Keith Enevoldsen

The gearbox is inside the rover body, so you never see it. No wonder it is hard to figure out how it works! In my Lego model rover shown here, I use a simple three-gear differential. Two gears connect to the two rockers and the third (middle) gear connects to the body. If you hold the model rover body steady in midair and tilt one rocker up, the gears will turn and the other rocker will tilt down (see the animations below).

The real Mars Exploration Rovers use more complicated gearboxes with more gears but they are functionally equivalent to this simple three-gear differential.

Differential Bar

The Mars Science Laboratory (Curiosity) uses a differential bar. This is the big black bar that you see across the deck of the rover.

Curiosity’s Differential
Image and Model © 2012 Keith Enevoldsen

The middle of the bar is connected to the body with a pivot and the two ends are connected to the two rockers through some short links. If you hold the model rover body steady in midair and tilt one rocker up, one end of the bar will go back, the other end will go forward, and the other rocker will tilt down (see the animations below).

The Mars Exploration Rovers did not use a differential bar because it would interfere with the solar panels. But the Mars Science Laboratory does not have that problem because it is nuclear powered and has no solar panels.

Animations

Animations of a Rocker-Bogie with a Differential Gearbox

Mars Exploration Rover (Spirit and Opportunity)

Body held steady in midair:

Wheels on the ground:

Animations of a Rocker-Bogie with a Differential Bar

Mars Science Laboratory (Curiosity)
Body held steady in midair:

Wheels on the ground:

No Springs

The rocker-bogie mechanism has no springs. The absence of springs helps it keep all six wheels on the ground with approximately equal pressure on each wheel. This is a good thing when you are driving on sand!

Want More? (and References)

“Mars Exploration Rover Mobility Assembly Design, Test and Performance”, JPL

“A Capable and Temporary Test Facility on a Shoestring Budget: The MSL Touchdown Test Facility”, JPL

I hope you enjoyed my dad’s post. He sure had fun figuring out those mechanisms and creating them out of Lego!

I’ll be posting a Curiosity-landing-“watching” event here very soon.

~ A l i c e !

It’s time for the 13th seasonal sunset watch!!

• When: Wednesday, June 20 at 9:01pm (so come at 8:45pm)
• Actual sunset is supposed to be at 9:11, but at the summer solstice we noticed that the Sun set about 10 minutes earlier than the USNO says it does, so I’ve moved the time of our sunset watch up so we don’t miss it. In Autumn the timing seemed to line up correctly – what will it do this time?
• Where: Solstice Park – all the way up the hill from the tennis courts (or, if you’re not in Seattle, wherever you have a view of the western horizon!)
• Who: Everyone welcome, as usual.

Sunset on September 23, 2011 Image credit: Jason Gift Enevoldsen

Come watch the summer solstice sunset at Solstice Park in West Seattle on Wednesday the 20th. We’ll see if the sunset lines up with the placed marker. I’ll be there even if it is cloudy because sometimes the Sun peeks through just as it begins to set, but if it is driving rain or sleet I’m staying home with some hot tea!

If you’re interested – here’s the timing of various celestial events  from Seattle, courtesy of the U.S. Naval Observatory Astronomical Applications Department:

Sun and Moon Data for One Day

The following information is provided for Seattle, King County, Washington (longitude W122.3, latitude N47.6):

Wednesday 20 June 2012 Pacific Daylight Time

SUN
Begin civil twilight 4:31 a.m.
Sunrise 5:11 a.m.
Sun transit 1:11 p.m.
Sunset 9:11 p.m.
End civil twilight 9:52 p.m.

MOON
Moonset 9:14 p.m. on preceding day
Moonrise 6:22 a.m.
Moon transit 2:11 p.m.
Moonset 9:53 p.m.
Moonrise 7:24 a.m. on following day

Phase of the Moon on 20 June: waxing crescent with 1% of the Moon’s visible disk illuminated.

New Moon on 19 June 2012 at 8:03 a.m. Pacific Daylight Time.

This event is my part of the NASA’s Solar System Ambassador program, and thanks to West Seattle Blog for publicizing the last few!

Everyone is welcome, see you there!

~ A l i c e !

It was cloudy and raining here in West Seattle. I gotta say though, perseverance won out. We had three “sun breaks.” The first two almost didn’t count the clouds were still so thick, but we did see the Sun. Everyone else saw Venus during the first break, but I didn’t see it until the second  break. Then we had a good one about 10 minutes long about halfway through the event.

Photos

 Photo (c) 2012 Jason Ayres Gift Enevoldsen (please ask permission before copying)

Photo by Spencer Laube

We spent hours waiting in the rain.

Yes that’s an umbrella over the telescope.

But, we did see it!

Want More?

And yes, I owe you photos from the eclipse too. My friend Dave Ingram hosted the viewing here at Solstice Park while I gallivanted off to California. It was socked in up here, but beautiful in Redding, CA.

~ A l i c e !

The last transit of Venus in your lifetime will happen on Tuesday, June 5 (here in Seattle). Come to Solstice Park and share the experience. NEVER, EVER, EVER look directly at the Sun. Even when there is something cool to see. You risk damaging your eyes. To find safe ways to observe please visit this website. Sunglasses are not enough. Build a pinhole projector instead.

• When: Tuesday June 5, 2012 at 3:06pm to 9:03pm
• Where: Solstice Park – all the way up the hill from the tennis courts (or, if you’re not in Seattle, wherever you have a view of the Sun! Go to this website to find the times for your location.)
• Who: Everyone welcome, as usual.

What is it?

The transit of Venus is when Venus passes in front of the Sun from the point of view of the Earth. In some ways it is like an eclipse, but since Venus is so far away it will look like a small dark dot on the face of the Sun.

Visibility from Seattle
Start: 3:06pm
Max: 6:27pm
SUNSET: 9:03pm (You can’t see the rest)

If you’re interested – here’s the timing of various celestial events on Saturday from Seattle, courtesy of the U.S. Naval Observatory Astronomical Applications Department:

Sun and Moon Data for One Day

The following information is provided for Seattle, King County, Washington (longitude W122.3, latitude N47.6):

Tuesday 5 June 2012 Pacific Daylight Time

SUN
Begin civil twilight 4:33 a.m.
Sunrise 5:13 a.m.
Sun transit 1:08 p.m.
Sunset 9:03 p.m.
End civil twilight 9:43 p.m.

MOON
Moonrise 9:36 p.m. on preceding day
Moon transit 2:03 a.m.
Moonset 6:33 a.m.
Moonrise 10:28 p.m.
Moonset 7:45 a.m. on following day

Phase of the Moon on 5 June: waning gibbous with 97% of the Moon’s visible disk illuminated.

Full Moon on 4 June 2012 at 4:12 a.m. Pacific Daylight Time.

This event is my part of the NASA’s Solar System Ambassador program, and thanks to West Seattle Blog for publicizing the last few!

Everyone is welcome!

~ A l i c e !

Transit of Venus—Tuesday, June 5, 2012

There is a transit of Venus in early June. Venus will pass between the Earth and the Sun, and appear as a small dot slowly crawling across the face of the Sun.

Viewing Safety

Never look directly at the Sun. Safe viewing is paramount. Always use appropriate safety equipment to view the Sun. This can be:

• Eclipse glasses (available at the Pacific Science Center Store and www.rainbowsymphony.com)
• #14 Welders goggles
• A pinhole projector (make: http://www.exploratorium.edu/eclipse/how.html)

Do not use stacked sunglasses, smoked glass, or rely on cloudcover to protect you.

Frequently Asked Questions

Visibility from Seattle Start:               3:06pm Max:    6:27pm SUNSET:          9:03pm You can’t see the rest

Transits of Venus have a strange pattern of frequency: 121 ½  years, 8 years, 105 ½ years , 8 years, 121 ½  years, 8 years, 105 ½ years , 8 years, ….)

When was the last/next one visible from Seattle?Other transits of Venus:1874 1882 2004 *2012* 2117

Some Diagrams

Links

Where can I watch it from Seattle?

Pacific Science Center Courtyard – http://www.pacificsciencecenter.org/Education/Programs/join-us-for-two-major-celestial-events

Solstice Park in West Seattle – https://www.alicesastroinfo.com/2009/06/solstice-park-west-seattle/

What time should I look from ­­­­another city?

http://transitofvenus.nl/wp/where-when/local-transit-times/

When is sunset for another city?

http://aa.usno.navy.mil/cgi-bin/aa_pap.pl

Oh no! It’s cloudy!

• Live broadcast http://www.exploratorium.edu/venus/

Give me more data!

http://www.transitofvenus.org/

http://eclipse.gsfc.nasa.gov/OH/transit12.html

http://eclipse.gsfc.nasa.gov/transit/venus0412.html

http://www.timeanddate.com/worldclock/converter.html

There is a partial annular solar eclipse this Sunday. It’s visible from Seattle. A solar eclipse is when the Moon moves between the Earth and the Sun. A total eclipse is when, from this position, the Moon completely blocks our view of the Sun. An annular eclipse is when the Moon is just a little farther away, and so although it is completely in front of the Sun, it doesn’t completely block our view, we can still see a ring of Sun around the edge of the Moon. For this eclipse the most anyone will be able to see is an annular eclipse.

Viewing Safety

Never look directly at the Sun. Safe viewing is paramount. Always use appropriate safety equipment to view the Sun. This can be:

• Eclipse glasses (available at the Pacific Science Center Store and www.rainbowsymphony.com)
• #14 Welders goggles
• A pinhole projector (make: http://www.exploratorium.edu/eclipse/how.html)

Do not use stacked sunglasses, smoked glass, or rely on cloudcover to protect you.

Frequently Asked Questions

Some Diagrams

Links

Where can I watch it from Seattle?

Pacific Science Center Courtyard – http://www.pacificsciencecenter.org/Education/Programs/join-us-for-two-major-celestial-events

Solstice Park in West Seattle (bring your own viewing equipment) – https://www.alicesastroinfo.com/2012/05/partial-solar-eclipse-viewing-2012-solstice-park/ –

What time should I look from ­­­­another city? http://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/SE2012May20Agoogle.html (“Magnitude” is the percent coverage: 0.82 means 82% coverage)

When is sunset for another city?

http://aa.usno.navy.mil/cgi-bin/aa_pap.pl

Oh no! It’s cloudy!

• Live broadcast from Japan at 2pm Seattle time: http://www.panasonic.net/eclipselive/
• Check nasa.gov for more live broadcast links

Give me more data!

http://eclipse.gsfc.nasa.gov/eclipse.html

http://eclipse.gsfc.nasa.gov/JSEX/JSEX-index.html

http://eclipse.gsfc.nasa.gov/OH/OH2012.html#SE2012May20A

 http://eclipse.gsfc.nasa.gov/OH/OHfigures/OH2012-Fig01.pdf

Did you know there’s going to be a partial solar eclipse visible from Seattle in just over a week? Come to Solstice Park and share the experience. NEVER, EVER, EVER look directly at the Sun. Even when it is partially eclipsed. You risk damaging your eyes. To find safe ways to observe please visit this website. Sunglasses are not enough. Build a pinhole projector instead.

• When: Sunday May 20, 2012 at 5:01pm-7:25pm (maximum eclipse is at 6:17pm)
• Where: Solstice Park – all the way up the hill from the tennis courts (or, if you’re not in Seattle, wherever you have a view of the Sun! Go to this website to find the times for your location.)
• Who: Everyone welcome, as usual.

This is a partial annular solar eclipse. This is the first solar eclipse visible from Seattle in ten years.

Unfortunately, I will not be in attendance this time, but I will be around for the June 5 Transit of Venus. I am simply facilitating this gathering from afar for the solar eclipse. I’m hoping to find an astronomy friend to show up and talk about safe solar viewing (anyone?).

If you’re interested – here’s the timing of various celestial events on Saturday from Seattle, courtesy of the U.S. Naval Observatory Astronomical Applications Department:

Sun and Moon Data for One Day

The following information is provided for Seattle, King County, Washington (longitude W122.3, latitude N47.6):

Sunday
20 May 2012 Pacific Daylight Time

SUN
Begin civil twilight 4:48 a.m.
Sunrise 5:25 a.m.
Sun transit 1:06 p.m.
Sunset 8:47 p.m.
End civil twilight 9:25 p.m.

MOON
Moonset 7:54 p.m. on preceding day
Moonrise 5:11 a.m.
Moon transit 12:58 p.m.
Moonset 8:52 p.m.
Moonrise 5:50 a.m. on following day

This event is my part of the NASA’s Solar System Ambassador program, and thanks to West Seattle Blog for publicizing the last few!

Everyone is welcome!

~ A l i c e !

It’s time for the 12th seasonal sunset watch (We’re at 3 years of these, folks)!!

• When: March 19, 2012 at 7:05pm (so come at 6:45pm)
• Actual sunset is supposed to be at 7:22, but at the summer solstice we noticed that the Sun set about 10 minutes earlier than the USNO says it does, so I’ve moved the time of our sunset watch up so we don’t miss it. I added another 5 minutes for the tall trees of Lincoln Park blocking our view. In Autumn the timing seemed to line up correctly – what will it do this time?
• Where: Solstice Park – all the way up the hill from the tennis courts (or, if you’re not in Seattle, wherever you have a view of the western horizon!)
• Who: Everyone welcome, as usual.

Sunset on September 23, 2011 Image credit: Jason Gift Enevoldsen

Come watch the spring equinox sunset at Solstice Park in West Seattle on Monday the 19th. We’ll see if the sunset lines up with the placed marker (it did for summer, but we weren’t sure in autumn or winter). I’ll be there even if it is cloudy because sometimes the Sun peeks through just as it begins to set, but if it is driving rain or sleet I’m staying home with some hot tea!

If you’re interested – here’s the timing of various celestial events on Saturday from Seattle, courtesy of the U.S. Naval Observatory Astronomical Applications Department:

Sun and Moon Data for One Day

The following information is provided for Seattle, King County, Washington (longitude W122.3, latitude N47.6):

Monday 19 March 2012 Pacific Daylight Time

SUN
Begin civil twilight 6:42 a.m.
Sunrise 7:13 a.m.
Sun transit 1:17 p.m.
Sunset 7:22 p.m.
End civil twilight 7:52 p.m.

MOON
Moonset 3:40 p.m. on preceding day
Moonrise 5:38 a.m.
Moon transit 11:08 a.m.
Moonset 4:47 p.m.
Moonrise 6:02 a.m. on following day

Phase of the Moon on 19 March: waning crescent with 8% of the Moon’s visible disk illuminated.

New Moon on 22 March 2012 at 7:38 a.m. Pacific Daylight Time.

This event is my part of the NASA’s Solar System Ambassador program, and thanks to West Seattle Blog for publicizing the last few!

Everyone is welcome, see you there!

~ A l i c e !

Eddy off South Africa

I have to share. NASA released a couple of somewhat mind-blowing photos earlier this month. This is a 150-km-wide vortex off the coast of South Africa. You can see it because it is full of plankton.

The Eddy

There are currents in the ocean that are as big as the ocean itself (look up the North Pacific Gyre if you don’t believe me), so I shouldn’t be surprised to see a little whirlpool that’s a bit larger than Connecticut. It’s also the size of the Puget Sound area – defined as Olympia to Bellingham, Seattle to Snoqualmie Pass.

But, it looks like something out of science fiction.

The Satellite

This image is from NASA’s Terra satellite, and is a natural-color image (in case you were wondering). You have probably seen images from Terra that were put together using the same “near-real-time”  technology before–many of the satellite images of wildfires are from Terra or another of NASA’s Earth Observing System satellites.

The Plankton

Eddies like this one can stir up the ocean, bringing warmer water into colder places, and kicking up nutrients from deeper in the sea. Like yeast, when you provide the right environment you’ll get a “bloom” happening. As you know if you’ve ever tried to make bread, the key is warm, well-fed organisms. A bloom is a visible increase in the amount of microscopic stuff in water. In the case of yeast the bloom is from byproducts of the process of living (i.e. yeast “poop”), but for plankton, this bloom is more likely reproduction – millions of new “baby” plankton.

Want More?

NASA’s Earth Observatory

NASA’s Visible Earth

~ A l i c e !

In light of the Lunar Eclipse this past weekend, it’s time for the 11th seasonal sunset watch!!

• When: December 21, 2011 at 4:05pm (so come at 3:30pm)
• Actual sunset is supposed to be at 4:20, but at the summer solstice we noticed that the Sun set about 10 minutes earlier than the USNO says it does, so I’ve moved the time of our sunset watch up so we don’t miss it. I added another 5 minutes for the tall trees of Lincoln Park blocking our view. In Autumn the timing seemed to line up correctly – what will it do this time?
• Where: Solstice Park – all the way up the hill from the tennis courts (or, if you’re not in Seattle, wherever you have a view of the western horizon!)
• Who: Everyone welcome, as usual.

Sunset on September 23, 2011 Image credit: Jason Gift Enevoldsen

Come watch the winter solstice sunset at Solstice Park in West Seattle on Wednesday the 21st. We’ll see if the sunset lines up with the placed marker (it did for summer, but will it for winter?). I’ll be there even if it is cloudy because sometimes the Sun peeks through just as it begins to set, but if it is driving rain or sleet I’m staying home with some hot tea!

If you’re interested – here’s the timing of various celestial events on Saturday from Seattle, courtesy of the U.S. Naval Observatory Astronomical Applications Department:

Sun and Moon Data for One Day

The following information is provided for Seattle, King County, Washington (longitude W122.3, latitude N47.6):

Wednesday 21 December 2011 Pacific Standard Time

SUN
Begin civil twilight 7:19 a.m.
Sunrise 7:55 a.m.
Sun transit 12:07 p.m.
Sunset 4:20 p.m.
End civil twilight 4:56 p.m.

MOON
Moonset 1:07 p.m. on preceding day
Moonrise 4:30 a.m.
Moon transit 9:11 a.m.
Moonset 1:47 p.m.
Moonrise 5:45 a.m. on following day

Phase of the Moon on 21 December: waning crescent with 11% of the Moon’s visible disk illuminated.

This event is my part of the NASA’s Solar System Ambassador program, and thanks to West Seattle Blog for publicizing the last few!

Everyone is welcome, see you there!

~ A l i c e !