Comet Offpoint Test Looked Great!

The comet offpoint test SDO ran very smoothly yesterday. Here is an AIA 171 Å image from 1441 UTC (10:41 am ET). The image of the Sun has been shifted back to the middle of the frame so the offpoint doesn't look so jarring in the daily movies.

If this had been a real comet observation the scientists would want to examine the missing right-hand side for the comet tail. AIA 171 Å was our best channel for looking at the comets.

My thanks to the SDO Flight Operations Team for making the test look easy.

Comet Offpoint Test, August 8, 2018

Tomorrow morning, from 1430-1505 UTC (10:30-11:05 ET), SDO will perform a comet offprint test maneuver. This is only a test, there is no comet that will be visible. However, it has been almost 6 years since we performed such a maneuver and its good to keep in practice. We only get a day or two notice when a Kreutz sun-grazing comet might be visible. This test allows us to put the instructions in a easy to access location for the next comet.

The Sun will appear to shift to the left during the test. That means it is useful for Kreutz comets in July and August, when the comets appear to come from the right and pass across the face of the Sun. Some science data, such as magnetograms and Dopplergrams, will not be produced while the Sun is shifted from the center of the images.

When a sun-grazing comet arrives, we will be ready to go comet watching!

Comet has Faded

The incoming sungrazing comet has faded and will most likely not be visible this afternoon in SDO AIA images. This is common in Kreutz comets, they evaporate before reaching the Sun. The light curve looks like a small comet evaporating.

Here's the latest from Karl Battams: "As I expected, the comet "pulled an ISON". I won't even describe it as a comet any longer. It's a rubble pile." (SunGrazerComets).

The Comet is Still There, Waiting for the Perihelion Time Update

This morning we should get better information about the orbit of the incoming comet. The orbit from last night is shown at left (times are in UTC). The plus signs are spaced 5 minutes apart. They get a little farther apart as the comet moves from lower right to the upper left. This means the comet is speeding up. At perihelion (the closest it gets to the Sun) the comet will be moving at about 600 km/s (1.3 million mph).

We see comets because they evaporate ices and other compounds from their surface. This cools the comet, but makes it disappear. The water ice that comes off the comet quickly turns into Hydrogen and Oxygen atoms. SDO sees comet tails when the oxygen atoms hit the electrons in the corona. We can use the tails to explore the Sun's magnetic field and corona.

Most Kruetz sungrazing comets are too small to make it to perihelion (Comet Lovejoy in December 2012 was the only exception). We can only watch as this comet goes behind the LASCO occulter disks and hope it continues to evaporate and be seen in SDO/AIA images.

A New Sungrazing Comet, Will SDO See It?

Karl Battams (@SungrazingComets on Twitter) has announced a new sungrazing comet. It is seen in a LASCO image taken earlier today. The image here shows what the comet looked like at 1430 UTC (12:30 pm ET) today.

This is a Kruetz comet, like the first comet seen by SDO back in 2012. The SDO team has decided that our normal pointing at the Sun is all we need to do to capture this comet as it passes across the disk. We used the picture at left to guide us. The comet will reach perihelion (where the plus signs end) just before it goes over the limb of the Sun and disappears behind the Sun. The comet is brightest just before perihelion (if it makes it that far), there is no reason to point SDO to another part of the orbit.

We will try to post images as fast as we can tomorrow afternoon. Perihelion is at 1947 UTC (3:47 pm ET).

Where Was Comet ISON?

By now I am sure people are aware that something continues to move along the orbit of Comet ISON in the LASCO observations. It is being discussed whether it is debris from Comet ISON or a remnant. In either case we should try to understand why we didn't see that debris or remnant as it went through perihelion yesterday.

When SDO looks for a comet it does not see the dust and ion tails seen in a comet far from the Sun. We see oxygen ions, oxygen atoms with some electrons stripped off, glowing when they are hit by the hot electrons in the corona. The AIA telescopes normally look at iron ions, but the enormous amount of oxygen that comes off a comet allows that element to emit brighter for a while after the comet passes.

The images we showed yesterday were our best guesses for which AIA passbands would show bright emissions from Comet ISON as it flew past the Sun. They are the passbands that showed Comet Lovejoy in December 2011 and the earlier comet in June 2011. All four passbands would show emission from O IV, O V, or O VI, oxygen atoms with 3, 4, or 5 electrons removed. This is a chain, where O V is produced from O IV and then O VI is produced from O V.

We pointed SDO at three locations along the projected orbit. If you compare the predicted orbit with the SOHO images it looks like we had pretty good alignment. I was watching how the comet moved along its predicted orbit as we changed the SDO pointing and the comet moved through the frames we provided. Here is a snapshot of what I was watching, at 1844 UTC yesterday, about the predicted time of perihelion. You can see that the position of Comet ISON is in the upper left part of the square the size of the Sun showing where SDO is pointed. The AIA telescopes have a larger field of view, because they need to see the corona around the Sun. So the comet is well inside the AIA images at this time. There are some movies posted under "Processed Movies" that show the predicted position of Comet ISON along with the frames taken during the perihelion point. Check out the Comet ISON AIA 171 movies (.mov) or Comet ISON AIA 171 Perihelion (.mp4). The color table in these movies has been fixed to bring out dim features in the corona. You can watch the corona loops extending out farther into the corona then we would normally see.

To estimate the brightness of the comet we had to assume a composition of the stuff coming off the comet and the density of the corona it was moving through. Even though the comet is heated by the glow of the Sun to sublimate the material, it is the corona that strips off the electrons to create the oxygen ions. This means we have a couple of things to check as we try to understand the lack of any visible trail in the SDO images. Did Comet ISON have a strange composition? By strange it would have to have very little oxygen. Many molecules that are found in comets have oxygen atoms in them. Water, carbon dioxide, and most rocks have oxygen in them. Comet ISON appeared normal as it moved through the solar system. It may have been running out of ice as it moved into perihelion, but the dust particles should also have sublimated and contributed oxygen to the corona as they flew past.

Perhaps the corona was not as dense as we assumed. The reactions that cause oxygen ions need fast electrons. Each step in the ion chain takes a little time. If the density of the fast electrons is smaller each step takes a little longer. At the same time the oxygen ions are moving away from the orbit of the comet. As the new oxygen ions get further away from the comet their density goes down and they become more difficult to see. This will require us to go look more carefully at those places in the frames where the comet material has been the longest. Maybe a faint signal can be detected.

We have now seen two sungrazing comets in SDO data. The first disappeared before its perihelion and Comet Lovejoy broke up after perihelion and eventually dissipated. We have also looked for at least two others. One never appeared, and now Comet ISON, which appears to have snuck by perihelion without a trace. We will work to understand what happened as Comet ISON, either as a remnant or debris, flew by the Sun.

We are tied at 2-all for sungrazing comets that we saw and didn't see in AIA data. I hope another sungrazing comet comes by soon to break to tie.

SDO has Repointed to the Sun

And the dust trail of Comet ISON has moved around the Sun.

Thanks for taking part in our SDO Data Event. A lot of people have help, especially Phil Scherrer, the HMI PI, Karel Schrijver, the AIA PI, Barbara Thompson, Leila Mays, and Kevin Addison. Many thanks to Judy Pepoy for keeping track of SDO during the off-points. The Google+ hangout had a lot of fun!

Thanks for joining us on Thanksgiving Day 2013 for Comet ISON's perihelion breakdown!

SDO is at Departure Pointing

SDO has moved to point at our Departure Pointing box. Let's keep watching!

A Pretty Dim Comet!

We are not seeing anything in 171 or 193.

Now we are hearing that the comet evaporated well before perihelion. I would think that the rocky bits would still move along the orbit and evaporate, but maybe that isn't true.

The latest SOHO LASCO C2 movies show that Comet ISON appears to have run out of steam.

SDO is Moving to the Perihelion Point Position

The Approach Pointing images will always be available for viewing.

It takes a while for the images to move through the system, about 10-15 minutes right now.

SDO is in the Approach Pointing Position

SDO has pointed toward the Approach Pointing box to look for Comet ISON. The images are moving through the data processing and should be viewable in a couple of minutes.

We are seeing images but so far no signal in 171 or 193.

Still looking here, but no comet sightings. If you look at these images long enough you can see anything!

SOHO LASCO Movies Available from our website

You can download the SOHO LASCO C2 and C3 movies of Comet ISON at http://cometison.gsfc.nasa.gov/ and selecting "Processed Movies".

Comet ISON Moving into Position for Perihelion

This image from the LASCO C3 Coronagraph on the SOHO satellite shows Comet ISON moving toward its encounter with the Sun. On Thursday Comet ISON will go through perihelion (the closest point of its orbit to the Sun) at about 1:45 pm ET (1845 UTC). During the next two days the comet will go very close to the Sun and make a 140° turn, almost reversing its direction and heading out into interstellar space. After several million years of heading in almost a straight line, it will change course in 2 days!

Join us for the SDO Comet ISON Perihelion Data Event tomorrow from 1 to 3:30 pm ET (1800-2030 UTC). There is also a Google+ hangout running during the perihelion passage.

Come out and watch a comet!