What’s NASA crashing into next?

Last year the infamous NASA LCROSS mission gained attention as the unmanned space probe was set on a collision course with the lunar surface.  On October 9 2009, viewers watched as footage of the crash event was streamed back to Earth.  The mission crashed a rocket into the moon’s southern pole while the LCROSS craft with all the sensors and recording equipment followed behind, analyzing the cloud of material kicked up by the impact, looking for water.  And water was found: on November 13 2009, scientists confirmed the presence of water in data collected from the mission.

It seems that sometimes you just have to be there. Being in direct contact with the subject of research (termed “in situ” observations) has its benefits. 

Amateur astronomers lined up telescopes and hundreds of people watched the LCROSS impact on the internet. Although not visually as exciting as many hoped for, the data obtained seems to have justified NASA’s methods of “bombing the Moon” as so many media outlets described. If you were underwhelmed by LCROSS, hold onto your hats because NASA is at it again. And this time it’s shooting for the stars. Or star, actually: our Sun. 

Artist's impression of SPP. Credit: JHU/APL

NASA’s “Solar Probe Plus” (SPP) is slated for launch in 2018 and involves sending an unmanned probe into the Sun’s atmosphere (the corona) in order to better study its properties. The probe itself will be roughly the size of a small car and as you might imagine, will face scorching temperatures during the planned 6 year, 321 day mission. 

At its closest approach the SPP will come within 4 million miles from the Sun’s surface where it will be subject to intense bursts of radiation and temperatures in the range of 2550 degrees Farenheit (1399 °C,  1672 K).  Compare this with the previous record-setters for closest solar approach, the Helios probes, which came within roughly 27 million miles (43.5 million km) of the Sun in 1976.  At SPP’s closest approach, the intensity of solar radiation that it will experience is over 500 times greater than at the Earth’s distance from the Sun.

To protect the SPP from such intense temperatures, the probe will be equipped with a revolutionary carbon-composite heat shield to protect it during its fiery orbits. This material will be similar to that which protects the Space Shuttle from the heat of atmosphere reentry. The probe itself will be solar powered, getting its electricity from liquid-cooled solar panels that can retract behind the heat shield whenthe sunlight becomes too intense. 

The sun's wispy corona can be seen during a solar eclipse.

Before the science can begin, the Solar Probe Plus has a journey ahead to bring it into an oval orbit around the Sun. This involves 7 “fly-by” approaches to Venus to adjust its orbit.  This can be seen as the opposite of the traditional “gravity assist” where probes like Cassini have orbited planets to increase their speed so as to reach further out into the Solar System.  When we send spaceprobes towards the Sun, they are greatly accelerated by the Sun’s gravitational well and so they must be slowed down to enter into a useful orbit.  As the schedule stands, the SPP would make its closest approach to the Sun in late 2024. During its 24 orbits of the Sun the probe comes ever closer to the Sun so that at its closest position it will be well within Mercury’s orbit, about 8.5 solar radii or 3.7 million miles from the Sun’s surface. This is 7 times closer than any previous spacecraft. 

Another first will be its speed of orbit: on its closest approach the SPP will be flying past the sun at a speedy 125 miles per second (450 000 mph, 724000 km/h). 

That’s fast. Roughly 3 times as fast as any other space probe has travelled. Compare with the Space shuttle and International Space Station which orbit the Earth at approximately 17,500 mph (28000Km/h). 

Five scientific experiments have been selected for the mission and range from counting the most abundant particles present in the solar wind to a telescope that will make 3D images of the Sun’s corona. 

So why the Sun? Don’t we already know a lot about our closest star? The answer is yes – and no. Some important questions which scientists studying the Sun (heliophysics) have been searching for answers to for decades remain. 

Two of the most important of these questions may be answered by NASA’s Solar Probe Plus mission: 

  1. Why is the sun’s outer atmosphere, the corona,  so much hotter than the sun’s visible surface? (the coronal heating problem)
  2. What drives the solar wind – the streams of highly energetic particles that blow out at speeds of a million miles an hour, affecting the Earth and entire Solar System

In addition, during its time orbiting the Sun, the SPP is likely to encounter several “solar storms”.  Researchers suspect that many of the most dangerous particles produced by solar storms are energized in the corona—just where Solar Probe Plus will be. The spacecraft will have an up close and personal view of these Solar Energy Particle (SEP) events, enabling scientists to better understand, characterize and forecast the radiation levels that might threaten the heath and safety of future space explorers. 

I go into some of the science involved below. This is without doubt an exciting mission and the first time humankind has dared to rendezvous with a star.  In itself that makes this adventure quite remakable. 

The anatomy of our sun. (image: original by C Qualtrough)

Some of the science behind the mission 

The first  of the two questions stated above arises when we look at the Sun’s composition: 

The Sun’s Core is at a temperature of approximately 13.6 million Kelvin (~25 million degrees Farenheit). The optical surface of the sun (the photosphere) is known to have a temperature of approximately 6,000 K ( 10340 degrees Farenheit, 5700°C).  Above it lies the solar corona, rising to a temperature of 1,000,000–2,000,000 K. 

Herein lies the problem: how can the corona of the sun be millions of kelvin hotter than the lower surface of the sun (photosphere)? 

The second law of thermodynamics can be stated in the form attributed to Rudolf Clausius: 

“Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature.”

In other words, heat would normally be unable to flow from the solar photosphere to the hotter corona, so we must conclude that something other than direct heat conduction must be responsible for the high temperatures in the corona. 

At present the two theories considered the most likely involve 

1)    Wave heating 

Several different types of waves can exist in the Sun’s plasma environment and are similar to sound waves in air. Two specific types of waves: Alfvén waves and magneto-acoustic waves could potentially carry energy from the Sun’s photosphere up into the corona. However, magneto-acoustic waves seem unable to carry enough energy upwards into the corona without  being reflected back to where they came from due to low pressure. 

And Alfvén waves may reach the corona but dissipate energy far too slowly and insufficiently to account for how much heating we observe. 

We’ve also seen quite a lack of direct observations of uniformly energetic waves in the Sun’s corona so far — which we’d expect if the theory holds true. 

2)    Magnetic reconnection (nanoflares) 

This theory relies on the Sun’s magnetic field present in the Corona. In fact, as well as the magnetic field loops we associate linking active parts of the Sun, we now know that the Sun’s entire surface contains small “patches” of magnetic fields. A patchwork of small magnetic fields and loops, now dubbed the “magnetic carpet”, can appear and disappear again on timescales on the order of ~40 hours (see figure below). 

Credit: SOHO consortium, ESA, NASA
The "magnetic carpet" of the sun's surface. Black and white spots represent magnetic field concentrations with opposite orientations, called polarity. Each spot is roughly 5,000 miles across. The loop joining these regions extend from surface into the corona.

A magnetic field induces an electric current (as we see in generators). Sometimes in a plasma, their electric currents “collapse” and the magnetic field lines which were maintained by those currents “reconnect” to other poles and energy is released as waves and heat. 

No one really knows what could produce these magnetic patches, which vary on such short timescales. Professor Edward Spiegel from Columbia University has suggested that the patches are produced by small dynamos located just beneath the surface of the Sun. But there is no proof so far. 

It is also possible that a combination of these theories provides the answer. 

The second question that the Solar Probe Plus hopes to shed light on is that of how the solar wind is driven. The solar wind streams  off of the Sun in all directions at speeds of about 1 million miles per hour (400 km/s). Originating in the corona, the temperature of the solar wind helps it escape the sun’s gravity. Determining how the corona is heated is important in also understanding how and where the coronal gas is accelerated to such high velocities. Additionally the solar wind is NOT uniform and has both slower and faster components. Determining the mechanisms for both the slow and fast components of the solar wind is of great scientific interest. 

In both problems, the fundamental role of the sun’s magnetic field in shaping dynamical processes on all scales has become very apparent. 

Want to know more about spacecraft that have crashed into or landed on solar system bodies? Check out a list here