Tuesday, February 19, 2019

Another Nifty Lunar Transit from SDO

The previously scheduled station-keeping maneuver is delayed until next week (February 27). It's time to talk the lunar transit of March 6-7, 2019.

From 2200 UTC on March 6,2019, until 0207 UTC on March 7 (5:00-9:06 pm ET, March 6) SDO will experience a lunar transit. The Moon will cover up to 82% of SDO's view of the Sun. This four-hour transit looks similar to the double transit of September 9-10, 2018, except that the shadow of the Moon is visible through the entire transit. Here is a movie from the Flight Operations Team that shows SDO, the Sun, and the Moon during the transit.

The Moon moves from lower left to upper right in the images during the first half of the transit and upper right to lower left for the second half. The first part of the transit is caused by SDO overtaking the Moon as SDO moves in the afternoon part of its orbit. (SDO orbits over the Mountain Time Zone of the USA so the transit happens from 3:00-7:06 MT.) SDO's velocity of about 3 km/s is faster than the Moon's of 1 km/s and SDO overtakes and moves past the Moon-Sun line. The second part of the transit happens as SDO moves into the dusk part of SDO's orbit around the Earth and is now moving mostly away from the Moon. The Moon's velocity takes it past SDO and the shadow appears to move from right to left across the Sun.
When I first saw this movie I thought we were going to talk about retrograde motion. Other planets, notably Mars, move in retrograde when the Earth moves between them and the Sun with our faster orbital velocity. But it isn't just that. The first part of the transit is like retrograde motion as SDO passes by the Moon-Sun line with its faster velocity and the Moon appears to move backwards. But the second part of the transit happens because SDO is moving mostly away from and even a little in the opposite direction of the Moon.
I created two videos to help understand what’s happening. The first shows why Mars appears to have retrograde motion and the second explains the extended lunar transits seen in September 2018 and March 2019.

Retrograde motion is the apparent backwards motion of an outer planet among the stars as the Earth moves past the line connecting the Sun and the outer planet. In this movie Mars is a red disk and the Earth is a blue disk moving in their orbits around the yellow disk of the Sun. A line is drawn connecting the Earth and Mars and extended out into space. A colored dot is drawn where an observer on the Earth will see Mars against the distant stars. The color of that dot then changes from red to blue as the Earth overtakes Mars in it's orbit. Those dots show that the path of Mars seen from Earth traces out a lazy Z in the stars. In this example the Z is centered about the time the Sun, Earth, and Mars are lined up.

The other video shows the situation for SDO's lunar transit on March 6. There is no line connecting SDO and the Moon, only the shadow of the Moon. SDO and the Moon are started at about the right times in their orbits and you can see the shadow line hits the grey disk of SDO as the satellite moves through the afternoon and dusk parts of the orbit. In the first part of the transit SDO moves across the black line from left to right across the Sun (so the Moon appears to move from left to right across the Sun.) During the second part of the transit SDO moves across the black line from right to left and the Moon moves in the opposite direction from the first.
During the total solar eclipse in August 2017 the Moon's shadow moved from the West coast of the US towards the East. This is because the speed of the rotation of the Earth (less than 0.5 km/s) is slower than the speed of the Moon. So the Moon overtakes the people on the rotating Earth. The shadow follows the Moon and moves from West to East, like the second part of the transit.
Although you can see the Moon throughout the movie SDO's instruments cannot see the Moon when it is not covering the Sun. The little white flash seen in the Moon is the word &ldquot;Moon&rdquot; being written by the software and then quickly covered. The boxes drawn on and around the Sun help the FOT run the spacecraft. The time is displayed in the lower left corner of the movie. The first seven numbers are the year (2019) and the day of year (065 and 066). The six numbers after the period are the hour, minutes, and second of UTC (2 numbers each).
One detail was left out of the discussion. A transit can only if the Sun, Moon, and SDO are in a line. The Moon's shadow has a small angle rather than the straight line in the movie. This means the model shown here is too simple, but it still explains why we see the long lunar transits with the Moon changing direction. Look at the FOT movie for a better simulation.

Once again, a lunar transit shows how complicated the motions of objects can appear even as they move along simple orbits.


Monday, February 11, 2019

Happy 9th Anniversary SDO!

Nine years ago today, at 10:23 am ET, SDO rose into the sky atop an Atlas V 401 launch vehicle. Since that day SDO has returned over 350 million images of the Sun, produced over 3000 scientific articles, and allowed millions to enjoy the Sun. Solar Cycle 24 was just starting in 2010. SDO watched it grow to maximum in 2014 and now the Sun is often spotless.

Here is a summary slide of what SDO saw. The AIA 193 Å shows the lower corona and the HMI magnetogram shows the surface magnetic field. We try to understand the Sun by watching what happens at the poles. The dark areas near the North and South poles in the October 2010 and February 2019 193 Å images are the polar coronal holes. They have disappeared by solar maximum (the February 2014 images). The magnetograms show how the Sun's magnetic field gets more complicated at maximum and quite simple at minimum. What happens in the poles at solar minimum seems to be one of our best predictors of future solar activity.

SDO data is the basis of accurate models of the solar corona. Machine Learning algorithms are being used to squeeze even more information out of our 12 PB of data. We are looking forward to another lengthy lunar transit next month and the Mercury transit in November.

You can make your own SDO Anniversary movie at our website sdo.gsfc.nasa.gov or the Helioviewer website.

Happy Anniversary SDO!

Tuesday, February 5, 2019

Welcome to 2019!

The replacement of the computers serving https://sdo.gsfc.nasa.gov have arrived at Goddard! The system will be replaced and the data moved to the new drives over the next few weeks. We will be ready to serve the near-realtime images during the Mercury transit on November 11, 2019.
The highlights of SDO's maneuvers for the first 2/3's of 2019 are:
  1. 01/31 @ 0506 UTC (00:06 am ET) - Spring Handover Season Starts
  2. 02/06 @ 0723 UTC (02:23 am ET) - Spring Eclipse Season Starts
  3. 02/20 @ 2210 UTC (10:10 pm ET) - Station Keeping maneuver #18
  4. 03/01 @ 0713 UTC (02:13 am ET) - Spring Eclipse Season Ends
  5. 03/06 2200 UTC to 03/07 0206 UTC (03/06, 05:00 to 09:06 pm ET) - Lunar Transit (movie released next week)
  6. 03/27 @ 0310 UTC (10:10 pm ET) - Spring Handover Season Ends
  7. 05/01 @ TBD - Momentum Management maneuver #34 (Tentative Date)
  8. 07/17 @ 0445 UTC (12:45 am ET) – Fall Handover Season Begins
  9. 07/24 @ TBD - Station Keeping maneuver #19 (Tentative)
  10. 08/08 @ 0709 UTC (03:09 am ET) – Fall Eclipse Season Starts
  11. 09/01 @ 0658 UTC (02:58 am ET) – Fall Eclipse Season Ends