Near-Earth asteroid (NEA) 2012 TC4 made a very close pass by the Earth last night on October 11, 2017 at 10:40 pm Pacific Daylight Time. It came within roughly 27,000 miles (44,000 km) from Earth’s surface or roughly five times closer to Earth than the Moon! It was travelling across the sky at roughly the diameter of the Moon every minute during its closest distance to the Earth, making it a tricky target to observe as it moved so quickly across the night sky.
2012 TC4 was discovered in 2012 and was next visible in the night sky in early August 2017. When it was discovered in 2012, astronomers didn’t know its orbit well enough to know if it would potentially have an impact with the Earth in 2050, so getting to observe it again as it came so close this time around was a chance to either rule out or confirm the future impact. The vast amount of data collected on this object over the last month, which includes data from LCO, has ruled out an impact that had a 1 in 250 chance of hitting the Earth in 2050. NASA and asteroid trackers around the world have been using this encounter with 2012 TC4 to test their ability to operate as a coordinated International Asteroid Warning Network.
Observations ramped up in early September 2017 to get ready for the close approach in October. LCO observed 2012 TC4 from mid-September through its close approach last night (with a heavy emphasis on the past week) using LCO telescopes in Chile, Texas, and South Africa. We also re-measured old LCO data from 2012 when the asteroid was last seen to better pinpoint where in the sky 2012 TC4 would first be seen in 2017. The NEO community collected 540+ observations of this object over a period of five weeks leading up to the close approach. These observations were mostly in optical (visible) light and included two radar observations that helped to constrain the object’s size and rotation period.
2012 TC4 is an elongated asteroid that is about twice as long as it is wide and roughly 33 feet (10 m) across. It has a rotation period of about 12 minutes and has what’s called non-principal axis rotation, which means it rotates similarly to a tumbling football that topples end over end.
2012 TC4 appears to have a low density, which points to is being what is called an enstatite achondrite, which is a fancy way of saying it’s a rock with bits of melted metal and organic material inside, making it an igneous or metamorphic rock. These types of asteroids have had a violent history and experienced an event in their past that have caused parts of their interiors to be partially melted. This can sometimes happen when two asteroids impact each other, which releases enough energy in the form of heat to partially melt the insides of the asteroids.
Using LCO’s multiple sites around the globe, we can measure the distance to 2012 TC4 during our observations using was is called a parallax measurement. Parallax is the difference in apparent motion of an object along two different lines of sight. Farther away objects move a shorter distance than does a nearer object when viewed from different positions on the Earth. Our parallax measurements made with LCO data from our Texas site and Chile site two days before the closest approach showed the distance to be 1,204,980 km which is within 180 km of the JPL/NASA distance measurement. Below is a time lapse movie of 2012 TC4 from two sites at LCO (Texas and Chile) from last night. Seeing different stars in the backgrounds of the two time lapses is an example of parallax, similar to the way background objects appear to move when you hold a finger at arm’s length and close one eye and then the other.
Download hi-res movie of 2012 TC4 (6.2Mb).
This work is supported in part by a NASA Near Earth Object Observations (NEOO) Grant (NNX14AM98G) to LCO.
