Satellite Trails and the Problems They Cause: An Astrophotography Perspective

Understanding Satellite Trails and Their Impact

As more satellites are launched into low Earth orbit—often in the hundreds at a time—the night sky is becoming increasingly crowded. For astrophotographers, this isn’t just about visual clutter; it's a technical challenge that can significantly impact the quality of astronomical imaging.

Satellite trails—those thin, bright lines that cut across your exposures—are now appearing in a wide range of shots, from wide-field Milky Way images to narrowband deep-sky photographs. What used to be a rare disruption is fast becoming a regular and unavoidable issue.

Unlike light pollution, which tends to affect contrast and background brightness, satellite trails interfere directly with the data you’re trying to capture. They don’t only ruin the aesthetic of your image—they often compel you to discard valuable subframes. This can compromise your stacking, photometry, and astrometry efforts. As our orbital space gets more crowded, achieving a clean, uninterrupted view of the universe becomes more challenging.

The Growth of Satellite Constellations

Over the next five years, the global satellite industry is poised for unprecedented growth. According to Goldman Sachs Research, approximately 70,000 low Earth orbit (LEO) satellites are expected to be launched during this period. A staggering 53,000 of these launches are projected to come from China. This surge is primarily driven by the expansion of broadband megaconstellations, such as SpaceX's Starlink and Amazon's Project Kuiper.

The European Space Agency (ESA) forecasts that by 2030, there could be as many as 100,000 satellites in orbit. This rapid increase raises urgent concerns about space congestion, potential collisions, and the proliferation of space debris.

What Are Satellite Trails?

Satellite trails are unwanted streaks that appear in long-exposure astrophotography images. They are caused by sunlight reflecting off satellites as they pass through the field of view. Unlike star trails, which are predictable and due to Earth’s rotation, satellite trails are unpredictable and abrupt.

The image below, taken by amateur astronomer and astrophotographer Aleixandrus (@aleixandrus.astro on Instagram and @aleixandrus on Picastro), highlights the massive issue of satellite trails.

With the rise of mega-constellations like Starlink, the number of satellites in orbit has increased dramatically over the last few years, making trail contamination a more common issue—even for short exposures.

How They Affect Image Quality

At a fundamental level, satellite trails are unwanted light intrusions that disrupt your frame. However, their impact goes beyond mere aesthetics:

Stacking Disruption

Stacking multiple exposures improves the signal-to-noise ratio, but satellite trails introduce artifacts that remain unless you aggressively reject those frames. Even with sophisticated stacking software that includes outlier rejection, strong satellite streaks can leave residual patterns or noise behind. And we don’t want this.

Data Loss

If a satellite crosses a crucial area of your image—like the core of a galaxy or a bright nebula—you might have to discard the whole subframe to maintain data integrity. During multi-hour sessions, losing even a few frames can reduce the depth and detail of your final stack.

Photometric Interference

For those conducting photometry, satellite trails can elevate background levels and contaminate star measurements, especially when a streak overlaps your target star. This leads to inaccurate brightness readings and undermines the scientific value of your capture.

Understanding Photometry

Photometry in astronomy is the precise measurement of brightness, or light intensity, from celestial objects like stars, planets, galaxies, and nebulae. It involves recording how much light an object emits or reflects, usually through different filters (e.g., red, green, blue, infrared). This helps astronomers:

• Track variable stars and supernovae

• Detect exoplanets by observing dips in a star’s brightness during transit

• Study stellar evolution and temperatures

• Estimate distances to objects

• Analyze light curves for scientific modeling

  
  

Accurate photometry requires clean, well-calibrated images. This is why trails from satellites or poor tracking can contaminate results and yield unreliable data.

Astrometric Distortion

Astrometry, which relies on precisely identifying star positions, can also be affected by satellite trails. These trails can confuse detection algorithms or obscure fainter stars, skewing the coordinate data and complicating efforts to match field stars with known catalogs.

Can Software Fix It?

While some modern tools attempt to correct satellite trails during stacking—by rejecting outliers or masking affected pixels—these solutions aren’t foolproof. Masking may blur details or introduce processing artifacts, particularly if the trail crosses an area with valuable data.

Often, the best solution is prevention rather than correction. Ideally, satellites wouldn't be reflective—perhaps that's the way forward!

How to Reduce Satellite Trails in Your Astrophotography Images

While it's challenging to avoid satellites altogether, some strategies can help minimize their impact on your astrophotography:

1. Take Multiple Shorter Exposures

By taking shorter subs, you reduce the likelihood that a trail will ruin a large portion of your data. If a trail does appear, you waste less time by discarding just one frame.

2. Use Stacking Rejection Carefully

Sigma-clipping or other outlier rejection methods can eliminate weaker trails. However, strong reflections may still leave behind artifacts. Be sure to review your subs before stacking.

3. Check Satellite Predictions

Tools like Heavens-Above or Stellarium can help you anticipate bright satellite passes. While not entirely foolproof, they assist in avoiding imaging during peak trail times.

4. Shoot at Darker Angles

Satellite brightness is typically highest shortly after sunset or before sunrise when the satellites are still illuminated by the sun. Imaging later in the night may reduce the chance of bright trails appearing in your shots.

Conclusion: Adapting to a Changing Sky

Satellite trails are no longer rare intrusions—they're an everyday reality in modern astrophotography. While some post-processing techniques can help minimize the damage, there's no true substitute for clean, uninterrupted subs. For those striving to produce high-quality, scientifically useful images, satellite trails pose an increasing challenge.

Have you been battling satellite trails in your images? Share your experience with the Picastro community on the Picastro app—compare techniques, learn from others, and help us adapt to this changing sky.

---wix---