September 17, 2023
Sunday, September 17. 2023
This Thursday, September 21, will be the start of the fall season of shows at the William M Thomas Planetarium with “Mars One Thousand One” (or Mars 1K1). Mars 1K1 is the story of one possible mission to Mars. Space reporter Miles O'Brien guides you through the first human mission to Mars — a daring 1000-day mission to fly an international crew to the red planet and return them safely to Earth. MARS 1K1 is a bit longer than the other full-dome films we show at the planetarium, so the entire presentation, with the tour of the evening night sky using the Goto Chronos star projector that we do with every presentation, will run from 7:30 to 9:00 p.m. The doors open at 7 p.m. and are locked when the show begins at 7:30 p.m. Two weeks later, on October 5, we will present “Incoming!” about asteroid and comet impacts on Earth. See the Planetarium’s new website for more information about the shows and purchasing tickets.
On the morning of September 24, the sample return capsule from the OSIRIS-REx spacecraft will land at the Utah Test and Training Range near Salt Lake City. When OSIRIS-REx gets within 63,000 miles of Earth, it will release the sample return capsule. The capsule will take about 4 hours to reach the top of Earth’s atmosphere at 7:42 a.m. PDT. It will pierce Earth’s atmosphere at about 27,650 mph. At about one minute later, the capsule’s exterior temperature will reach over 5000 deg F from the air friction. About two minutes later, a drogue parachute will deploy to slow the capsule even more. By then it will be about 19 miles above the surface. After another five minutes, the main parachute will deploy followed by another five minutes of slow descent to what we hope is a gentle landing. From initial atmosphere entry to landing is thirteen minutes.
The capsule will have about 250 grams (8.8 ounces) of material from the near-Earth asteroid, Bennu. That amount of material is 50 times more than Japan’s Hayabusa2 probe collected from another near-Earth asteroid, Ryugu and many thousands of times greater than NASA’s previous sample return mission, Stardust at comet Wild 2. About 30% of the Bennu sample will be distributed to scientists all over the world. The remaining 70% will be stored for analysis by future generations of scientists with even more advanced instruments than the state-of-the-art equipment of today.
Bennu is a very dark, carbon-rich asteroid about 524 meters across that also has a significant amount of water molecules embedded in the rocks. Bennu is a rubble-pile of rock chunks that coalesced from the remains of an earlier violent collision. The chunks are held together by Bennu’s weak gravity, so the surface is as loosely packed as a bowl of popcorn or a pit of plastic balls that children (or the young at heart) jump into. We were surprised at how loosely it was packed together. The sampler was supposed to go just a few centimeters into the surface but it instead sunk almost half a meter beneath the surface. In fact, there’s even the possibility that if the thrusters had not fired, the entire spacecraft might have been swallowed up by Bennu.
The rock chunks of Bennu are thought to be primitive material that hasn’t changed since the formation of the solar system and carbon-rich asteroids, with a lot of water molecules in the rock matrix, might have brought the organic compounds and water to the early barren Earth—giving Earth the materials needed for life. Besides the abiogenesis sorts of reasons for studying Bennu, Bennu is also of interest because it is a potentially-hazardous asteroid with among the highest probabilities of hitting Earth sometime in the future.
On September 4, 2135 Bennu will pass close enough to Earth for Earth’s gravity to significantly change Bennu’s path. The next close encounter will be September 24, 2182. Depending on how its path is tweaked on the 2135 close encounter, Bennu will either hit Earth on September 24, 2182 or miss us. The OSIRIS-REx’s very detailed observations and measurements of Bennu’s internal properties and its orbit now give us a 1 in 2700 (0.037%) chance of hitting Earth in 2182 and a 1 in 1750 (0.057%) chance of hitting Earth sometime through the year 2300. After OSIRIS-REx releases its sample return capsule, it will fire up its thrusters for a rendezvous with another potentially-hazardous asteroid, Apophis, in 2029.
In this evening’s sky the moon will be a thin waxing crescent low in the west just above Spica after sunset. It will be at first quarter phase on September 22 and pass under Saturn as a waxing gibbous the night of September 26. It will be full on the night of September 28/29. One definition of “supermoon” says that this full moon will be a supermoon. Saturn is shining brightly among the dim stars of Aquarius in the southeast after sunset. Jupiter rises in the east with the stars of Aries and the head of Cetus around 9:30 p.m. One last astronomy event: autumnal (fall) equinox on September 22 at 11:50 p.m. PDT.
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Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
