Phobos has an anomalously low density. 1.876 g/cm^3.

In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow . The issue is still not settled – Phobos has a surface composition similar to that of carbonaceous chondrites, with spectra & albedo very similar to those of C- or D-type asteroids.

Ordinary chondrites (think of them as “rocks”) have densities of 3.2 to 3.4 g/cm^3.
Carbonaceous chondrites have densities of 2.1 to 3.4 g/cm^3.
So some of the interior of Phobos is probably empty space.

Mars Express is going in for the closest ever flyby of Phobos tomorrow (29/12/2013), just 45 km from the surface . It’s going to be so close that it can’t take pictures with its highest resolution camera. Instead, pictures have & will be taken on 21 Dec 2013 and 4 Jan 2014. The deflection of the track will be used, over the next few weeks, to find out what the gravity field of Phobos is like and to use this to probe the structure of Phobos’s interior for high and low density regions, to see if it’s a pile of rubble for example.

On the other hand, Phobos is far from being the asteroid with the lowest density . Asteroid 15 Eunomia may have a density as low as 0.96. 45 Eugenia has a density near 1.2. 90 Antiope and 253 Mathilde have densities near 1.3.

> On the other hand, Phobos is far from being the asteroid with the lowest density . Asteroid 15 Eunomia may have a density as low as 0.96. 45 Eugenia has a density near 1.2. 90 Antiope and 253 Mathilde have densities near 1.3.

Perhaps those asteroid density estimates are wildly wrong. Other sources give:
15 Eunomia – 3.1 or 3.8 g/cm^3
45 Eugenia – 1.1 g/cm^3
90 Antiope – 1.25 g/cm^3
253 Mathilde – 1.3 g/cm^3
So only the density given for 15 Eunomia was wildly wrong.

>It’s going to be so close that it can’t take pictures with its highest resolution camera

They need a little snap happy camera like mine. For close-ups it even has a “cuisine” setting.

Does NASA have an Instagram account?

Living inside a hollowed-out Phobos. One of those Space Age predictions that incredibly seemed practical at the time:

I like her space skirt and the little dude fishing.

The detailed tracking of Mars Express during the Phobos flyby was done using the ESA 35 metre antenna near the town of New Norcia in Western Australia, 140 km north of Perth, along the Great Northern Highway.

The reason the scientific instruments couldn’t be used to image Phobos during the flyby was because the high-gain antenna had to be pointed at Earth during the entire flyby, which left the scientific instruments pointed at Mars rather than Phobos.

How did this go then?

dv said:

How did this go then?

Apparently some pilot near Heathrow saw it.