2 Pallas

2 Pallas (minor-planet designation: 2 Pallas) is the third-largest asteroid in the Solar System by volume and mass. It was the second asteroid ever discovered, found after Ceres. Scientists think it might be a leftover piece of a small planet that never finished forming. Like Ceres, it is made of materials found in certain meteorites, but it has less water than Ceres. Pallas is very big—it’s about three-quarters the size of Vesta and makes up around 7% of all the material in the asteroid belt.

Long ago, when planets were being made, many pieces like Pallas came together to form bigger worlds. Most of these pieces became part of the planets we see today, but some didn’t. Pallas, along with Vesta and Ceres, is one of the few big pieces that survived from that early time, staying within the orbit of Neptune.

Pallas was discovered in 1802 by a German astronomer named Heinrich Wilhelm Matthias Olbers. At first, it was thought to be a planet, just like the other early asteroids. But as more and more of these small worlds were found, scientists began to call them “asteroids” instead.

Pallas has a very tilted path around the Sun, making it harder for space ships to visit. Because of this tilt, Pallas can get very close to some stars—on October 9, 2022, it passed very near the bright star Sirius, closer than any planet ever can.

History

On the night of April 5, 1779, the astronomer Charles Messier saw an object that looked like a star while tracking a comet. Later, in 1801, another astronomer named Giuseppe Piazzi found a moving object that wasn’t a comet. This was the first asteroid ever discovered and was named Ceres.

Not long after, another astronomer named Heinrich Olbers noticed yet another moving object nearby. This was the asteroid Pallas, the second asteroid to be discovered. Finding Pallas sparked excitement because some astronomers had guessed there might be a planet between Mars and Jupiter. Scientists estimated Pallas to be very large at first, but later measurements gave a more accurate size.

The orbit of Pallas was calculated by Carl Friedrich Gauss, showing it takes about 4.6 years to go around the Sun, similar to Ceres. Pallas also has a tilted path compared to most objects in space.

Orbit and rotation

Pallas moves in a special way around the Sun. Its path is tilted and not a perfect circle, even though it is far from the Sun like most objects in the asteroid belt. Pallas also tilts on its side by 84°, meaning that during its "year," some parts of it get constant sunlight or darkness for a long time — even up to two of our years near the poles.

Near resonances

Pallas shares its orbit with another big object called Ceres, though this is likely by chance. Pallas also has special patterns in its orbit with Jupiter, happening roughly every 91,000 years and every 83 years.

Animation of the Palladian orbit in the inner Solar System 

-     Pallas
-     Ceres
-     Jupiter
-     Mars
-     Earth
-     Sun

-

An animation of Pallas's near-18:7 resonance with [Jupiter](/wiki/Jupiter). The orbit of Pallas is green when above the ecliptic and red when below. It only marches clockwise: it never halts or reverses course (i.e. no [libration](/wiki/Libration)). The motion of Pallas is shown in a reference frame that rotates about the [Sun](/wiki/Sun) (the center dot) with a period equal to Jupiter's orbital period. Accordingly, Jupiter's orbit appears almost stationary as the pink ellipse at top left. [Mars](/wiki/Mars)'s motion is orange, and the Earth–Moon system is blue and white.

Transits of planets from Pallas

From Pallas, we can sometimes see planets like Mercury, Venus, Mars, and Earth passing in front of the Sun. Earth last did this in 1968 and 1998 and will do so again in 2224. Mercury did so in October 2009. Venus will next do this in 2123, and Mars will in 2759.

Physical characteristics

The mass of 2 Pallas (dark green) compared to other large asteroids and the remainder of the Main Belt. The unit of mass is ×10¹⁸ kg.

1. 2 Pallas 204 (8.52%)
2. 4 Vesta 259 (10.8%)
3. 10 Hygiea 87 (3.63%)
4. 704 Interamnia 35 (1.46%)
5. 15 Eunomia 30 (1.25%)
6. other ≈ 841 (35.1%)
7. 1 Ceres 938 (39.2%)

Pallas is the third-largest asteroid in our solar system. It is slightly smaller than Vesta but still very big. Pallas is made of materials similar to certain meteorites found on Earth, but it has less water than Ceres, another large asteroid.

Pallas is covered in craters from impacts. It is not perfectly round, suggesting it may have been hit by a large object in the past. Its surface has one bright spot and many large craters, some named after ancient weapons.

Satellites

Scientists thought they saw a small moon around 2 Pallas in 1978, but later studies showed this was not true. In 1980, they thought they found a bigger moon, but that idea was also proven wrong with more data.

Exploration

The asteroid 2 Pallas has never been visited by a spacecraft, but there have been ideas to send one. One idea was for a spacecraft called Dawn, which visited 4 Vesta and 1 Ceres, to also fly by Pallas. This wasn't possible because Pallas orbits at a steep angle. Another idea was a mission called Athena SmallSat, which would have flown by Pallas in 2022. However, this mission wasn't chosen because other ideas were picked instead. The people who suggested it called Pallas the "largest unexplored" main-belt protoplanet.