High density asteroid our ancestor?

In July 2010, the ESA’s Rosetta spacecraft flew by the asteroid 21 Lutetia, the largest asteroid visited by mankind until the Dawn mission went into orbit around Vesta in July 2011.

Now, over a year later, scientific analysis from Rosetta’s data has been completed and results published in the JournalScience suggest Lutetia formed during the birth of the Solar System.

 Most asteroids are large piles of rubble, weakly bound by gravity and easily torn apart by collisions. Large asteroids (more than 100km in diameter) however, survive most collisions with smaller objects intact, so it is believed they remain unchanged since the birth of the solar system.

As Rosetta approached Lutetia, the gravitational pull of the massive asteroid accelerated it. The additional Doppler shift of the spacecraft radio signals imposed by 21 Lutetia’s gravitational perturbation on the flyby trajectory was used to determine the mass of the asteroid at 1.7 × 10¹⁸ kilograms. The volume was then estimated using Infrared remote imaging on board the craft. This allowed the Rosetta team to calculate the asteroid’s density, which at 3400 kg/m3, has surprised everyone, as similar high bulk densities are known for the asteroids 4 Vesta, 16 Psyche, and 20 Massalia, all of which are much larger than Lutetia. Bulk densities of more primitive asteroids are in the range of 1200 kg/m3 to 2700 kg/m3.

This supports the suggestion that Lutetia formed as a solid object and does not have a porous, debris-like structure. Moreover, the density measurements indicate a metal core – possibly iron rich. Other surface processes such as landslides and the presence of regolith – a dusty material formed over a long time by the shattering of rock by cosmic rays and micrometeorites, bolster confidence in this theory.

Rosetta is now in hibernation till 2014, but its data should help us have a clearer picture of the nature of bodies involved in planetary formation.