October 2, 2022

Early October at 9:30 PM looking south-southeast

Sunday, October 2, 2022

The DART hit its mark! At 4:14 PM on September 26, the Double Asteroid Redirection Test spacecraft successfully collided with the small asteroid Dimorphos that is orbiting the larger asteroid Didymos. It was a test of nudging an asteroid, which is a technique we may need to do in the future with some other asteroid on a collision course with Earth. Although there is no known asteroid larger than 140 meters across heading our way in the next 100 years, we haven't discovered all of the Potentially Hazardous Asteroids (PHAs) yet.

The 140-meter diameter is the lower bound for an asteroid strike that could penetrate Earth's atmosphere to create a crater and wipe out a metro or state-size area. About 39% of the estimated 25,000 PHAs this size have been found. Such a collision is estimated to occur roughly once every 20,000 years but nature has its own timetable, so don't expect nature to keep to a regular 20,000-year time interval. 

Asteroids one kilometer or larger would create a crater ten kilometers wide and cause global devastation, probably causing the collapse of civilization. Fortunately, there are only about 900 PHAs and we've found 95% of those. They strike about once every 500,000 years. The mass extinction PHAs are those at least 10 kilometers in size and there are only four of them.

The larger the lead time we have in redirecting an asteroid heading our way, the smaller is the amount of energy required to give it the necessary nudge out of Earth's way. DART was a test of one possible way to nudge an asteroid. Other ways include detonating nuclear warheads near an asteroid or using the gravitational force of a nearby spacecraft traveling next to the asteroid for several years.

DART is actually NASA's second successful attempt at hitting a small solar system body. The first was when Deep Impact hit the comet Tempel 1 in 2005. That mission was designed to investigate the interior of a comet by crashing a 370-kg copper impactor into the nucleus of Tempel 1 and analyzing the material shot out from the impact (copper is an element not expected to be in comets). Tempel 1 is several miles across, so we were not expecting to change its course at all (and it's not ever going to come close to Earth anyway). 

DART is the first mission designed to change a small solar system body's motion and this was to be a small prototype test of the process. The 570-kg DART spacecraft hit Dimorphos at about 22,530 kilometers per hour (about 14,000 mph). Dimorphos has a mass of approximately six billion kilograms and orbits Didymos once every 11.92 hours. DART's collision is expected to shorten Dimorphos' orbit around Didymos by about 1% or roughly ten minutes. The change of Dimorphos' orbit around Didymos is much easier to measure than if we tried to redirect a single asteroid's orbit around the sun. It takes at least several orbits to measure any sort of change, with many orbits required to get a precise measurement of the change, and there will be extensive cross-checking of the measurements, so it'll probably be a few weeks before the results are ready to be made public. Sorry, this doesn't fit the Hollywood movie timeframe but science deals in reality not movie fiction.

DART took pictures all the way to one second before collision. The pictures show that Dimorphos looks similar to the rubble-pile asteroid Bennu we visited with the OSIRIS-REx spacecraft in 2019 to 2021. OSIRIS-REx collected a sample of Bennu and will return it to Earth on September 24, 2023. Bennu is a PHA with a 1 in 2700 chance of hitting Earth in 2182.

I described Bennu as a rubble-pile which means it consists of small rock chunks held together with very little cohesion by Bennu's weak gravity. Bennu is about 500 meters in diameter. The surface is as loosely packed as a bowl of popcorn or a pit of plastic balls that children jump into. Would such an asteroid survive passage through Earth's atmosphere? Probably not as a single body but millions of small chunks burning up at once in the atmosphere would cause other problems. Basically, ANY asteroid of 140 meters or larger hitting Earth, whether a solid chunk of rock or a rubble pile, is not a good thing!

Tickets continue to be on sale for the October 6th showing of “Dynamic Earth” that describes the various parts of Earth's climate system. Tickets for the following show “Earthquake” on October 20 are also on sale now. 

Tonight the moon is at first quarter phase in the Teapot part of Sagittarius. Between the nights of October 4 and 5, the waxing gibbous moon will pass under Saturn in Capricornus and a bright moon, one day shy of full phase, will be next to Jupiter in Pisces. Despite the abundant reflected sunlight off of the nearly full moon, bright Jupiter will still be visible next to the moon.

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com