Swift Descending

If you look in the right-hand column of this blog, you’ll see a new widget titled “Swift Observatory.” That’s because of a speedy satellite that is in need of some speedy attention.

The Neil Gehrels Swift Observatory satellite was originally called the “Swift Gamma-Ray Burst Explorer” when NASA launched it in 2004. It has since been renamed to honor its longtime principle investigator, but that original name pretty much explains what it does: It’s a space telescope built specifically to study gamma-ray bursts. These brief but highly-energetic bursts of energy from outer space were discovered in the 1960s by satellites designed to detect nuclear bomb testing, and they have remained somewhat mysterious for decades. In the 1990s astronomers observed that gamma-ray bursts were sometimes followed by an “afterglow” in the X-ray, ultraviolet, and visible parts of the spectrum.

The Swift Observatory was built specifically to observe these emissions. It consists of three instruments: The gamma-ray detector, an X-ray telescope, and an ultraviolet/visible light telescope. When a gamma-ray burst is detected, the satellite can rapidly slew around to point the X-ray and visible light telescopes at the origin point in deep space to gather more data.^[1]The “Swift” in the name refers not to its flight but to the satellite’s ability to quickly rotate to point at a target, which it does with a cluster of flywheels: Spin the flywheel clockwise and the satellite reacts by spinning counter-clockwise. When it’s not observing gamma-ray bursts, astronomers around the world can request that it look at other things in space.

We generally think of space as beginning 100 kilometers (62 miles) above the Earth’s surface. That’s roughly the altitude at which the atmosphere becomes too thin to have aerodynamic effects on a human scale. It’s also the altitude you have to reach to be called an astronaut. But the Earth’s atmosphere doesn’t end at a sharp line. Above 100km the atmosphere is very, very thin, but it still matters to satellites.

As a satellite plows through the scattered air molecules, it bumps them out of the way, and the reaction force from bumping air molecules slowly drains away the satellite’s speed. That loss of speed causes the satellite to lose altitude, descending into a lower orbit. As the satellite descends, its gravitational potential energy is converted to kinetic energy and it actually speeds up.^[2]Orbital flight is counterintuitive: Going faster makes you go higher, getting higher makes you slow down, slowing down makes you go lower, and getting lower makes you go faster. It sounds weird, but that’s the physics. The combination of a higher speed and deeper immersion in the atmosphere makes the drag even worse. Eventually, the satellite’s orbit decays far enough that it re-enters the thick atmosphere near the surface and the extreme drag forces convert all that speed to pressure and heat, causing the satellite to break up, burn up, or both.

To prevent this from happening, many long-duration satellites in low orbit below about 2000 km (1240 mi) have some sort of station-keeping engines that adjust the orbit to counter the effects of drag. Unfortunately, the Swift Observatory’s mission was not originally intended to last long enough for orbital decay to be a factor, so it was never equipped with such engines.

The Swift Observatory has been traveling through the wisps of low Earth orbit atmosphere at about 7.5 kilometers per second (about 17,000 miles per hour) for a little over 21 years. That’s over 3 billion miles of circular flight around the Earth. The atmospheric drag from that long flight has brought the Swift Observatory down from its initial orbital altitude of 600 km (370 mi) to about 400 km (250 mi) today. The effects of drag are uncertain because upper atmospheric “weather” varies over time depending on a variety of conditions, but aerospace engineers currently think it will re-enter the atmosphere and burn up by the end of 2026.

Somewhat surprisingly, there’s a rescue plan. A relatively new company called Katalyst received a $30 million contract in September to try to move the Swift Observatory to a higher orbit. They’re going to use a Pegasus XL rocket, dropped from a plane at about 39,000 feet, to boost Katalyst’s robotic repair spacecraft into orbit to rendezvous with Swift, grab onto it, and push it into a higher orbit. The robot has been used before to repair some satellites, but this is the first time it will attempt to move a satellite. This is also the first time anyone has attempted to move a satellite that was never intended to be moved.

Most of the cost of the Neil Gehrels Swift Observatory, about $500 million, was spent at the beginning to build it and put it into orbit. Since then, the operating costs have been comparatively small, and even this $30 million mission is well worth it, compared to the cost of launching a new observatory.

This is a race against time, or rather against orbital decay. Engineers working on the project estimate that once the Swift satellite’s orbit drops below 300 km — which is expected to happen in the fourth quarter of 2026 — a rescue will no longer be feasible with the technology available. The current plan is to launch the rescue mission in June of 2026, which is a crazy short lead time for a space flight. Apparently, Katalyst was planning a test flight in June anyway, so they’re repurposing it to be an actual mission, which allows them to reuse a lot of the planning and equipment. Still, this is a very ambitious mission.

In honor of the Swift Observatory, and the plan to rescue it, I’ve added a widget in the right-hand column that tracks its orbit. The data comes as standard two-line element sets (TLEs) from U.S. Space Force, which maintains watch on tens of thousands of objects in low Earth orbit. The TLEs are then fed into an SGP4 propagation model, which continuously calculates the instantaneous position of the satellite for display in the widget. There’s a polling agent on the back end that checks for updates from Space Force several times per day.

The positioning data is pretty standard — latitude and longitude are the position on Earth over which the satellite is passing, altitude is how high it is above the surface, heading is the direction of travel, with 0 being due north, and speed is the rate of travel along that heading. The orbital elements are a little more technical, and I’m only displaying some of the simpler ones, but I do want to draw your attention to perigee, which is the height above the Earth at the point of closest approach.

You may notice that the displayed altitude is often below the perigee. This seems like it should be impossible because the perigee is supposed to be the lowest point of the orbit. The short explanation is that the elements in a TLE set are idealized mean values intended to serve only as input to the SGP4 model, which adjusts for some of the details of orbital flight around the Earth. So sometimes the altitude of the more realistic model strays below the altitude implied by the perigee element. To help clarify, I’ve added a Lowest Altitude value that is calculated by finding the lowest calculated altitude over a 24 hour period, which is what I think we really want.

Note that there’s a menu in the upper right corner of the widget that lets you get more information about the Swift Observatory’s orbit, including a website showing you the HTML to add this widget to your website…assuming I got it right. I’ve never tried something like this before.

Finally, I wish everyone at NASA, Katalyst Space Technologies, and the Swift Mission Operations Center the best of luck. Here’s hoping all goes well and the Swift observations keep rolling in for a long, long time.