Holiday space shows: an asteroid and a comet


Caption: Comet Hale-Bopp, 1997.


On December 24 – Christmas Eve for many parts of the world – near-Earth asteroid 2003 SD220 will make a so-called “close approach” to Earth.

On that day, this asteroid will pass by Earth at a distance of 6.8 million miles (10.9 million kilometers – or, using observers’ preferred unit of measurement, 28.4 lunar distances – the distance from Earth to the Moon, or 238,900 miles/384,000 kilometers).

A near-Earth asteroid is an asteroid whose orbit periodically brings it within approximately 121 million miles (195 million kilometers) of the Sun – that’s within about 30 million miles, or 50 million kilometers, of Earth’s orbit.

So why is the predicted Christmas Eve pass of 2003 SD220 called “close”?

For asteroid watchers, a “close approach” is a predicted event in which an asteroid passes within the orbit of Earth’s Moon. Some passes of larger NEOs – such as 2003 SD220 – close to the Earth-Moon system but not between the two bodies are also called close approaches.

The December 24 flyby (my preferred term, though “pass” is probably more accurate) of 2003 SD220 on its orbit around the Sun has been drawing a lot of media attention for the past few weeks. However, this asteroid is only one of six known near-Earth asteroids that are predicted to make “close approaches” to Earth on December 24. See the Jet Propulsion Laboratory’s (JPL’s) Close Approach Tables for details. (FYI: JPL measures close approach distances as the distance between the Earth center and asteroid center.)

So why is 2003 SD220 drawing all the attention?

It’s likely due to its size. It’s in the one-kilometer range. Asteroids that are one kilometer or larger in size are the ones that scientists and politicians have decided to worry about – or, rather, they’re not worrying about the asteroids but about possible impacts of such objects with Earth. (FYI: According to JPL’s Sentry impact risk table, no known asteroids are predicted to impact Earth for the next 100 years.)

NASA started its near-Earth object (NEO) observations program in 1998 in response to congressional direction: a 1994 request from the House Committee on Science, Space, and Technology to develop a plan to discover, characterize and catalog within 10 years comets and asteroids larger than one kilometer in diameter; and a 1998 congressional directive to conduct a program to discover at least 90 percent of one-kilometer or larger NEOs within 10 years. (This goal was met in 2010.)

JPL’s Close Approach Tables report 2003 SD2003’s size as 1.1-2.5 kilometers. However, this size estimate has been refined due to further observations. It may well be that 2003 SD220 could be smaller than one kilometer.

According to a web site maintained by JPL’s Lance Benner – an expert at radar observations of near-Earth asteroids – “nothing is known about [2003 SD220] other than its absolute magnitude [brightness] of 16.9, which suggests a diameter of about 1.3 km.” Benner has also posted an update on the estimate of 2003 SD220’s size. NASA’s NEOWISE mission detected the asteroid on November 16, and preliminary results of that observation, he reports, “suggest the diameter is roughly 0.7 km.”

Astronomers, professional and amateur around the world, are keeping an eye on this asteroid. By December 24, we may have a more accurate assessment of its size.

According to Benner, “The 2015 apparition [of 2003 SD220] is the first of five encounters by this object in the next 12 years when it will be close enough for a radar detection. By obtaining radar ranging measurements at each observing opportunity, it may be possible to…obtain an estimate of the object’s mass, information that is invaluable for understanding the object’s bulk density and internal structure…. In 2018…the asteroid will make a much closer approach within 0.019 au….”

FYI: As of November 1st, 2015, 13,280 NEOs (including 13,176 near-Earth asteroids) have been discovered – about 98 percent of them by NASA-funded surveys. About 875 of these NEOs are one kilometer or larger in size – a quantity that accounts for more than 90 percent of the estimated population of NEOs in this size range.

Now on to comets….

In the early hours of January 1, 2016, Comet Catalina (a.k.a. Comet C/2013 UQ4) is predicted to reach peak brightness, making it visible with binoculars or even to the naked eye, depending on where you are and what the weather’s like. This report on includes links to a number of sources of information about the comet.

My last two comet sightings were Hyakutake in January 1996 (I still remember how cold it was outside, but it was worth it) and Hale-Bopp – for days and days – in spring 1997. Wow.

Happy holidays, and enjoy the space show!


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