Bakersfield Night Sky – April 20, 2013
By Nick Strobel
This past week was the 2013 Planetary Defense Conference of the International Academy of Astronautics (IAA). The IAA fosters the development of astronautics for peaceful purposes and provides a program through which member countries can contribute to international endeavors for the advancement of aerospace science. They met in Flagstaff, AZ, less than a hour's drive away from Meteor Crater, a kilometer-wide crater produced by a large iron asteroid about 50,000 years ago. Meteor Crater is a good reminder about the ever-present possibility of a nasty impact but the February surprise of the Russia asteroid airburst over the city of Chelyabinsk followed several hours later of the very close fly-by of asteroid 2012 DA14 are even more potent reminders of the large space rocks that have Earth in their cross-hairs. The evening before the conference started was devoted to a special session about the Chelyabinsk Event.
The asteroid that airburst over Chelyabinsk was an ordinary stony chondritic asteroid about 17 to 20 meters (56 to 66 feet) in size with a mass of about 11,000 tons. It entered the Earth's atmosphere at a shallow angle and at a high speed of at least 11.6 miles/sec. At peak brightness, it was at an altitude of 14.5 miles. The tremendous air pressure it experienced fragmented the asteroid explosively and produced a shock wave that shattered windows for miles around. The airburst explosion was equivalent to about 440 kilotons of TNT. It was the biggest impact since the 1908 Tunguska blast in Siberia that leveled the forest for several hundred square miles. Piecing together all of the various video footage of the Russia asteroid's path in the air as seen from many different locations enabled astronomers to calculate its orbit before impact. Its elliptical orbit stretched from the asteroid belt at its farthest distance from the Sun to near Venus' orbit at its closest distance from the Sun (and crossing the Earth's orbit). The asteroid had probably been following this orbit for many thousands of years. As the asteroid approached the Earth, it remained within 15 degrees of the Sun's direction so ground-based telescopes would not have been able to spot it in any case (too much glare from the Sun).
We're undoubtedly going to be surprised again by something the size of the Chelyabinsk asteroid because we have found less than 1400 of the several million (at least) Near-Earth asteroids of that size. The larger DA14 asteroid (150 feet in diameter) was discovered in 2012 and its orbit and composition are very different than the Chelyabinsk asteroid. DA14's orbit around the Sun was very similar to the Earth's, taking 368 days to orbit the Sun. With February's very close fly-by, its orbit has shrunk so it now takes only 317 days to orbit the Sun. DA14 is a C-type asteroid with a lot of dark carbon in it. Something like the 150-foot diameter asteroid DA14 passes near the Earth every 40 years on average and an impact happens roughly once every 1200 years. An impact of something as small as DA14 would be equivalent to 2.4 megatons of TNT---enough to wipe out a city the size of Bakersfield but not have a global effect. DA14 has no chance of hitting Earth for at least the next century. Are there others though heading our way? Well, there are over a million Near-Earth asteroid of DA14's size and we have found only approximately 2200 of them so far.
The Near-Earth Object Program has found over 90% of the near-Earth objects larger than one kilometer (something that size hitting the Earth would wipe out life around the globe). The program is now focusing on finding 90% of the near-Earth objects larger than 140 meters (or roughly three times the size of DA14). Ground-based searches are limited to finding objects farther out from the Sun than we are and they are limited to essentially just the visible light wavelengths. Furthermore, the searches can occur only when there is no bright gibbous or full moon (so that's just two weeks of the month). We have probably found just 20% of the near-Earth objects larger than 140 meters. To meet the 90% goal (and to find many more of the smaller city-buster size asteroids), we need to use space-based infrared telescopes. The best location for a space-based infrared Near-Earth asteroid detector would be an orbit inside Earth's orbit---an orbit similar to Venus' orbit.
Canada launched the first satellite dedicated for finding and tracking near-Earth objects in the inner solar system in late February called NEOSSat. It is a "micro-satellite": less than a meter in size with a mass of just 74 kilograms. However, it is a visible light telescope (not infrared) just 15-centimeters (about 6 inches) across and the satellite orbits only 500 miles above the Earth because it is also tasked to look for orbiting man-made debris that can threaten working satellites. During asteroid-hunting mode, its search scan sweeps a narrow cone between 45 and 55 degrees from the Sun for near-Earth asteroids with orbits inside the Earth's orbit. The small micro-satellites are cheap and relatively quick to build. Go to http://neossat.ca to learn more about NEOSSat.
Space-based infrared telescopes specially tasked to find the near-Earth asteroids are still at least several years off. Infrared surveys are able to filter out better all of the confusion from the bright stars to see the asteroids and comets clearly and they can easily find the very common but dark asteroids missed by the visible light telescopes. Infrared telescopes can make much more accurate measurements of the asteroid's size. A space-based platforms can scan the Earth's environment continously 24/7 every day. One infrared mission called NEOCam (Near Earth Object Camera) is a NASA-funded project in the early design stage. Earlier this week they announced that the infrared sensor passed a critical design test that simulated the conditions of deep space. The sensor is the key component for the proposed NEOCam mission and the sensor itself has been in development for almost ten years! The NEOCam satellite would be positioned between the Earth and the Sun at one of the Sun-Earth gravity balance points called "L1", about 4 times further away from the Earth than is the Moon. It is expected to find approximately two-thirds of the potentially hazardous asteroids. The NEOCam satellite package is only in very preliminary design stages and it is competing for very limited NASA dollars to continue on. The next design proposal stage is in 2015. The close call of asteroid DA14 and the surprise of the Chelyabinsk Event might push it up higher in priority.
Another special purpose mission is being developed by the privately-funded B612 Foundation with a launch date in mid-2018. Called "Sentinel", the infrared space telescope will orbit the Sun in a Venus-like orbit to discover and catalog at least 90% of the near-Earth asteroids larger than 140 meters in diameter as well as finding a large number of those as small as 30 meters in diameter. The first ten years of the B612 Foundation's history was dedicated to figuring how we would deflect an asteroid heading toward Earth but by 2011 they realized that all of the deflection technology would be useless against asteroids that hadn't been detected yet, so they changed their focus to finding the potentially hazardous asteroids. They also realized that the shrinking NASA budget wouldn't allow a mission like Sentinel to be built, so they began a push to raise money privately. Go to http://b612foundation.org to find out more (and pass the link on to any rich relatives or friends who want to save humanity). To find out more about asteroid impacts, including information on a larger near-Earth asteroid called Apophis that has also been in the news, see the William M Thomas Planetarium's Asteroid Impacts page.
Approximately 100 tons of space debris falls to Earth every day. It is mostly in the form of very tiny particles about the size of a grain of sand or smaller and it is distributed over the entire surface of the Earth so it doesn't make the news like the big rocks discussed above. At least once a month, the Earth passes through a trail of such particles left behind by a comet. This coming week we will pass by one trail of dust particles left behind by comet C/1861 G1 Thatcher to make the Lyrid meteor shower. The Lyrids are expected to peak in the pre-dawn hours of Monday morning, April 22nd. However, the strength of this meteor shower is hard to predict and we will be contending with a Waxing Gibbous Moon that will be up most of the night. The Moon will set at about 4 AM and there will be slightly more than an hour of dark sky to look for meteors. The attached star chart below shows the eastern sky at around 4 AM. The meteors will appear to streak out of a point to the upper right of the bright star Vega in Lyra. If the night of April 21/22 doesn't fit your schedule, then try the few nights before or after the peak as the shower is active between April 16th and 25th. Get up out of the valley to escape the dust-filled sky of the Bakersfield metro area that scatters all of the city lights and the light of that bright Moon.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
last updated: April 16, 2013
Webpage contact: Nick Strobel