National Aeronautics and Space Administration Logo
Follow this link to skip to the main content NASA Banner
Solar System Exploration
Science & Technology
Join NASA in Measuring an Asteroid
Artist concept of a narrow asteroid belt orbiting a star.
Artist concept of a narrow asteroid belt orbiting a star. (NASA/JPL-Caltech)

By Janet Anderson
Marshall Space Flight Center, Huntsville, Ala.

Rhea and Titan
A Saturn moon, Rhea, emerges after being occulted by the larger moon Titan. (NASA/JPL/Space Science Institute)

July 3, 2011: On the evening of July 3 at 11:05:30 p.m. EDT -- at a distance of 280 millon miles into space that poses no threat to Earth -- 52 Europa will pass in front of star TYC 0292-00339-1 in the constellation Virgo. The asteroid will eclipse the star's light for 17.9 seconds in a process known as occultation. Here on Earth, astronomers can measure the exact length of time the star's light is blocked and use those calculations to help verify the size of 52 Europa. They'll also have a live Web chat to discuss how to measure the asteroid 52 Europa on the evening of July 3, 2011, from 9 p.m. to midnight EDT. You can view a map of the occultation's path across North America below.

Ahead of the live Web chat on July 3, you may already have some questions about occulations, eclipses, transits, and 52 Europe. Below is some information that you may find useful ahead of the event.

What is asteroid 52 Europa?

By volume, asteroid 52 Europa is the seventh largest asteroid in the asteroid belt, with a diameter of about 300 kilometers/186 miles. It was discovered in 1858 by Herman Goldschmidt from his balcony in Paris and named for Europa, a conquest of Zeus in Greek mythology. Highly porous, the asteroid is classified as a dark C-type asteroid, meaning the surface contains a large amount of carbon. This gives 52 Europa a dark surface that reflects only a small amount of light that falls on it.

Generally speaking, what is an occultation? How is that different from an eclipse or transit?

For practical purposes, an occultation is a celestial event in which a larger body covers up a distant object. Compare this to an eclipse, where the bodies are generally nearly equal in angular diameter, and a transit, where a much smaller body passes in front of a large object, such as transits of the sun by Mercury and Venus. During an occultation the occulting body blocks the light of the more distant celestial object.

As an example, when an airplane flies directly between an observer on the Earth's surface and the sun, its shadow passes right over the observer blocking the sun's light momentarily. The airplane has occulted the sun. Now move the observer just 100 meters away, and the airplane's shadow would miss the observer entirely. If the observer had arrived 30 seconds later to that position he or she would not see the occultation of the sun by the airplane.

What is an asteroid occultation?

An asteroid occultation occurs when an asteroid passes directly in front of a star causing the star to disappear for some span of time -- usually from several seconds to a minute or more. An instantaneous drop in the light level of the star of the merged object -- star + asteroid -- can be observed. This fading is caused by the asteroid's own light replacing the light of the brighter star. If the asteroid is too faint to be seen, then the star vanishes completely while the asteroid is in front of it. Asteroid occultation events provide a unique opportunity for ground-based observers to directly determine the sizes and shapes of these mysterious objects.

With thanks to the International Occultation Timing Association's Manual (PDF, 10.58 MB).

What is the path of the July 3 occultation of asteroid 52 Europa and star TYC 0292-00339-1?

Below is a map showing the path of occultation. This event will not be viewable with the naked eye, but you can see it through your telescope -- or visit this NASA page on July, starting at 9 p.m. EDT, for a Ustream view of the occultation. The 17.9-second event will occur on July 3 at 11:05:30 p.m. EDT.

Map showing the path of occultation for 52 Europa.

Last Updated: 28 June 2011

Science Features
Astronomy Features
Technology Assessment Reports
Sungrazing Comets


Best of NASA Science
NASA Science Highlights
Technology Features
Lectures & Discussions

Awards and Recognition   Solar System Exploration Roadmap   Contact Us   Site Map   Print This Page
NASA Official: Kristen Erickson
Advisory: Dr. James Green, Director of Planetary Science
Outreach Manager: Alice Wessen
Curator/Editor: Phil Davis
Science Writers: Courtney O'Connor and Bill Dunford
Producer: Greg Baerg
Webmaster: David Martin
> NASA Science Mission Directorate
> Budgets, Strategic Plans and Accountability Reports
> Equal Employment Opportunity Data
   Posted Pursuant to the No Fear Act
> Information-Dissemination Policies and Inventories
> Freedom of Information Act
> Privacy Policy & Important Notices
> Inspector General Hotline
> Office of the Inspector General
> NASA Communications Policy
> NASA Advisory Council
> Open Government at NASA
Last Updated: 28 Jun 2011