What is RHESSI: The satellite that helped scientists study solar flares will crash on Earth after 21 years in orbit/ How ‘space junk’ became a problem for human activities in space

The Rhessi satellite, launched into space by the US National Aeronautics and Space Administration (NASA) more than 20 years ago, will crash to Earth sometime Wednesday, but experts say there is very little chance of anyone being hit, reports the US newspaper New York Post.

The device, called the Reuven Ramaty High Energy Solar Spectroscopic Imager, or RHESSI for short, weighs more than 270 kilograms and is estimated by the agency to burn up almost entirely on re-entry, but there is a possibility that pieces of the satellite could “survive” the fall and hit the ground. The odds of hurting someone are 1 in 2,467.

The satellite that made history

Although it’s now basically just a hunk of metal that can’t do anything except hurt someone, RHESSI started out as an important step in exploring and understanding space.

The spacecraft was launched aboard a Pegasus XL rocket from Orbital Sciences Corporation with a mission to image the high-energy electrons that carry much of the energy released in solar flares. It did this using its only instrument, an imaging spectrometer, which recorded the Sun’s X-rays and gamma rays. Before RHESSI, such images had never been taken.

The RHESSI data provided vital clues about solar flares and their associated coronal mass ejections. These events release energy equivalent to billions of megatons of dynamite into the solar atmosphere in a matter of minutes and can have effects on Earth, including disrupting electrical systems. Understanding these phenomena, NASA explains, has proven challenging.

Tracking and interpreting the data collected by RHESSI has been the subject of several scientific papers over the years, such as in 2009 and 2012 and 2015, but even though the satellite was decommissioned in 2018, the data collected during its 16 years of operation retains its scientific value. In 2020, for example, the University of California points to the importance of the data collected by RHESSI for understanding and predicting solar phenomena.

Earth’s orbit, a graveyard of iron

The fact that after more than a decade and a half, RHESSI has simply been deactivated and left to drift around the Earth might seem surprising, but in reality, such an episode is not the exception, but rather the rule.

Simply launching a satellite into space is a monumental and complex achievement for mankind, but no piece of equipment, however impressive and advanced, can function forever. When something breaks down thousands of kilometres up, often the simplest solution is… to do nothing.

According to Statista, as of March, the Earth was orbited by more than 8,000 active satellites, most of them launched into space by the United States, followed by the states of the former Soviet Union, China, the United Kingdom and Japan.

In addition to the satellites that are still operational, there are many that are no longer working. Britain’s Natural History Museum identifies more than 3,000 “dead” satellites in space, and in January 2023, The Washington Post noted that 1,800 of them were in low-Earth orbit, the result of fairly lax regulations, especially in the United States, by far the world’s largest satellite producer.

The criticism is not just made on theoretical grounds – space junk has actually become a problem for human activities in space. Since the launch of the first satellite in 1957 by the Soviet Union, humanity has carried so many objects into orbit that millions of man-made metal objects now circle the planet, points out NBC News.

Most are tiny, bolts, nuts and metal fragments smaller than a coin, but an estimated 30,000 such objects are large enough and fast enough to pose a serious problem in the event of a collision.

There have already been several near mishaps of this kind. In 2022, the International Space Station had to change course to avoid colliding with pieces of an old Soviet satellite that had been destroyed by a controversial test of a new anti-satellite rocket carried out by Russia.

Since its launch in 1998, the Station has had to avoid space debris more than 30 times, has been damaged in the few instances it has failed, and in 2021 the astronauts came close to an emergency depature and return to Earth in the event of a serious collision, which was thankfully avoided.

In this context, it is not surprising that in September 2022, the US government tightened the rules in this regard, imposing a five-year limit on the removal from orbit of satellites whose mission has ended. The change is a dramatic one – until then, the deadline was 25 years, CNN points out.

When satellites return home

RHESSI is far from the first satellite to return to Earth under such circumstances. In fact, it’s not even the first in 2023. In January, the ERBS satellite, also launched by NASA, entered the atmosphere over the Bering Sea, where it burned up almost entirely without causing an unpleasant incident.

ERBS has spent 38 years in space, but has been operational for only 21 of those. The data collected by the satellite during that time, since its launch in 1984, have been vital in confirming that the ozone layer was thinning globally and in the 1987 signing of the Montreal Protocol, an international treaty that limited the emission of harmful gases into the atmosphere.

A more sceptical or less global warming-minded mind might argue that these contributions are not enough to justify the risk of waking up with a satellite on your head during your morning walk, but the risk is not so great unless you are an astronaut. According to the science website Phys.org, on average, around 100 tonnes of space junk re-enters the atmosphere each year, but so far, no deaths have been recorded from space junk.

Moreover, the BBC notes, only one person has ever been hit by such an object. In 1997, Lottie Williams of Tulsa, Oklahoma, was struck in the shoulder by a piece of metal believed to have come from a Delta II rocket. The woman was not injured, took the object home and reported it to police the next day.

In the end, although the return of a satellite to Earth tends to get the world’s attention, the truth is that such an event is far less spectacular than one might think. More dramatic and pressing is the problem of satellites that remain in orbit. From this point of view, people who have dreamed of becoming astronauts can breathe a sigh of relief – life on the ground may be duller, but at least the danger of being hit by a clandestine satellite is much smaller.