Halley’s Comet particles
Every year between mid-April and the end of May, the Earth encounters the outbound debris from Halley’s Comet. Outbound describes the motion of each particle as it moves away from the sun and passes near the Earth on its way back to the outer realms of the solar system. Halley’s comet passes through the inner solar system every 76 years and each one of those passages creates a new stream of particles as they separate from the comet due to heating from the sun. These streams each have a slightly different orbit due to planetary perturbations and the fact that the comet is never exactly in the same every location time when it returns to the inner solar system. When a planet passes close to one of these streams the particles get moved ever so slightly out of their previous orbit. Since many of these particles have been orbiting for over two thousand years they can now encounter the Earth anytime during a six week span from mid-April to the last of May. The Earth encounters the mean position of these many orbits on May 5th or 6th of each year. That is when the particle concentration is greatest and we witness the maximum activity of the eta Aquariid meteor shower. It should be noted that the particles we see as meteors separated from the comet many hundreds of years ago as the current orbit of the comet does not cross the orbit of the Earth.
Eta Aquariids in 2023 will peak on May 6
Since the eta Aquariids are the outbound particles of the Halley’s comet, we see them radiate from a point west of the sun. This positioning only allows these meteors to be seen on the morning side of Earth. To make matters more restrictive, the source of these meteors, located in the constellation of Aquarius, does not clear the horizon until 2:00 to 3:00am local daylight saving time as seen from the lower northern latitudes. Viewing circumstances are even worse for mid and upper northern latitudes as the rising of the radiant and the start of morning twilight become closer together as one moves northward. Viewing circumstances for the eta Aquariids are best for those located in the southern tropics where the source of these meteors rises highest in a dark sky. Most observers in the northern hemisphere have only a one to two hour window prior to dawn to view these meteors.
In 2022, the waxing crescent moon was located in the evening sky during the shower’s maximum and the display was well seen. This year, conditions are much worse at maximum as the moon is full on May 5th. This means that the bright moonlight will obscure all but the brighter meteors. To best see this activity one should view during the last two hours prior to dawn. For most of us located in mid-northern latitudes, this will roughly be 3-5am local daylight saving time. This will allow you to see the most possible activity which should be near 10-15 per hour from mid-northern latitudes and up to 30 per hour for southern tropical locations. While the peak is predicted to occur on May 6, the week centered on this date will also provide rates near 10 per hour. By May 10, hourly rates will fall below 10 per hour and will slowly fall as the month progresses.
This year may prove interesting as we enter a two year period in which the eta Aquariids should experience increased activity. This is the reason the expected rates are similar to last year when no moon was present. The reason for this increased activity is that during the next two years the Earth will encounter particles of Halley’s Comet that have been perturbed by the giant planet Jupiter into an orbit that is closer to the Earth. According to Bill Cooke, the lead for the Meteoroid Environment Office at NASA’s Marshall Space Flight Center in Huntsville, Alabama, these meteors were produced by the return of Halley’s comet in the year 390 BC.*
*Eta Aquarid meteor shower 2023: Where, when and how to see it, https://www.space.com/36502-eta-aquarid-meteor-shower-guide.html Accessed 2023/04/29
The potential increase in activity may also interest radio meteor scatter observers located in mid-northern latitudes as the radiant lies in a favorable zone in the sky for approximately six hours centered at 08:00. Note that this timing is entirely during daylight this time of year when the meteors cannot be seen visually. This zone is centered at an elevation of 45 degrees above the horizon. Radio rates at higher or lower elevations will be less than when the radiant is located near 45 degrees elevation. For more information on radio meteor scatter visit: https://www.imo.net/observations/methods/radio-observation/
This year serious observers can help us determine the effect that bright moonlight has upon observers of meteor activity. The International Meteor Organization is conducting a project for observers who view under bright moonlight during major meteor shower activity. You simply need to view for at least one hour and estimate your limiting magnitude every 30 minutes. This is easily done by counting the number of stars visible with certain areas of the sky. Charts for these areas are available at: https://www.imo.net/observations/methods/visual-observation/major/observation/#table1. Area 6 on chart #6 would be the easiest to use. Using more than one area is also encouraged to provide more estimates. Of course these limiting magnitude estimates can be attempted at all hours of the night, whether the moon is visible or not. You should expect to achieve better results (higher magnitude estimates) when the moon is below the horizon. The conversion table is available in Table 2 on the link provided above. Helpful tips for visual observing are also available at: https://www.imo.net/observations/methods/visual-observation/ and https://www.imo.net/observations/methods/visual-observation/major/observation/. In order to submit your observations we suggest that you fill out a visual meteor report form provided by the International Meteor Organization. You must register to provide your data, but there is a free option for those not wishing to subscribe to the IMO Journal. ALPO also accepts observations emailed directly to: Robert Lunsford
We wish you good luck and look forward to seeing your viewing results!
