Mercury Spacecraft Crashes with the Energy of Two Thousand Falling Trees

For the past eight years NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft has been sending back gigabytes of images and data from the Sun’s closest companion, Mercury. Today this will come to a fiery end when the fuel-depleted spacecraft crashes into the surface of Mercury in a planned end to the highly-successful mission.

Craters on Mercury colored by the type of material. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The resulting crater, estimated to be about 50 feet wide, will join the countless other craters that MESSENGER has imaged in great detail. The data from MESSENGER has revealed many surprises over the years including ice in the shadowy craters at Mercury’s south pole, a misaligned magnetic field compared to the spin axis of Mercury, an excess of volatile elements like potassium and sulfur that shed light on how Mercury may have formed, and unusual “hollows” or depressions etched into the surface that haven’t been seen on any other planet.

An enhanced-color view of Mercury. Credit: Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

To get a sense of scale of MESSENGER’s impact, let’s compare it to a falling tree (as an Oregonian from a logging town, this was the first analogy that came to mind).

We need two things to make this comparison: the energy delivered by MESSENGER on impact, and the energy delivered by a typical tree as it falls.

1. Energy of MESSENGER impact

NASA estimates that the speed of MESSENGER just before impact today will be 8,750 miles per hour (or 3.91 km/s). The mass of the spacecraft (assuming fuel depletion) is 484 kilograms, which is about the same as a Harley-Davidson motorcycle. From the speed and the mass we can work out the kinetic energy upon impact, which will be the same as the energy delivered to Mercury’s surface.

This is roughly equivalent to exploding a ton of TNT, but let’s continue with a tree comparison.

2. Energy of a falling tree

The energy of a falling tree is a little bit trickier to work out because we have to assume a few things about a “typical” tree in order to work out its mass.

Let’s say we’re measuring the energy of a large pine tree with a height (L) of 30 meters and radius (r) of half a meter as it comes crashing to the ground. For simplicity, let’s say the diameter is constant so that the tree is a perfect cylinder. The density pine wood is about 500 kg/m3. With the density and volume of a cylinder we can work out the tree’s mass:

The energy of the falling tree will be the same as its potential energy when it stands upright. This comes from the tree’s mass, the acceleration due to gravity (g), and the average height (h) of the tree above the earth (in our case, this is half the tree’s height, or 15 meters).

Thousands of Falling Trees

Comparing the energy of the MESSENGER crash to the energy of a falling pine tree, it would take about 2,200 large pine trees crashing to the ground simultaneously to match the energy of the spacecraft’s impact.

That said, the energy of MESSENGER’s impact is relatively small compared to man-made explosives here on Earth (the atomic bombs dropped in WWII released more than ten thousand times more energy than today’s impact) and will form only a small crater about 50 feet across.

The impact is estimated to occur about 3:30 PM Eastern Time today but it will not be visible from Earth at the time of impact. However you can watch a live stream of the events leading up to the impact from 3 PM Eastern Time here.

By Tamela Maciel, also known as “pendulum”

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