Tales of Two Asteroid Occultations: Io, Nemausa Cast Shadows Over North America

Up for a challenge? Over the next two weekends, two asteroid occultations pass over North America. These are both occulting (passing in front of) +7th magnitude stars, easy targets for even binoculars or a small telescope. These events both have a probability score of 99-100%, meaning the paths are known to a high degree of accuracy. These are also two of the more high profile asteroid occultations for 2016.

Here’s the lowdown on both events:

Image credit:
The path of the 85 Io event. Image credit: Steve Preston/Asteroid Occultation Updates.

On the morning of Saturday, August 27th , the +10th magnitude asteroid 85 Io occults a +7.5 magnitude star (TYC 0517-00165-1). the maximum predicted duration for the event is 28 seconds, and the maximum predicted brightness drop is expected to be 3 magnitudes. The ‘shadow’ will cross North America from the northeast to the southwest starting over Quebec at 4:27 Universal Time (UT), crossing Ontario and Michigan’s upper peninsula at 4:30 UT, and heading south over Oklahoma, Texas, and Mexico at 4:36 UT. The action takes place in the constellation Aquarius, with the Moon at a 28% waning crescent.

Image credit: Stellarium
A wide field finder view for the 85 Io event. Image credit: Stellarium

Discovered by C.H.F. Peters on September 19th, 1865, 85 Io is about 180 kilometers in diameter, as measured by an occultation in late 1995.

Image credit:
The path of the 51 Nemausa event. Image credit: Steve Preston/Asteroid occultation updates.

Next, on the morning of Saturday, September 3rd, the +11.5 magnitude asteroid 51 Nemausa occults a +7.6 mag star (HIP 8524). The maximum duration of the event along the centerline is expected to be 32 seconds in duration, with a maximum drop of four magnitudes. Said shadow will cross western Canada at9:42 UT, and the U.S. crossing runs from 9:49 to 9:55 UT. The action takes place in the constellation Pisces. The Moon phase is a slim 4% waxing crescent during the event.

Image credit: Stellarium
A wide field finder view for the 51 Nemausa event. Image credit: Stellarium

Discovered in 1858 by A. Laurent observing from Nîmes, France, 51 Nemausa occulted a bright star in 1979. In fact, there’s evidence from previous occultation to suggest the 51 Nemausa may possess a tiny moon… could it show up again during the September 3rd event?

Observing asteroid occultations is really a modern sub-specialty of amateur and even professional astronomy. To predict such an occurrence, the orbit of the asteroid or occulting body and the precise position of the star need to be known to a pretty high degree of precision. This required the advent of modern astrometry and massive computing power. If any casual sky observer noticed a naked eye star wink out way back when pre-mid 20th century, it’s lost to history.

The first successfully predicted and observed occultation of a star by an asteroid was the +8.2 magnitude star SAO 112328 by 3 Juno on February 19, 1958. Less than two dozen such events were observed right up through to 1980. Today, hundreds of such events are predicted worldwide each year.

Next month’s expected data release from the ESA’s Gaia mission should refine our stellar position and parallax knowledge even further, and fine-tune predictions of future asteroid occultations.

And speaking of occultations, some great observations were made of the July 29th, 2016 lunar grazing occultation of Aldebaran across the United States. The Moon occults Aldebaran for every lunation in 2016 into 2017, and we get another shot favoring the southern U.S. in the daytime on August 25th.

Observing an asteroid occultation is a challenge, requiring an observer acquiring and monitoring the correct star at the precise time of the event. If possible (i.e. weather permitting) familiarize yourself with the star field a night or two prior to the event. I usually have a precise audio time signal such as WWV radio running in the background.

Image credit: Occult 4.2.
The shape of 51 Nemausa from the 2014 event. Image credit: Occult 4.2.

Why occultations? Well, if enough observations can be gathered, a sort of shadow profile of the occulting space rock can be made, with each observation representing a chord. Even negative ‘misses’ along the edge of the path help. Tiny moons of asteroids have even been discovered this way, as the distant star winks out multiple times.

The International Occultation Timing Association wants your observation. You can, with a little practice, make usable observations visually, though most observers tend to video record events.

Don’t miss these asteroid occultations and more, coming to a sky worldwide near you!

Observing Challenge: Watch Asteroid 3 Juno Occult a +7th Magnitude Star Tonight

One of the better asteroid occultations of 2014 is coming right up tonight, and Canadian and U.S. observers in the northeast have a front row seat.

The event occurs in the early morning hours of Thursday, November 20th, when the asteroid 3 Juno occults the 7.4 magnitude star SAO 117176. The occultation kicks off in the wee hours as the 310 kilometre wide “shadow” of 3 Juno touches down and crosses North America from 6:54 to 6:57 Universal Time (UT), which is 12:54 to 12:57 AM Central, or 1:54 to 1:57 AM Eastern Standard Time.

Steve Preston
The path of tomorrow’s occultation along with the circumstances. Credit: Steve Preston’s Asteroid Occultation website.

The maximum predicted length of the occultation for observers based along the centerline is just over 27 seconds. Note that 3 Juno also shines at magnitude +8.5, so both it and the star are binocular objects. The event will sweep across Winnipeg and Lake of the Woods straddling the U.S. Canadian border, just missing Duluth Minnesota before crossing Lake Superior and over Ottawa and Montreal and passing into northern Vermont and New Hampshire. Finally, the path crosses over Portland Maine, and heads out to sea over the Atlantic Ocean.

Don’t live along the path? Observers worldwide will still see a close pass of 3 Juno and the +7th magnitude star as both do their best to impersonate a close binary pair. If you’ve never crossed spotting 3 Juno off of your astro-“life list,” now is a good time to try.

The position of the target star HIP43357/SAO 117176 is:

Right Ascension: 8 Hours 49’ 54”

Declination: +2° 21’ 44”

Starry Night
A finder chart for 3 Juno and HIP43357. Stars are noted down to +10th magnitude. Created using Starry Night Education software.

Generally, the farther east you are along the track, the higher the pair will be above the horizon when the event occurs, and the better your observing prospects will be in terms of altitude or elevation. From Portland Maine — the last port of call for the shadow of 3 Juno on dry land — the pair will be 35 degrees above the horizon in the constellation of Hydra.

NOAA
The projected sky cover at the time of the occultation. Credit: NWS/NOAA.

As always, the success in observing any astronomical event is at the whim of the weather, which can be fickle in North America in November. As of 48 hours out from the occultation, weather prospects look dicey, with 70%-90% cloud cover along the track. But remember, you don’t necessarily need a fully clear sky to make a successful observation… just a clear view near the head of Hydra asterism. Remember the much anticipated occultation of Regulus by the asteroid 163 Erigone earlier this year? Alas, it went unrecorded due to pesky but pervasive cloud cover. Perhaps this week’s occultation will fall prey to the same, but it’s always worth a try. In asteroid occultations as in free throws, you miss 100% of the shots that you don’t take!

IOTA
The path of the occcultation across eastern North America. Credit: Google Earth/BREIT IDEAS observatory.

Why study asteroid occultations? Sure, it’s cool to see a star wink out as an asteroid passes in front of it, but there’s real science to be done as well. Expect the star involved in Thursday’s occultation to dip down about two magnitudes (six times) in brightness. The International Occultation Timing Association (IOTA) is always seeking careful measurements of asteroid occultations of bright stars. If enough observations are made along the track, a shape profile of the target asteroid emerges. And the possible discovery of an “asteroid moon” is not unheard of using this method, as the background star winks out multiple times.

UT-Juno Occultation
3 Juno as imaged by the 100″ Hooker telescope at the Mt. Wilson observatory at different wavelengths using adaptive optics. Credit: NASA/JPL/The Harvard Smithsonian Center for Astrophysics.

3 Juno was discovered crossing Cetus by astronomer Karl Harding on September 1st, 1804 from the Lilienthal Observatory in Germany. The 3rd asteroid discovered after 1 Ceres and 2 Pallas, 3 Juno ranks 5th in size at an estimated 290 kilometres in diameter. In the early 19th century, 3 Juno was also considered a planet along with these other early discoveries, until the ranks swelled to a point where the category of asteroid was introduced. A denizen of the asteroid belt, 3 Juno roams from 2 A.U.s from the Sun at perigee to 3.4 A.U.s at apogee, and can reach a maximum brightness of +7.4th magnitude as seen from the Earth. No space mission has ever been dispatched to study 3 Juno, although we will get a good look at its cousin 1 Ceres next April when NASA’s Dawn spacecraft enters orbit around the king of the asteroids.

3 Juno reaches opposition and its best observing position on January 29th, 2015.

3 Juno also has an interesting place in the history of asteroid occultations. The first ever predicted and successfully observed occultation of a star by an asteroid involved 3 Juno on February 19th, 1958. Another occultation involving the asteroid on December 11th, 1979 was even more widely observed. Only a handful of such events were caught prior to the 1990s, as it required ultra-precise computation and knowledge of positions and orbits. Today, dozens of asteroid occultations are predicted each month worldwide.

Observing an asteroid occultation can be challenging but rewarding. You can watch Thursday’s event with binoculars, but you’ll want to use a telescope to make a careful analysis. You can either run video during the event, or simply watch and call out when the star dims and brightens as you record audio. Precise timing and pinpointing your observing location via GPS is key, and human reaction time plays a factor as well. Be sure to locate the target star well beforehand. For precise time, you can run WWV radio in the background.

And finally, you also might see… nothing. Asteroid paths have a small amount of uncertainty to them, and although these negative observations aren’t as thrilling to watch, they’re important to the overall scientific effort.

Good luck, and let us know of your observational tales of anguish and achievement!

Conjunctions to Watch For in July

The planets are slowly returning into view this month, bashfully peeking out from behind the Sun in the dawn & dusk sky. This month offers a bonanza of photogenic conjunctions, involving the Moon, planets and bright stars.

The action begins tonight on July 8th, as the waxing crescent Moon joins the planet Venus in the dusk sky. The razor thin Moon will be a challenge on Monday night, as it just passed New on the morning of the 8th at 3:14AM EDT/7:14 Universal Time (UT). The record for spotting the thin crescent with the naked eye currently stands at 15 hours and 32 minutes, completed by Stephen O’Meara on May 1990. Binoculars help considerably in this endeavor.  Wait until 15 minutes after local sunset, and then begin patiently sweeping the horizon.

Mr. Thierry Legault completed an ultimate photographic challenge earlier today, capturing the Moon at the precise moment of  New phase!

The Moon & Venus on the evening of July 9th from latitude 30 degrees north, about 30 minutes after sunset. (Created by the author using Stellarium).
The Moon & Venus on the evening of July 9th as seen from latitude 30 degrees north, about 30 minutes after sunset. (Created by the author using Stellarium).

This week  marks the start of lunation 1120. The Moon will be much easier to nab for observers worldwide on Tuesday night, July 9th for observers worldwide. The sighting of the waxing crescent Moon will also mark the start of the Muslim month of Ramadan for 2013. Due to the angle of the ecliptic in July, many northern hemisphere observers may not spot the Moon until Wednesday night on July 10th, about 6.7 degrees south west of -4.0 magnitude Venus.

Did you know? There are Guidelines for the Performance of Islamic Rites for Muslims aboard the International Space Station. It’s interesting to note that the timing of the rituals follows the point from which the astronaut originally embarked from the Earth, which is exclusively the Baikonur Cosmodrome in Kazakhstan for the foreseeable future of manned spaceflight.

Malaysia’s first astronaut, Sheikh Muszaphar Shukor observed Ramadan aboard the International Space Station in 2007.

From there, the crescent Moon fattens, meeting up with Saturn and Spica on the evenings of July 15th and 16th. The Moon will actually occult (pass in front of) the bright star Spica on the evening of July 15/16th at ~3:33UT/11:33PM EDT (on the 15th) for observers in Central America and western South America. The rest of us will see a near miss worldwide.

The waxing crescent Moon nearing Spica on the evening of the 15th at 10PM EDT. The Moon reaches 1st Quarter on the same evening at 11:18PM EDT. (Created by the author using Starry Night).
The waxing crescent Moon nearing Spica on the evening of the 15th at 10PM EDT. The Moon reaches 1st Quarter phase on the same evening at 11:18PM EDT. (Created by the author using Starry Night).

This is the 13th in a cycle of 18 occultations of Spica by our Moon spanning 2012-2013. Spica is one of four stars brighter than magnitude +1.4 that lie close enough to the ecliptic to be occulted by our Moon, the others being Antares, Regulus and Aldebaran. Saturn will lie 3 degrees from the Moon on the evening of July 16th.

Can you nab Spica and Saturn near the Moon with binoculars in the daytime around the 15th? It can be done, using the afternoon daytime Moon as a guide. Crystal clear skies (a rarity in the northern hemisphere summertime, I know) and physically blocking the Sun behind a building or hill helps.

The waxing gibbous Moon will also occult +2.8 Alpha Librae for South Africa on July 17th around 17:09UT & +4.4th magnitude Xi Ophiuchi for much of North America on the night of July 19th-20th.

And speaking of Regulus, the brightest star in the constellation Leo lies only a little over a degree (two Full Moon diameters) from Venus only the evenings of July 21st & the 22nd. 77.5 light years distant, Regulus is currently over 100 times fainter at magnitude +1.4. Can you squeeze both into the field of view of your telescope at low power? Venus’s mythical ‘moon’ Neith lives!

Venus can even occult Regulus on rare occasions, as last occurred on July 7th, 1959 and will happen next on October 1st, 2044.

But there’s morning action afoot as well. The planets Mars and Jupiter have emerged from solar conjunction on April 18th and June 19th, 2013 respectively, and can now be seen low in the dawn skies about 30 minutes before sunrise.

Mars and Jupiter in a close conjunction on the morning of July 22nd, about 30 minutes before sunrise as seen from latitude 30 degrees north. (Created by the author using Starry Night).
Mars and Jupiter in a close conjunction on the morning of July 22nd, about 30 minutes before sunrise as seen from latitude 30 degrees north. (Created by the author using Starry Night).

Mars approaches Jupiter in the dawn until the pair is only 0.79 degrees (about 48 arc minutes) apart on Monday, July 22nd. Mars shines at magnitude +1.6 and shows a tiny 3.9” disk, while Jupiter displays a 32.5” disk shining at magnitude -1.9 on this date. Conjunction occurs at about 7:00 UT/3:00 AM EDT, after which the two will begin to race apart. Mercury is visible beginning its morning apparition over 5 degrees to the lower right of the pair (see above).

Jupiter will reach opposition and reenter the evening sky on January 5th, 2014, while Mars won’t do the same until April 8th of next year. Weird factoid alert: neither Jupiter or Mars reach opposition in 2013! What effect does this have on terrestrial affairs? Absolutely none, well unless you’re a planetary imager/observer…

Mars also reaches its most northern declination of 2013 of 24 degrees in the constellation Gemini on July 16th at 7:00 AM EDT/11:00 UT.  Mars can wander as far as declination 27 degrees north, as last happened in 1993.

Finally, are you observing from southern Mexico this week and up for a true challenge? The asteroid 238 Hypatia occults a +7.4 magnitude star from 10:13-10:49 UT on July 10th in the constellation Pisces for up to 29 seconds. This event will be bright enough to watch with binoculars- check out our best prospects for asteroid occultations of stars in 2013 here and here.

Good luck, clear skies, and be sure to post those astro-pics in the Universe Today’s Flickr community!