Herschel meets asteroid – close encounter of the fast kind

November 16, 2011
It is the closest, fastest and smallest object so far seen by Herschel. In fact, such observations were not even foreseen due to technical constraints. The asteroid 2005 YU55 is moving on the sky with a speed exceeding by far Herschel's possibilities to track it. But with the help of a little trick scientists at the Max Planck Institute for Extraterrestrial Physics and at the European Space Astronomy Centre in Spain were able to use Herschel to catch a glimpse of this asteroid shortly after it had crossed the Moon's orbit. With these observations the astronomers could determine some thermal and physical parameters. As it turned out, 2005 YU55 is somewhat smaller than had been assumed and probably might be a loose assembly of boulders, pebbles and dust.

The minor planet (or asteroid) 2005 YU55 was first detected in December 2005, hence its name, and on 9 November 2011 passed the Earth at 00:28 CET in 0.85 lunar distances (324,600 km). The Herschel observations took place one day later from a safe distance of 805,000 km (see note 1).

As Herschel could not track the object on the sky, the scientists  observed an elongated fixed sky field on the asteroid's track at a pre-calculated time in a standard way. 2005 YU55 dashed - exactly as predicted - right through this field (see note 2). Data reduction was done in a special way, centering the more than 3500 individual images from the two four-minute observing runs on the true object position. The final images show 2005 YU55 perfectly focused and centred in all three frequency channels (see the reconstructed image for 70 microns in fig 1.).

This was only possible because the PACS instrument used for these  infrared observations was designed and built at MPE, and the scientists therefore have precise knowledge of the instrument and its capabilities. A careful analysis of the data in each of the three infrared filters (at 70, 100 and 160 microns) resulted in an estimate for several asteroid parameters.

Although 2005 YU55 is the smallest object seen by Herschel so far, it is one of the largest bodies with very close Earth-encounters. Surprisingly, however, the new measurements indicate that is has to be smaller than previously thought. Depending on its orientation the scientists estimate  about 310 m for the object's diameter (retrograde) or about 340 m (prograde). In any case the minor planet has to be smaller than the 400 m diameter derived from NASA radar observations in 2010; an object with this size would have produced much higher fluxes.

From the infrared observations the scientists also determined the albedo (6%), i.e. 2005 YU55 is very dark and reflects only 6% of the sunlight. The rest is absorbed and converted to infrared radiation. This indicates a dark, carbonaceous surface consistent with a previous classification as  C-type object. About 75% of all minor planets belong to this class.

The thermal properties seem to be very similar to the ones found for Itokawa, a small body visited by the Japanese mission Hayabusa, which turned out to be a rubble pile. Our findings indicate that 2005 YU55 might also be a loose assembly of boulders, pebbles and dust and not a single monolithic rock in space.

The Herschel observations also help to improve the orbit calculations of 2005 YU55 (see note 3). The minor planet does not pose a danger for Earth, at least within the next few decades, for which we can predict its orbit with high accuracy. In 2029 this little asteroid will have a close encounter with Venus (at 340 000 km), afterwards the orbit is not perfectly predictable and it is unlikely that 2005 YU55 will cross the Moon's orbit again in 2041. (See note 4.)

In case of 2005 YU55 we can rest assured that we are safe for the next few decades. And, instead of working out deflection concepts, we can concentrate on the more scientific aspects of characterising this exotic and very interesting minor body.


1. Herschel is orbiting the so-called Lagrange point L2, some 1.5 million kilometres outside the Earth orbit. The viewing direction towards the Earth and the Sun is off-limits for Herschel. Therefore the minor planet could be observed only one day after the Earth fly by, at a safe distance of 60 degrees from the Sun.  

2. The perfect execution of these challenging observations was made possible by the highly motivated and experienced Herschel Science Centre team at ESAC.

3. The physical and thermal properties of the object have to be taken into account when determining the non-gravitational forces to achieve a high precision for the orbit predictions.

4.If an object of this size were to hit the Earth, it would produce a crater of a few kilometre diameter and a few hundred metre depth (depending also on the internal material strength), accompanied by earthquakes, tsunamis and enormous dust pollution in the atmosphere with global effects over many years.

Müller, Thomas, MPE, Germany
Kidger, Mark, ESAC, Spain
Altieri, Bruno, ESAC, Spain
Lorente, Rosario, ESAC, Spain
O'Rourke, Laurence, ESAC, Spain
Metcalfe, Leo, ESAC, Spain
Pal, Andras, Konkoly, Hungary

Futher information:

Wikipedia website about 2005 YU55:

Radar observations by NASA:

last update 2011-11-16 by H. Steinle

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